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september 2011 doc id 6395 rev 4 1/33 AN1012 application note predicting the battery life and data retention period of nvrams and serial rtcs introduction standard sram devices have the advantage, over eeprom and flash memory, of high write-speed when used as main memory for a processor or microcontroller. their disadvantage is that they are volatile, and lose their contents as soon as the power supply is removed (whether this is for a prolonged period due to being turned off, or due to an unexpected glitch or loss of the power supply). stmicroelectronics manufactures a line of non-volatile srams (nvrams), known as zeropower ? or timekeeper ? nvrams, supervisors, and serial rtcs which offer the best of both worlds: memory devices that are non-volatile like eeprom, yet have the fast access of sram. these devices consist of an array of low-power cmos sram, plus a small long-life lithium power cell (along with a high-accuracy quartz crystal, in the case of the timekeeper). while the external power su pply is within its specified limits, the memory behaves as standard sram; but as soon as the external power supply strays out of tolerance, the sram becomes write-protected, and its contents are preserved by a small trickle current supplied by the internal power cell. unlike eeprom, where the data contents are gua ranteed to be preserved for 10 years (and typically last for much longer), the contents of nvram will only be retained while the internal cell is able to supply sufficient current to maintain the array. this document summarizes the factors involved in predicting the battery life, and consequently data retention under various operating conditions. many of the zeropower, timekeeper, su pervisor, and serial rtc devices are packaged in a 600 mil dip caphat?, a hybrid dip, or a 330 mil soic snaphat ? . the snaphat (shown in figure 1 ) has a removable top that includes both the long-life lithium cell and, in the case of the ti mekeeper, a high-accuracy crystal. stmicroelectronics has shippe d several million snaphats that have been used in a broad range of applications. from pc-based systems to high-end workstations, telecommunications, consumer, and automotive applications, these products have provided highly reliable data storage for the electronics industry. figure 1. standard zeropower, time keeper, supervisor, and serial rtc packages soic and snaphat ? to p caphat? hybrid dip www.st.com
contents AN1012 2/33 doc id 6395 rev 4 contents 1 process technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2 battery technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3 battery backup current - pr edicting data retention time . . . . . . . . . . . . 8 3.1 storage life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 3.2 calculating storage life . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3 capacity consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.4 calculating capacity consumption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 4 4t cell devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5 timekeeper products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 5.1 timekeeper ? register map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 5.2 timekeeper ? evolution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5.2.1 m48t02 and m48t12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 5.2.2 m48t08 and m48t18 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 5.2.3 m48t58 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 5.2.4 m48t35 and m48t37v/y . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 6 supervisor products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 7 choosing sram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 8 industrial temperature devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 9 u.l. recognition and recycling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23 10 summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 appendix a product data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 appendix b zeropower products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 appendix c timekeeper ? products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
AN1012 contents doc id 6395 rev 4 3/33 appendix d serial rtc products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 11 revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
list of tables AN1012 4/33 doc id 6395 rev 4 list of tables table 1. zeropower and timekeeper ? product categories . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 table 2. typical timekeeper (m48t37v/y) re gister map . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 table 3. typical i bat current for timekeeper devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 table 4. snaphat part numbers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 table 5. m40z300w (120mah snaphat) data retention life vs. sram type . . . . . . . . . . . . . . . . . 20 table 6. m48t201v/y (120 mah snaphat) data retention life vs. sram type . . . . . . . . . . . . . . . 21 table 7. data for zeropower ? and timekeeper ? devices . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 table 8. data from hybrid/module devices (v cc duty cycle = 0%) . . . . . . . . . . . . . . . . . . . . . . . . . . 25 table 9. data from m48z02/12 devices (available only in caphat? - br1225, 48 mah) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 table 10. data from m48z08/18, m48z58, and m48z58y devices . . . . . . . . . . . . . . . . . . . . . . . . . . 26 table 11. data from m48z35/y/av devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 7 table 12. data from m48t02/12 devices (available only in caphat? - br1632, 120 mah) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 table 13. data from m48t08/y/18 and m48t58/y devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 table 14. data from m48t35/y/av and m48t37v/y devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29 table 15. data from m41t56/94, m41st85w, m41st87w/y, and m41st95w ind. temp. (mh6) devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 table 16. data from m41t00/s, m41t11, and m41t81/s industrial temperature (mh6) devices . . . 31 table 17. document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
AN1012 list of figures doc id 6395 rev 4 5/33 list of figures figure 1. standard zeropower, ti mekeeper, supervisor, and serial rtc packages. . . . . . . . . 1 figure 2. four-transistor (4t) sram cell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 figure 3. (a) br1225 discharge rate and (b) br1632 discharge rate. . . . . . . . . . . . . . . . . . . . . . . . . 7 figure 4. predicted battery storage life versus temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 figure 5. block diagram of a timekeeper ? device . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 figure 6. m48t02/12 data retention lifetime vs. temperature (120 mah, 100% battery backup). . . . 14 figure 7. m48t08/18 data retention lifetime vs. temperature (120 mah, 100% battery backup). . . . 15 figure 8. m48t58 data retention lifetime vs. temperature (48 mah, 100% battery backup) . . . . . . . 16 figure 9. m48t58 data retention lifetime vs. temperature (120 mah, 100% battery backup) . . . . . . 16 figure 10. m48t35/37v/37y data retention lifetime vs. temperature (48 mah, 100% battery backup) 17 figure 11. m48t35/37v/37y data retention lifetime vs. temperature (120 mah, 100% battery backup) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
process technology AN1012 6/33 doc id 6395 rev 4 1 process technology the zeropower ? , timekeeper ? , supervisor, and serial rtc families consist of a broad range of products that encompass various technologies. these products can be divided into six categories, as shown in ta b l e 1 . the sram array is generally based on a 6-transistor or 4-transistor cell, as indicated by the categories (6t and 4t). figure 2 illustrates a one-bit st orage cell from a 4-transistor sram cell. the hybrid devices (also known as module de vices) contain individually packaged analog circuitry and sram. they are not covered in this document, except for the table of values for typical battery lifetimes in appendix a: product data on page 25 . table 1. zeropower and timekeeper ? product categories figure 2. four-transistor (4t) sram cell the first devices, released in 1982, were based on a conventional 6t, full-cmos, sram design. these were specified for low-voltage data retention, and were built to stringent manufacturing and test specifications. with data retention currents of less than 150 na at 70 c, these devices were designed to retain data in battery backup for at least 10 years over the full commercial temperature range. newer devices have since been released. they use 4t, cmos sram arrays. by using two poly-r resistors in place of the pull-up transistors of full-cmos design, the 4t cell is much smaller than the 6t equivalent. die size is dramatically reduced because the poly-r resistors can be stacked on top of n-channel pull-down mosfets in the cell. this leads to a net reduction in the device costs. although the current drawn from the lithium cell is increased, the devices have been specified to outlast the useful life of most equipment in which they are used. category devices zeropower (4t cell) m48z02, m48z12, m 48z08, m48z18, m48z 58/y, m48z35/y/av zeropower hybrid m48z128/y, m48z129v, m48z512a/ay, m48z2m1v/y timekeeper (4t cell) m48t08/y, m 48t58/y, m48t35/y/av, m48t37v/y timekeeper hybrid m48t128y, m48t129v, m48t512y supervisors m40z111/w, m40z300w, m48t201v/y serial rtcs (6t cell) m41t00/s, m41t11, m41t56, m41t81/s, m41t94, m41st85w, m41st87w bit-line bit-line row select gnd supply voltage poly-load resistors q1 q2 q3 q4 ai02457
AN1012 battery technology doc id 6395 rev 4 7/33 2 battery technology stmicroelectronics uses both the br1225 and the br1632 lithium button cell batteries. these have charge capacities of 48 mah an d 120 mah, respectively. their constituents have non-toxic and non-corrosive characteristic s, and are chemically and thermally stable before, during, and after discharge. this makes these cells particularly attractive for use in electrical components. they contain a solid carbon cathode that is pressed into a tablet of predetermined weight and height. the anode consists of high-purity lithium metal. the electrolyte is based on an organic solvent instead of the corrosive alkaline or acidic solution found in most conventional batteries. this greatly reduces the likelihood of internally-induced cell leakage, and reduces the ill effects in cases of externa lly-induced cell leakage. the cell is then crimp- sealed with a polypropylene grommet. st has conducted extensive tests on these cells, at temperatures up to 85 c. destructive analysis was conducted (post-stress), in order to measure such factors as weight loss and remaining charge capacity. the analysis determined that the cells were drying out, and that the weight loss was due to electrolyte evaporation. models were developed to predict the nominal rate of electrolyte loss, and how this would be reduced by adding a second level of encapsulation. this proprietary secondary seal encapsulation, adopted by st, has been found to provide up to a two-fold reduction of the electrolyte loss rate. both cells produce a nominal 2.9 v output with a flat discharge curve until the end of their effective lives, and thus confirms that both are suitable for providing battery backup to low leakage cmos srams (see figure 3 ). figure 3. (a) br1225 discharge rate and (b) br1632 discharge rate ai02519 0 1.0 1.5 2.0 2.5 3.0 3.5 voltage (v) load characteristics temp: 20c 1000 2000 3000 4000 5000 6000 duration (hrs.) (b) 15k 30k 50k 100k 0 1.0 1.5 2.0 2.5 3.0 3.5 voltage (v) load characteristics temp: 20c 200 400 600 1200 1600 1800 duration (hrs.) (a) 100k 800 1000 1400 2000 15k 30k
battery backup current - predicting data retention time AN1012 8/33 doc id 6395 rev 4 3 battery backup current - predicting data retention time a zeropower ? , timekeeper ? , supervisor, or serial rtc device will reach the end of its useful life for one of two reasons: capacity consumption it becomes discharged, having provided curr ent to the sram (and to the oscillator in the case of the timekeeper) in the battery backup mode. storage life the effects of aging will have rendered the cell inoperative before the stored charge has been fully consumed by the application. the two effects have very little influence on each other, allowing them to be treated as two independent but simultaneous mechanisms. the data retention lifetime of the device is determined by which ever fa ilure mechanism occurs first. 3.1 storage life storage life, resulting from electrolyte evaporation, is primarily a function of temperature. figure 4 illustrates the predicted storage life of th e br1225 battery vers us temperature. the results are derived from temperature-accelerated life test studies performed at stmicroelectronics. for the purpos e of testing, a cell failure is defined as the inability of a cell, stabilized at 25 c, to produce a 2.4 v closed-circuit voltage across a 250 k load resistor. the two lines, sl 1% and sl 50% , represent different failure rate distributions for the cell?s storage life. at 60 c, for example, the sl 1% line indicates that the battery has a 1% chance of failure 28 years into its life, and the sl 50% line shows that the battery has a 50% chance of failure at the 50 year mark. the sl 1% line represents the practical onset of wear out, and can be considered the worst case storage life for the cell. the sl 50% line can be considered to be the normal, or average, life. as indicated by the curves in figure 4 on page 9 , storage life does not become a limiting factor to overall battery life until temperatures in excess of 60 c to 70 c are involved. as an approximation, sl 50% = 14270 x (0.91) t , and sl 1% = 8107 x (0.91) t , when 20 c < t < 90 c.
AN1012 battery backup current - predicting data retention time doc id 6395 rev 4 9/33 figure 4. predicted battery storage life versus temperature 3.2 calculating storage life only the user can estimate predicted storage life in a given design because the ambient temperature profile is dependent upon application-controlled variables. as long as the ambient temperature is held reasonably constant, the expected storage life can be read directly from figure 4 on page 9 . if the battery spends an appreciable amount of time at a variety of temperatures, the following formula can be used to estimate predicted storage life: where, t i /t is the relative proportion (of the total time) during which the device is at ambient temperature ta i ; sl i is the storage life at ambient temperature ta i as illustrated in figure 4 ; and t is the total time = t 1 + t 2 + ... + t n . for example, consider a battery exposed to temperatures of up to 90 c for 600 hrs/yr, and temperatures of 60 c or less for the remaining 8160 hrs/yr. reading predicted t 1% values from figure 4 , sl 1 is about 1.8 yrs; sl 2 is about 28 yrs; t is 8760 hrs/yr; t 1 is 600 hrs/yr; and t 2 is 8160 hrs/yr. the predicted storage life evaluates to: this predicts that the storage lif e, in this particular case, is at least 14 years. this is, therefore, better than the normally accepted life time of 10 years. ai01024b 20 30 40 50 60 70 80 90 1 2 3 4 5 8 6 temperature (degrees celsius) 10 20 30 40 50 storage life (years) sl 50% (average) sl 1% 1 ? t 1 t ----- 1 sl 1 ---------- ?? ?? ?? t 2 t ----- 1 sl 2 ---------- ?? ?? ?? t n t ----- 1 sl n ---------- ?? ?? ?? +++ 1 ? 600 8760 -------------- - 1 1.8 --------- ?? ?? 8160 8760 -------------- - 1 28 ------ ?? ?? +
battery backup current - predicting data retention time AN1012 10/33 doc id 6395 rev 4 3.3 capacity consumption when v cc is being held by the external power supply within its specified range, the current drawn from the battery is zero. when v cc falls below the battery backup switchover voltage (v so ), the device goes into battery backup mode and draws all of its current from the battery. the v cc duty cycle represents the proportion of time, expressed as a percentage, that the device is supplied with power from the external supply, and therefore not drawing current from the battery. in its battery backup mode, the array of sram cells can be characterized by its data retention (i ccdr ) current, caused primarily by the current through the poly-r load resistors in the 4t technology, as well as also by juncti on leakage, sub-threshold current, and gate-to- substrate leakage. the total current is referred to as i bat (the current drawn during battery backup mode). for zeropower ? devices, this is the sum of leakage currents plus the current necessary to maintain the sram array. for timekeeper ? devices, it is the sum of the array current (including leakage) and the clock current: i bat = i array + i clock many factors need to be taken into account when calculating the i bat current, including process parameters, working temperature, and the v cc duty cycle. 3.4 calculating capacity consumption capacity consumption is simply calculated by: where: battery capacity is measured in ampere-hours; 8760 is the constant for the number of hours there are in a year; v cc duty cycle is measured as a percentage; and i bat is measured in amperes. for the m48t35y, a 32k x 8 timekeeper ? device with a 0.048 ah (48 mah) m4t28- br12sh1 battery, the typical battery current is approximately 2666 na at 70 c. so, if the v cc duty cycle is 50%, the predicted capacity life is: and therefore is about 4.11 years at 70 c. batterycapacity 8760 1 v cc dutycycle 100 ? ? () i bat ----------------------------------------------------------------------------------------------------- 0.048 8760 0.5 2666 10 9 ? ------------------------------------------------------------ -
AN1012 4t cell devices doc id 6395 rev 4 11/33 4 4t cell devices in moving to the newer process technologies (e.g., m48z58 (8k x 8) device), stmicroelectronics has chosen to reduce the active current as well as decrease the die size. the stmicroelectronics hcmos4pz process is a 0.6 m, double-level metal process. in the standard sram memory cell, 6 transistors are formed into a pair of cross-coupled inverters. in the 4t memory cell, the top two p-channel devices are replaced by poly-silicon load resistors (poly-r). this co mbination allows for significant die size reduction because the poly-r structures can be stacked on top of the active n-channel devices. there is always at least one direct path constantly leaking current to ground in each cell because of the poly-r structures in each sram cell. however, the value of the resistor is extremely high (about 3t at 25 c), so at a cell voltage of 3 v, this leads to a leakage current of 1 pa. multiplying by the number of cells within the array, the array standby current can be calculated (i.e. 65.5 na for a 65536-cell array). the poly-r structure values are dependent on temperature, so the entire array current is very strongly temperature-dependent. appendix b: zeropower products on page 26 shows the expected battery lifetime of an m48z58 device versus working temperature with a v cc duty cycle of 0%. the original specification was an expected lifetime of greater than 10 years at 25 c but, in fact, this target is typically achieved even at 70 c. by reducing the temperature, the expected lifetime rises to greater than 20 years (i.e., when the device is operated at 50 c). this change is defined entirely by the temperature sensitivity of the poly-r structures within each sram cell. the m48z35 also employs the stmicroelectronics hcmos4pz process, 4t sram cell technology. appendix b shows the expected battery lifetime of an m48z35 device versus working temperature with a v cc duty cycle of 0%. from this we can see that expected lifetime is typically greater than 20 years when operated at 30 c with no external v cc applied, and falls to approximately 2.6 years for continuous battery backup at 70 c. this is to be expected, due to the increased current consumption inherent in the 4t sram cell architecture. it should be noted that th is data is based on usage of the snaphat ? product which includes a 48 mah battery.
timekeeper products AN1012 12/33 doc id 6395 rev 4 5 timekeeper products timekeeper ? products are very similar in cons truction and operation to zeropower ? products. however, they must be evaluated separately. the current drawn is highly dependent not only on the temperature, but also on wh ether the oscillator is active. the main components of tim ekeeper devices are (see figure 5 ): a cmos ram array; voltage sense and switching circuitry; an analog oscillator and clock chain; a lithium power cell; and a high-accuracy quartz crystal. figure 5. block diag ram of a timekeeper ? device ai01 383 d lithium cell o s cillator and clock chain v pfd r s t v cc v ss 3 2,76 8 hz cry s ta l voltage s en s e and s witching circuitry 16 x 8 biport s ram array 8 176 x 8 s ram array a0-a12 dq0-dq7 e w g power irq/ft
AN1012 timekeeper products doc id 6395 rev 4 13/33 5.1 timekeeper ? register map ta bl e 2 shows a typical register map for the seconds, minutes, hours, date, day, month, and year fields. this information is stored in bi nary coded decimal (bcd) format. these basic functions are availabl e on all timekeeper devices. additi onal features (e.g., watchdog timer, alarms, battery low flag, and a wake-up function) have additional registers allocated to them (such as those shown for the m48t37v/y in ta b l e 2 ). the timekeeper register locations are constructed from biport? memory cells which allow data access from two sides. the on-chip system clock connects to one side (the system side) and the user data is output to connections on the other (the user?s side). at one-second intervals, clock pulses are generated by the oscillator and clock chain structure. the system side updates the new time in the timekeeper registers. each ti mekeeper register loca tion (e.g. minutes, hours, day) is then updated as necessary. when the user wants to write a new time, the ?w? bit (the write bit) of the control register is reset, causing the biport cells to upload the new system time. the user accesses the timekeeper and arra y data by executing standard read/write cycles. the oscillator and clock chain structure consists of a mixture of analog and digital circuitry, and account for the majority of the i bat current. ta b l e 3 gives conservative estimates of the currents drawn as a function of technology and working temperature. table 2. typical timekeeper (m48t37v/y) register map table 3. typical i bat current for timekeeper devices address data function range (in bcd format) d7 d6 d5 d4 d3 d2 d1 d0 7fffh 10 years year year 00-99 7ffeh 0 0 0 10m month month 01-12 7ffdh 0 0 10 date date date 01-31 7ffch 0 ft 0 0 0 day day 01-7 7ffbh 0 0 10 hours hours hours 00-23 7ffah 0 10 minutes 10 minutes minute 00-59 7ff9h st 10 seconds seconds second 00-59 7ff8h w r s calibration control 7ff7h wds bmb4 bmb3 bmb2 bmb1 bmb0 rb1 rb0 watch 7ff6h afe 0 abe 0 0 0 0 0 interrupt 7ff5h rpt4 0 ai 10 date alarm date a date 01-31 7ff4h rpt3 0 ai 10 hour alarm hour a hour 00-23 7ff3h rpt2 alarm 10 minutes alarm minutes a minute 00-59 7ff2h rpt1 alarm 10 seconds alarm seconds a second 00-59 7ff1h 1000 years 100 years century 00-99 7ff0h wdf af 0 bl z z z z flags typical at 20c typical at 70c capacity technology array clock array clock 64 kbit 4t cell 40 na 497 na 511 na 619 na
timekeeper products AN1012 14/33 doc id 6395 rev 4 5.2 timekeeper ? evolution timekeeper products have seen a continuous ev olutionary cycle since their initial market introduction in the 1990s. 5.2.1 m48t02 and m48t12 the first timekeeper products released were the mk48t02 and mk48t12 which offered 2k x 8 ram and employed the stmicroelectronics 2.0 m spectrum? cmos technology. when released, these products included a br1225 lithium cell with a specified 39 mah capacity. this combination offered the user approximately 3.5 years of continuous battery backup life. since that time, the devices have been moved to the 4t cell technology (hcmos4pz) and a caphat? package revision which includes a larger capacity lithium cell (120 mah br1632) capacity, and new part numbers (m48t02/12). these changes increased the expected battery life to 19 years at 60c. figure 6 shows expected battery lifetime versus temperature with 100% battery backup. the data shows that by operating the devices at various temperatures, designers can expect a battery life approaching 20 years under most conditions. figure 6. m48t02/12 data retention lifetime vs. temperature (120 mah, 100% battery backup)
AN1012 timekeeper products doc id 6395 rev 4 15/33 5.2.2 m48t08 and m48t18 the next timekeeper ? to be released was the mk48t08/18 family, which has an 8k x 8 sram array. by using the more advanced 1.2 m hcmos3 process and refining the on- board oscillator, stmicroelectronics was able to offer a nearly three-fold increase in battery lifetime, even though the array size had increased by a factor of four. this product was later converted to the 0.6 m, double-level metal hcmos4pz process for 4t sram cells. the battery was then upgraded to 120 mah for the caphat? package revision (part numbers m48t08/18), which provides a battery life of at least 10 years across the commercial temperature range (0 c to 70 c, see figure 7 ). in the m48t08/18 datasheet, the battery lifetime (t dr , data retention time) has been specified as 10 years or greater across the commercial temperature range (for a 0% v cc duty cycle). figure 7. m48t08/18 data retention lifetime vs. temperature (120 mah, 100% battery backup)
timekeeper products AN1012 16/33 doc id 6395 rev 4 5.2.3 m48t58 the next timekeeper ? product was the m48t58 which is fabricated on the 0.6 m, double-level metal hcmos4pz process for 4t sram cells. table 13 on page 28 , appendix c: timekeeper ? products on page 28 , figure 8 , and figure 9 on page 16 show the extent to which the data retention of these devices is more dependent on temperature. higher temperatures cause lower resistor values (and therefore, higher currents) because of the negative temperature coefficient of the poly-r resistors. data retention lifetimes typically range from 8.6 years (at 30 c) for devices in the caphat? package, with a 48 mah battery (see figure 8 ), and up to 20 years (and more) for the snaphat package with a 120 mah br1632 battery (see figure 9 ). as always, several factors affect battery lifetime, including the v cc duty cycle and temperature. figure 8. m48t58 data retention lifetime vs. temperature (48 mah, 100% battery backup) figure 9. m48t58 data retention lifetime vs. temperature (120 mah, 100% battery backup)
AN1012 timekeeper products doc id 6395 rev 4 17/33 5.2.4 m48t35 and m48t37v/y the m48t35 and m48t37v/y families use the same technology as the m48t58 device, but with a 32k x 8 sram array. figure 10 and figure 11 show the expected battery lifetime versus temperature. the expected battery lifetime (at 30 c with no periods of valid v cc ) is typically 6.8 years (this assumes that a 48 mah battery is used, see figure 10 ). devices in the larger m4t32-br12sh snaphat ? package have a data retention lifetime of greater than twice this (almost 17 years, see figure 11 ). figure 10. m48t35/37v/37y data retention lifetime vs. temperature (48 mah, 100% battery backup) figure 11. m48t35/37v/37y data retention lifetime vs. temperature (120 mah, 100% battery backup)
timekeeper products AN1012 18/33 doc id 6395 rev 4 if data retention lifetimes greater than those shown are required, the user is advised to choose the version of the device in a snaphat ? package. then, as the battery starts to reach the end of its useful life, it is possible to remove the snaphat top containing the nearly expended cell and replac e it with a fresh snaphat top. no data will be lost during the process, provided that the board remains powered up during the operation (although some time will be lost du e to the momentary removal of the 32 khz crystal). ta bl e 4 shows which snaphat top part numbers are available. table 4. snaphat part numbers part number description package m4z28-br00sh li batt ery (48mah) for zeropower products and supervisors snaphat m4z32-br00sh li batt ery (120mah) for zeropower products and supervisors snaphat m4t28-br12sh li ba ttery (48mah) for timekeeper pr oducts and supervisors snaphat m4t32-br12sh li batt ery (120mah) for timekeeper products and supervisors snaphat
AN1012 supervisor products doc id 6395 rev 4 19/33 6 supervisor products stmicroelectronics also has a family of zeropower ? and timekeeper ? supervisor devices. supervisors are self-contained units that allow standard low-power srams to be turned into non-volatile memory devices. they monitor and provide v cc input for one or more external srams the same way zeropower and timekeeper products do. they use a precision voltage reference and comparator to monitor the v cc input for going out-of- tolerance. when v cc becomes invalid, the supervisor?s conditioned chip-enable outputs (e con ) are forced to their ?inactive? state, thereby putting each external sram into its own write-protect state. during the power failure, the supervisor provides the power for the sram from the lithium cell within its snaphat top. the supervis or switches the power source back to the v cc supply as soon as the voltage returns to specified levels.
choosing sram AN1012 20/33 doc id 6395 rev 4 7 choosing sram most low power srams on the market today can be used with both zeropower ? and timekeeper ? supervisors, although there are some issues that need addressing before finally choosing which sram to use. the chip enable input, when taken inactive, must disable all the other inputs to the sram. this allows inputs to the external srams to be treated as ?don?t care? once v cc falls below v pfd (min). the sram should guarantee data retention when working at v cc = 2.0 volts. the chip-enable access time must be sufficient to meet the system needs, taking into account propagation delays on chip enable and output enable. most srams specify a data retention current (i ccdr ) at 3.0 v. manufacturers generally specify a typical condition for room temperature along with a worst case condition (generally at elevated temperatures). th e system level requirements will determine the choice of which value to use. the data retention current value of the srams can then be added to the i bat value of the supervisor to determine the total current requirements for data retention. the available battery capacity for the snaphat ? of your choice can then be divided by this current to determine the data retention period (see section 3.3: capacity consumption on page 10 ). for example, the m48t201v/y has an i bat value of 575 na at 25 c, and 800 na at 70 c. the m40z300w has an i bat value of 5 na at 25 c, and 100 na at 70 c. ta b l e 5 indicates typical data retention lifetimes for the m40z300w zeropower supervisor when it is used with a number of commercially ava ilable 1 mbit and 4 mbit srams. table6 on page21 shows the same kind of information fo r the m48t201v/y timekeeper supervisors. table 5. m40z300w (120mah snaphat) data retention life vs. sram type size (mbit) product i bat (sram) (na) i bat (total) (na) lifetime in years (1) 1. according to the respective manufactu rer?s datasheets at the time of writing. 25c 70c 25c 70c 25c 70c 1 hynix hy628100bllt1-55 1000 10000 1005 10100 13.6 1.4 hy62v8100bllt1-70 (2) 1000 10000 1005 10100 13.6 1.4 renesas m5m51008dvp-55h 500 10000 505 10100 > 20 1.4 m5m5v108dvp-70h (2) 2. 3 v device 1000 10000 1005 10100 13.6 1.4 4 renesas r1lp0408csb-5sc 800 8000 805 8100 17.0 1.7 r1lv0408csb-5sc (2) 500 8000 805 8100 > 20 1.7 8 renesas hm62v8100ltti-5sl 500 10000 505 10100 > 20 1.4 samsung k6x8008t2b-uf5500 n/a 15000 n/a 15100 n/a 0.9
AN1012 choosing sram doc id 6395 rev 4 21/33 table 6. m48t201v/y (120 mah snaphat) data retention life vs. sram type size (mbit) product i bat (sram) (na) i bat (total) (na) lifetime in years (1) 1. according to the respective manufactu rer?s datasheets at the time of writing. 25c 70c 25c 70c 25c 70c 1 hynix hy628100bllt1-55 1000 10000 1075 10800 8.7 1.3 hy62v8100bllt1-70 (2) 2. 3 v device 1000 10000 1075 10800 8.7 1.3 renesas m5m51008dvp-55h 500 10000 1075 10800 12.7 1.3 m5m5v108dvp-70h (2) 1000 10000 1575 10800 8.7 1.3 4 renesas r1lp0408csb-5sc 800 8000 1375 8800 10.0 1.6 r1lv0408csb-5sc (2) 500 8000 1075 8800 12.7 1.6 8 renesas hm62v8100ltti-5sl 500 10000 1075 10800 12.7 1.3 samsung k6x8008t2b-uf5500 n/a 15000 n/a 15800 n/a 0.9
industrial temperature devices AN1012 22/33 doc id 6395 rev 4 8 industrial temperature devices due to ever increasing requirements for portability and operati on under extreme environmental conditions, stmicroelectronics offers industrial temperature versions (?40c to +85c) of our serial rtc devices. this expanded operating range allows these products to perform under more extreme temperatures for applications such as: cell phone base stations; traffic control; portable equipment; land, water, and aircraft instrumentation; and industrial control equipment. these products are indicated by the digit ?6? at the end of the sales-type. the industrial temperature timekeeper ? snaphat ? top is also designated by the suffix ?6.? predicted data retention lifetimes are listed in appendix b: zeropower products on page 26 and appendix c: timekeeper ? products on page 28 .
AN1012 u.l. recognition and recycling doc id 6395 rev 4 23/33 9 u.l. recognition and recycling while providing innovative, leading edge pro ducts, stmicroelectronics remains committed to safety, including its products, its customers, and the environment. each device contains reverse-charge protection circuitry, and uses safe lithium mono-fluoride batteries. all zeropower ? , timekeeper, supervisor, and serial rtc components are recognized by underwriter?s laboratory under file number e89556, and are compliant to the ll-94-vo flammability rating. the unique snaphat packaging consists of a 330 mil soic device and a separate, ?snap- on? snaphat, which includes both the lithium power cell, and in the case of timekeeper product, a high accuracy crystal. the snaphat is removable and can be replaced, providing the added benefit of proper disposal or recycling that has not been available before with nvrams. various companies offer recycling and safe disposal of scrap lithium cells.
summary AN1012 24/33 doc id 6395 rev 4 10 summary battery life and data retention for zeropower ? and timekeeper ? products are primarily functions of two factors: capacity consumption, and storage life of the lithium button cell battery. due to the fact that storage life (caused by electrolyte evaporation) has little effect at temperatures below 60 c, the data retention of most app lications will be dependent upon the i ccdr of the sram being backed-up, as well as the v cc duty cycle. this allows a fairly simple calculation (see section 3.4: calculating capacity consumption on page 10 ) to be used to determine the lifetime. all st zeropower products are able to offer at least a 10 year data retention life, typically at 40 c. this may be increased by reducing the temperature, increasing the v cc duty cycle, or in the case of the surface mount snaphat ? products, using the larger 120 mah snaphat top. for the timekeeper family, batter y lifetimes are also affected by the percentage of time the oscillator is in operation. commercial devices fabricated in 4t technologies provide 7 years of continuous operation at 20 c using the 48 mah m4t28-br12sh snaphat top, and typically greater than 15 years with the 120 mah m4t32-br12sh snaphat top. the zeropower and timekeeper supervisor families allow the user to purchase commodity srams at the best available market price. however, overall data retention life will be determined by the i ccdr of the sram selected.
AN1012 product data doc id 6395 rev 4 25/33 appendix a product data note: the symbol ?>>? means, ?... much greater than...? table 7. data for zeropower ? and timekeeper ? devices table 8. data from hybrid/module devices (v cc duty cycle = 0%) note: these devices are not recommended for new design. please contact local st sales office for availability. device process technology sram cell battery type i bat (t = 20c) (na) typical data retention lifetime (1) (years) 1. the data retention lifetime can be significantly increa sed by using the snaphat (zeropower or timekeeper, as appropriate) with the higher capacity br1632 battery. snaphat (2) 2. the larger capacity br1632 (120 mah) battery is also available in the snaphat package. caphat m48z02/12 0.6 m, hcmos4pz 4t n/a br1225 9 10 m48z08/18 0.6 m, hcmos4pz 4t br1225 br1225 37 10 m48z35/y/av 0.6 m, hcmos4pz 4t br1225 br1225 148 10 m48z58/y 0.6 m, hcmos4pz 4t br1225 br1225 37 10 m48t02/12 0.6 m, hcmos4pz 4t n/a br1632 506 10 m48t08/18 0.6 m, hcmos4pz 4t br1225 br1632 535 10 m48t35/y/av 0.6 m, hcmos4pz 4t br1225 br1632 646 7/10 m48t37y 0.6 m, hcmos4pz 4t br1225 n/a 646 7 m48t58/y 0.6 m, hcmos4pz 4t br1225 br1225 535 7 device specification at t = 25c (years) experimental conditions (years) 0c 25c 70c m48z128/y 10 >> 20 > 20 2.3 m48z129v 10 >> 20 > 20 2.3 m48z512a/av/ay 10 >> 20 > 20 6.0 m48z2m1v/y 10 > 20 > 20 3.1 m48t128y 10 > 20 16.6 2.0 m48t129v/y 10 > 20 16.6 2.0 m48t512y 10 > 20 19.4 4.8
zeropower products AN1012 26/33 doc id 6395 rev 4 appendix b zeropower products the tables in this appendix use the terms ?typical? and ?worst case? to indicate the ?mean value at the given temperature? and ?mean value plus maximum expected deviation at the given temperature.? note: the symbol ?>>? means, ?... much greater than...? table 9. data from m48z02/12 devices (available only in caphat? - br1225, 48 mah) table 10. data from m48z08/18, m48z58, and m48z58y devices temperature (c) v cc duty cycle = 0% v cc duty cycle = 100%, shelf life (years) typical (years) worst case (years) 0 >> 20 >> 20 >> 20 10 >> 20 >> 20 >> 20 20 >> 20 >> 20 >> 20 25 >> 20 >> 20 >> 20 30 >> 20 >> 20 >> 20 40 >> 20 >> 20 >> 20 50 >> 20 >> 20 >> 20 60 > 20 > 20 > 20 70 11.0 11.0 11.0 temperature (c) caphat or snaphat (br1225, 48 mah) snaphat (br1632, 120 mah) v cc duty cycle = 100%, shelf life (years) v cc duty cycle = 0% typical (years) worst case (years) typical (years) worst case (years) 0 >> 20 >> 20 >> 20 >> 20 >> 20 10 >> 20 >> 20 >> 20 >> 20 >> 20 20 >> 20 >> 20 >> 20 >> 20 >> 20 25 >> 20 >> 20 >> 20 >> 20 >> 20 30 >> 20 >> 20 >> 20 >> 20 >> 20 40 >> 20 > 20 >> 20 >> 20 >> 20 50 > 20 16.4 >> 20 >> 20 >> 20 60 19.7 10.1 > 20 >20 > 20 70 11.0 5.8 11.0 11.0 11.0
AN1012 zeropower products doc id 6395 rev 4 27/33 table 11. data from m48z35/y/av devices temperature (c) caphat or snaphat (br1225, 48 mah) snaphat (br1632, 120 mah) v cc duty cycle = 100%, shelf life (years) v cc duty cycle = 0% typical (years) worst case (years) typical (years) worst case (years) 0 >> 20 >> 20 >> 20 >> 20 >> 20 10 >> 20 > 20 >> 20 >> 20 >> 20 20 >> 20 > 20 >> 20 >> 20 >> 20 25 > 20 17.2 >> 20 >> 20 >> 20 30 > 20 12.9 >> 20 > 20 >> 20 40 14.2 7.5 > 20 18.6 >> 20 50 7.4 3.8 18.4 9.5 >> 20 60 4.5 2.5 11.3 6.2 > 20 70 2.6 1.4 6.5 3.5 11.0
timekeeper ? products AN1012 28/33 doc id 6395 rev 4 appendix c timekeeper ? products table 12. data from m48t02/12 devices (available only in caphat? - br1632, 120 mah) table 13. data from m48t08/y/18 and m48t58/y devices temperature (c) v cc duty cycle = 0% v cc duty cycle = 100%, shelf life (years) typical (years) worst case (years) 0 > 20 > 20 >> 20 10 > 20 > 20 >> 20 20 > 20 > 20 >> 20 25 > 20 > 20 >> 20 30 > 20 > 20 >> 20 40 > 20 > 20 >> 20 50 > 20 18.5 >> 20 60 19.0 17.0 > 20 70 11.0 11.0 11.0 temperature (c) caphat or snaphat (br1225, 48 mah) caphat (1) or snaphat (br1632, 120 mah) 1. only available in m48t08 and m48t18 caphat?. v cc duty cycle = 100%, shelf life (years) v cc duty cycle = 0% typical (years) worst case (years) typical (years) worst case (years) 0 11.0 10.0 > 20 > 20 >> 20 10 10.1 9.2 > 20 > 20 >> 20 20 9.4 8.5 > 20 > 20 >> 20 25 9.0 8.1 > 20 > 20 >> 20 30 8.6 7.6 > 20 19.0 >> 20 40 7.9 6.8 19.7 16.9 >> 20 50 6.9 5.6 17.1 13.9 >> 20 60 5.9 4.5 14.8 11.3 > 20 70 4.8 3.4 11.0 8.4 11.0
AN1012 timekeeper ? products doc id 6395 rev 4 29/33 table 14. data from m48t35/y/av and m48t37v/y devices temperature (c) snaphat (br1225, 48 mah) caphat or snaphat (br1632, 120 mah) v cc duty cycle = 100%, shelf life (years) v cc duty cycle = 0% typical (years) worst case (years) typical (years) worst case (years) 0 10.4 9.0 > 20 > 20 >> 20 10 9.0 7.6 > 20 19.1 >> 20 20 8.1 6.7 > 20 16.6 >> 20 25 7.4 6.0 18.6 14.9 >> 20 30 6.8 5.3 16.9 13.2 >> 20 40 5.5 4.0 13.8 10.0 >> 20 50 4.0 2.6 10.0 6.6 >> 20 60 2.9 1.9 7.4 4.8 > 20 70 2.0 1.2 5.0 3.0 11.0
serial rtc products AN1012 30/33 doc id 6395 rev 4 appendix d serial rtc products table 15. data from m41t56/94, m41st85w, m41st87w/y, and m41st95w ind. temp. (mh6) devices temperature (c) snaphat (br1632, 120 mah) v cc duty cycle = 100%, shelf life (years) v cc duty cycle = 0% typical (years) ?40 > 20 >> 20 ?30 > 20 >> 20 ?20 > 20 >> 20 ?10 > 20 >> 20 0> 20 >> 20 10 > 20 >> 20 20 > 20 >> 20 25 > 20 >> 20 30 > 20 >> 20 40 > 20 >> 20 50 > 20 >> 20 60 > 20 > 20 70 11.0 11.0 80 4.3 4.3 85 2.7 2.7
AN1012 serial rtc products doc id 6395 rev 4 31/33 table 16. data from m41t00/s, m41t11, and m41t81/s industrial temperature (mh6) devices temperature (c) snaphat (br1632, 120 mah) v cc duty cycle = 100%, shelf life (years) v cc duty cycle = 0% typical (years) ?40 > 20 >> 20 ?30 > 20 >> 20 ?20 > 20 >> 20 ?10 > 20 >> 20 0 > 20 >> 20 10 > 20 >> 20 20 > 20 >> 20 25 > 20 >> 20 30 > 20 >> 20 40 > 20 >> 20 50 > 20 >> 20 60 > 20 > 20 70 11.0 11.0 80 4.3 4.3 85 2.7 2.7
revision history AN1012 32/33 doc id 6395 rev 4 11 revision history table 17. document revision history date revision changes 13-oct-1998 0.0 document written 14-dec-1998 1.0 1st edition of zeropo wer and timekeeper application note book 07-mar-2000 1.1 data changed from that of 49 mah and 130 mah batteries to that of 48 mah and 120 mah batteries 25-apr-2000 1.2 controllers renamed as supervisors 26-jun-2000 1.3 m48t35 typ data retention lifetime changed to 7/10 years (tab-7 on p15) 08-may-2001 2.0 reformatted, text, graphics, values updated ( figure 6 , 7 , 8 , 10 ; ta bl e 3 , 5 , 6 , 7 , 15 , 13 , 14 , 16 , 17 ) 15-may-2001 2.1 change trend colors to black ( figure 6 , 7 , 8 , 10 ) 31-may-2005 3.0 update information ( figure 1 , 6 , 7 , 8 , 9 , 10 ; ta b l e 1 , 3 , 5 , 6 , 7 , 8 , 9 , 11 , 12 , 13 , 14 , 15 , 16 ) 15-sep-2011 4 product updates; minor textual updates; revised document presentation.
AN1012 doc id 6395 rev 4 33/33 please read carefully: information in this document is provided solely in connection with st products. stmicroelectronics nv and its subsidiaries (?st ?) reserve the right to make changes, corrections, modifications or improvements, to this document, and the products and services described he rein at any time, without notice. all st products are sold pursuant to st?s terms and conditions of sale. purchasers are solely responsible for the choice, selection and use of the st products and services described herein, and st as sumes no liability whatsoever relating to the choice, selection or use of the st products and services described herein. no license, express or implied, by estoppel or otherwise, to any intellectual property rights is granted under this document. i f any part of this document refers to any third party products or services it shall not be deemed a license grant by st for the use of such third party products or services, or any intellectual property contained therein or considered as a warranty covering the use in any manner whatsoev er of such third party products or services or any intellectual property contained therein. unless otherwise set forth in st?s terms and conditions of sale st disclaims any express or implied warranty with respect to the use and/or sale of st products including without limitation implied warranties of merchantability, fitness for a parti cular purpose (and their equivalents under the laws of any jurisdiction), or infringement of any patent, copyright or other intellectual property right. unless expressly approved in writing by two authorized st representatives, st products are not recommended, authorized or warranted for use in milita ry, air craft, space, life saving, or life sustaining applications, nor in products or systems where failure or malfunction may result in personal injury, death, or severe property or environmental damage. st products which are not specified as "automotive grade" may only be used in automotive applications at user?s own risk. resale of st products with provisions different from the statements and/or technical features set forth in this document shall immediately void any warranty granted by st for the st product or service described herein and shall not create or extend in any manner whatsoev er, any liability of st. st and the st logo are trademarks or registered trademarks of st in various countries. information in this document supersedes and replaces all information previously supplied. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners. ? 2011 stmicroelectronics - all rights reserved stmicroelectronics group of companies australia - belgium - brazil - canada - china - czech republic - finland - france - germany - hong kong - india - israel - ital y - japan - malaysia - malta - morocco - philippines - singapore - spain - sweden - switzerland - united kingdom - united states of america www.st.com

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