application note 1 of 4 www.xicor.com an 121 xicor x1243 real time clock oscillator requirements by carlos martinez, march 1999 introduction with any real time clock, there needs to be a quartz crystal controlling the oscillator frequency. this is necessary, because variations of even 20 parts per million in the oscillator frequency result in a clock that is off by almost a minute a month. typically this oscillator consists of an inverting ampli?r and a 32.768 khz crystal network. the constraints on this oscillator circuit are many. it should: 1. be simple, 2. oscillate at the desired frequency and not vary in frequency over voltage or temperature, 3. readily oscillate on application of power at any oper- ating voltage and temperature. 4. be a ?lean?oscillation, with no instabilities that can be interpreted as extra clocks. 5. not consume much current. often, meeting one requirement makes it harder to meet another. this application note looks at the xicor real time clock oscillator dircuit and examines the network requirements for maximum coverage of the desired features. oscillator accuracy the real time clock operates at 32.768 khz, because this frequency divided by 2 15 equals one clock per second. the oscillator accuracy depends primarily on providing the proper capacitive load to the crystal oscillator element. a typical quartz crystal oscillator is the cfs-206 from citizen. this crystal expects a 12.5 pf load for operation at exactly 32.768 khz. if the load varies, the oscillator frequency varies. this relationship is shown graphically in figure 1. problems arise when process variations or component tolerances change the capacitive loading, hence affecting frequency accuracy. since this oscillator provides the timebase for the real time clock counters, any variations in frequency translates into inaccuracies in the measure- ment of the real time. xicor s ?st real time clock makes use of external components to allow adjustments for greater accuracy. oscillator network the xicor real time clock requires the use of two external resistors and 2 external capacitors in addition to the crystal itself (see figure 2) the resistors set the level of feedback in the oscillator circuit. the capacitors adjust the loading on the crystal for both stability of the oscillator and proper loading of the crystal element. 5 6 8 10 12 14 16 18 20 22 24 26 28 0 -20 -40 60 40 20 100 80 figure 1. frequency accuracy vs. load capacitance frequency deviation (ppm) load capacitance (pf) march, 1999
2 of 4 an 121 application note www.xicor.com march, 1999 for the circuit of figure 2, the crystal sees the loading of the two capacitors in series. this is represented as: where c1 = 18 pf and c2 = 43 pf. this works out to a loading of 12.7 pf. similar loading is achieved with c1 = 18 pf and c2 = 39 pf (12.31 pf) or c1 = 16 pf and c2 = 56 pf (12.4 pf). a 12.5 pf load capacitance yields an expected error of about 0 ppm operating conditions the xicor real time clock is functional over an indus- trial temperature range (-40? to +85?). the crystal in an external network, however, typically operates over a much narrower range. seiko speci?s that their crystal operates from -10? to +60?. also, as the temperature varies from 25?, the accuracy of the crystal degrades. figure 3 shows a frequency-temperture curve for the seiko vt-200. this curve shows that changing the temperature by plus or minus 20? results in a 10 ppm reduction in the frequency of the crystal (and a comparable error in the real time clock reading.) in order to ?iden?the temper- ature range over which there is less than 10 ppm change in frequency, the values of the capacitors in the external network can be changed. for example, the capacitors can be chosen to give a +10 ppm error at 25?. this will effectively shift the curve up, so instead of ?0 ppm from 7? to 43?, there is an accuracy of ?0 ppm from 0? to 50?. rtc oscillator characterization characterization of the xicor real time clock indicates that the oscillator frequency changes over temperature with a curve similar to that of the crystal, though the temperature impact is not as great in the circuit as with an individual crystal. there is also a variation in frequency that is dependent on the operating voltage applied to the device. this variation is much less signi? cant than the temperature and is linear with respect to voltage. characteristic curves over temperature (0? to 60?) and voltage are shown in figure 4. an alternative to using an external crystal and network is the use of an external temperature controlled crystal oscillator (tcxo) module. typically this would be used in an application that requires operation over an indus- figure 2. rtc oscillator network and power supply x1 x2 3v lithuim v back v ss v cc 12 pf 68 pf 10 m ? 360 k ? 2.7 - 5.5 v 1 ctot ----------- - 1 c1 ------- - 1 c2 ------- - + = 0 10 20 30 40 50 60 -20 -10 0 -10 -20 -30 -40 -50 -60 temperature ( o c) frequency deviation (ppm) figure 3. crystal frequency-temperature curve 0 10 20 30 40 50 60 -20 -10 -10 -20 -30 -40 -50 -60 temperature ( o c) 0 10 5v 2.7v 1.8v frequency deviation (ppm) figure 4. rtc oscillator frequency vs. temperature and voltage
3 of 4 an 121 application note www.xicor.com march, 1999 trial temperature range or more accuracy over temperature. tcxos are highly accurate and consume little current. current consumption the current consumption of the real time clock circuit can be an important consideration in the design of a system. the xicor real time clock device itself consumes very little current. when driven by an external 32.768 khz oscillator the rtc draws less than 1ua from the supply source. when using the external network, however, the current consumption is somewhat higher. this current consumption is greatly dependant on the power supply voltage and not by the temperature. the curves in figure 6 show typical current consumption over operating voltage. in a typical application (see figure 2) the main system supply connects to v cc and a primary (non-recharge- able) battery connects to v back . when v cc is greater than v back , an internal switch from the battery is open and no current ?ws out of the battery. when v cc fails, the real time clock circuits automatically connect to the backup supply source. a common backup battery is a lithium type that provides between 3v and 2v. an ener- gizer cr2032 lithium battery provides 225 mahr (from 3v down to 2v) and would maintain the time and date in the xicor rtc for a cummulative 10 years of main power failure. alternatively, a supercapacitor can be used to provide voltage to the clock during main power failures. a supercap can provide several days of operation during a power outage and does not pose the environmental or manufacturing problems of lithium batteries. since the capacitor needs to be charged, an external diode is required in the circuit as shown in figure 7. supercaps are available with capacities to 1 f and above. using a standard capacitor discharge equation, with the following assumptions, table 1 shows maximum dura- tion of a power outage that various supercaps can maintain the rtc. this capability is often all that is required. figure 5. rtc wiring tcx0 x1 x2 v ss v cc 2.7 - 5.5 v open tcxo 1.8v 3.6v 5v 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 operating voltage 2.7v current consumption (ua) figure 6. current consumption vs. operating voltage (recommended external crystal network) table 1: rtc operating time using a supercap capacitance power outage duration capacitor physical size [1] 1. panasonic sg series gold capacitors 0.1 f 0.8 days 12.5mm diameter, 5.5mm height 0.22 f 1.8 days 0.47 f 3.9 days 20mm diameter, 6mm height 1.0 f 8.3 days figure 7. use of a supercap for v back supercap v back v ss v cc 2.7 - 5.5 v ic dv dt ------- = dv = 4.3v to 1.8v i = 3.5a (average current from 4.3v to 1.8v)
4 of 4 an 121 application note www.xicor.com march, 1999 board layout while xicor s real time clock is less sensitive to board layout than some clocks, there is still reason to be cautious in the layout of the external components. the problem exists because of the internal threshold levels on the x1 input are necessarily narrow. noise on the x1 pin can therefore cause oscillations as the input compar- ator tries to track the noise. these oscillations are perceived as additional clocks, so the rtc appears to run fast. while there are many ways to implement this layout, one suggestion is provided in figure 8. this layout includes a bypass capacitor from v cc to v ss and a pull-up resistor (4.3 k ? ) on the irq output. not included are pull up resistors on the sda and scl lines. summary the xicor real time clock integrates a clock calendar, alarm, battery backup circuit, eeprom and (in some versions) a cpu supervisor into a single package, however the primary role of this device is to maintain the real time. to do this, a crystal controlled oscillator provides an accurate timebase. by using external components, this oscillator achieves better than ?0 seconds a month accuracy over 0? to 50? and consumes so little current the clock operates for up to 10 years from a single lithium battery. figure 8. pcb layout for x1243 x1243 soic 1 vss vcc pcb layout component placement 360 k 10 m 68 pf 12 pf 0.1 f 4.3 k
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