1/8 december 2003 AN1790 application note how to connect st reset circuits to a microprocessor contents introduction reset thresholds reset output manual reset input connecting the reset circuits in an application ? connection in a noisy environment ? connection to a bi-direc- tional microprocessor reset ? connection with reset output valid down to vcc = 0v conclusion references revision history this application note describes how to connect the stm809, stm810, stm811 and stm812 reset circuits to microproces- sor systems. further information on reset circuits can be found in the stm809, stm810, stm811, stm812 datasheet on www.st.com . introduction all digital systems require the power supply to be controlled during power-up and normal operations. noise resulting from ground loops or switching many signals at once, can create glitches in the power supply which can cause the system to malfunction. for this reason st has developed a new type of low-power su- pervisory device, called reset circuits, specifically to monitor power supplies. the reset circuits, stm809, stm810, stm811, stm812, assert a reset signal whenever the power supply drops below a preset threshold value, and keep it as- serted until the voltage rises above that threshold for a mini- mum period of time. the stm811 and stm812 also provide a push-button reset input signal (mr ).
AN1790 - application note 2/8 reset thresholds the stm8xx reset circuit devices have different reset threshold levels. table 1, lists the thresholds, v rst , for each device and each temperature range. when considering the threshold level required for an application, it is also necessary to consider the max- imum transient duration, not causing a reset pulse, with respect to voltage threshold overdrive. a large transient voltage overdrive needs less time to generate the reset pulse, than a small transient voltage overdrive, which needs much longer. refer to the characterization graph shown in figure 1. table 1. reset thresholds figure 1. max transient duration not causing reset pulse vs. reset comparator overdrive part temperature range ( c) v rst (v) min typ max stm8xxl 25 4.54 4.63 4.72 -40 to 80 4.50 4.75 stm8xxm 25 4.30 4.38 4.46 -40 to 80 4.25 4.50 stm8xxt 25 3.03 3.08 3.14 -40 to 80 3.00 3.15 stm8xxs 25 2.88 2.93 2.98 -40 to 80 2.85 3.00 stm8xxr 25 2.58 2.63 2.68 -40 to 80 2.55 2.70 ai07883 0.00 100.00 200.00 300.00 400.00 500.00 600.00 700.00 1 10 100 1000 reset comparator overdrive, v rst ? v cc (mv) maximum transient duration (s) r/s/t l/m
3/8 AN1790 - application note reset output st reset circuits feature a push-pull reset output with the option of an active high (rst) or active low (rst ) output (see figure 2 and table 2). the push-pull output minimizes the external connections be- tween the reset circuit and the microprocessor or other logic devices. it offers high speed, almost rail-to- rail response, and the capability to source or sink current. figure 2. reset output table 2. device options part reset output active manual reset (mr ) stm809 low no stm810 high no stm811 low ye s stm812 high ye s ai08632 reset circuit v cc rst/rst v rst v ss v cc rst rst trec trec
AN1790 - application note 4/8 manual reset input the stm811 and smt812 devices feature a manual reset (mr ) input, which activates the reset output for a predefined time, t rec . the simplest way to use this feature is to connect a push-button between the mr and v ss (ground), as shown in figure 3. the manual reset has an internal 20 k ? pull-up resistor and a debounce circuit, which solves the problem of any noise generated by switching the push-button. the timing diagram represented in figure 4 shows how the manual reset input functions. when mr is driven low, the reset (rst or rst ) output becomes active after a delay of t mlrl , and remains active dur- ing the time that mr is low. when the switch is released, mr goes high (thanks to the internal pull-up resistor), but the reset output remains active for a time of t rec . the minimum pulse width of the mr signal (t mlmh(min) = 10 s) must be taken into account in a design. if the mr pulse width is shorter than t mlmh(min) , the rst/rst pulse generated is equal to the pulse width of mr , which can lead to the system not functioning correctly. figure 3. manual reset connection figure 4. manual reset waveform note: 1. rst for stm810/812 2. refer to stm8xx datasheet for the timing values. ai08637 v cc v ss v cc rst push-button reset v cc mr v ss reset input microprocessor ai08640 rst (1) mr tmlrl trec tmlmh
5/8 AN1790 - application note connecting the reset circuits in an application the standard connection of the reset devices is very simple, as normally no other circuits are required. the standard connections for the stm809, stm810, stm811 and stm812 are shown in figure 5. figure 5. standard reset device connections ai08633 microprocessor stm812 v cc v ss v cc rst push-button reset v cc mr v ss reset input v cc v ss v cc rst push-button reset v cc mr v ss reset input stm811 microprocessor microprocessor stm810 v cc v ss v cc rst v cc v ss reset input v cc v ss v cc rst v cc v ss reset input stm809 microprocessor stm810 connection stm809 connection stm811 connection stm812 connection
AN1790 - application note 6/8 connection in a noisy environment the internal debounce circuit in the st reset devices is especially designed to remove any noise gener- ated by switching the push-button at the manual reset input. however the internal debounce circuit may not be sufficient to filter external noise in extremely noisy environments, for example, long wires in a noisy environment or high speed buses near the manual reset input. in such cases, an external capacitor can be added to solve the problem (see figure 6). the recommended value of the capacitor is 0.1f. figure 6. reset device connection in a noisy environment connection to a bi-directional microprocessor reset sometimes designers need to connect a reset circuit device to a microprocessor?s bi-directional reset input. in this case a resistor should be connected between the rst output and the microprocessor?s re- set input. the value of the resistor should be high enough to limit the current when the microprocessor pulls down the reset input and low enough to respond when rst goes low. the recommended value of the resistor is 4.7k ? (see figure 7). figure 7. reset device connection to a bi-directional microprocessor reset ai08634 microprocessor stm812 v cc v ss v cc rst push-button reset v cc mr v ss reset input c = 0.1f ai08635 microprocessor stm809 v cc v ss v cc rst v cc v ss reset input r = 4.7k ? to other devices
7/8 AN1790 - application note connection with reset output valid down to v cc = 0v when v cc falls below 1v, the rst (stm809/811) output no longer sinks current, but becomes an open circuit. in most applications this is not a problem, as most microprocessors do not operate below 1v. how- ever, in applications where rst must be valid down to 0v, a pull-down resistor should be added to hold the rst output low. the value of the resistor must be large enough not to load the rst output, and still be small enough to pull the output to ground. the recommended value of the resistor is 100k ? (see figure 8). figure 8. reset device connection with rst valid down to v cc = 0v conclusion st reset circuits are low-power supervisory devices, that have been specifically designed to monitor power supplies in microprocessor systems. in standard applications, they can be directly connected to the microprocessor, as no other external cir- cuits are required. however, they can also be used in specific application environments, with the simple addition of a resistor or capacitor. they are available in small sot23 and sot143 packages; they require only a low supply current, typically in the 5-10 a range and offer a wide range of voltage thresholds. references stm809, stm810, stm811, stm812 datasheet revision history table 3. document revision history date version revision details 04-dec-2003 1.0 first issue. ai08636 microprocessor stm809 v cc v ss v cc rst v cc v ss reset input r=100k ?
AN1790 - application note 8/8 if you have any questions or suggestion concerning the matters raised in this document please send them to the following electronic mail address: ask.memory@st.com (for general enquiries) please remember to include your name, company, location, telephone number and fax number. information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequence s of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is grante d by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publicati on are subjec t to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics prod ucts are no t authorized for use as critical components in life support devices or systems without express written approval of stmicroelectro nics. the st logo is a registered trademark of stmicroelectronics. all other names are the property of their respective owners ? 2003 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 - singapore - spain - sweden - switzerland - united kingdom - united states www.st.com
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