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AN1366/0601 1/11 AN1366 application note how to use the st92163 microcontroller reset by microcontroller division applications introduction the st92163 mcu can be reset in several ways. this application note explains the different reset mechanisms available on the st92163, so you can make the best use of them when you design your application. there are 3 available reset sources that you can either use independently or in conjunction with each other, depending on your application. hardware reset sources: C an external reset pin C an internal low voltage detector, or lvd on some devices software reset source: C embedded watchdog activated by software 1 selecting which reset source to use operating voltage range both the external reset pin and the watchdog reset can be used over the whole st92163 voltage range. the lvd function however, requires the mcu supply voltage to remain over 3.8v in its stable phase. low power modes if you intend to use the watchdog to reset the micro, you should note that, once it is activated, the watchdog stays on permanently and cannot be deactivated (other than by a reset). while active, it prevents using any of the st92163 low power modes (such as stop mode or usb suspend mode). 2 use of the lvd in the power up phase the lvd works in conjunction with the external reset in the power-up phase. an external reset pulse activates the initialization logic which activates the lvd. then the lvd ensures the core does not start running before the mcu supply has reached a precise threshold. in other words, the lvd extends the internal reset signal triggered by the external reset pin 1
2/11 how to use the st92163 microcontroller reset until the static voltage comparator of the lvd detects that the mcu supply has reached the v it threshold (see figure 1 ). figure 1. lvd block diagram based on a static voltage comparator, the lvd protects the microcontroller from any changes in the supply voltage, oscillations included, as long as the supply voltage does not fall below 2v. below this voltage, the internal logic controlling the lvd is not guaranteed anymore (shaded area in figure 2 ). therefore, it is required to externally keep the reset pin at a low logic level when the mcu supply voltage is below 2v in order to safely use the lvd. - + v it initialization logic v dd lvd enable reset pin internal reset 1
3/11 how to use the st92163 microcontroller reset 3 use of the lvd in the power down phase when the mcu supply voltage drops, the lvd acts as brown-out detection circuit: as soon as the supply goes below a certain threshold, an internal reset is generated. in the same way as in the power-up phase, the threshold detection is based on a static voltage comparator, and the reset triggering does not depend on the falling speed. figure 2. lvd timing diagram lvd threshold 2v internal reset reset pin lvd enable (see note) note: a hysteresis is implemented between the rising and falling lvd thresholds. this is not shown in the drawing. = undefined logic state v dd t micro is unstable until ext. reset is asserted low caution!
4/11 how to use the st92163 microcontroller reset 4 external reset circuits there are several external reset implementation schemes to choose from based on your ap- plication-specific parameters, such as the power supply behaviour. whatever the solution chosen, the idea is to keep the reset pin at a low logic level until the supply voltage has reached C 2v if the lvd is used, after which point the internal logic is guaranteed C or 3v if the lvd is not used, in order to guarantee a safe start at 8 mhz. internal frequency. 4.1 dynamic reset circuit this concept is the most cost-effective external reset solution if the supply waveform is mo- notonous and the maximum rising time is known figure 3. example of rc circuit the principle is to let the reset pin rise with the mcu supply after a delay. a safe design should allow the reset pin to rise when the mcu supply has reached 2v if the lvd is used, or 3v if the lvd is not used. the basic solution is to use an rc delay determined by the rise rate of the supply itself: a delay corresponding to 30% of the rise time is generally safe. due to statistical dispersion of the r and c values, the delay is not stable: care must then be taken to ensure that the reset pin rises while the supply voltage is between 2v if the lvd is used, or 3v if the lvd is not used (internal logic guaranteed) and the minimum voltage re- quired for the application in its stabilized phase (4.05v in case of a usb application). see figure 4 . reset st92163 1 m f v dd 10k w
5/11 how to use the st92163 microcontroller reset figure 4. good example of dynamic reset circuit note also that this scheme requires a certain delay between a power-down and the next power-up, because the delay generator has to be reinitialized: in practice, capacitor c needs to be discharged. this kind of scheme can be used in conjunction with the lvd. the delay holds down the ex- ternal reset pin, until the lvd takes over and releases the mcu core when the lvd threshold is reached. 4.2 external mcu power supply monitor this solution is necessary if the power supply oscillates during power on, or if the power rise time is unspecified. there are two reasons for this: C firstly, the rc delay will be insufficient if it is too small compared to the total rise time: the reset pin rises before the mcu supply reaches 2v if the lvd is used, or 3v if the lvd is not used (see figure 5 ). C secondly, the supply may drop momentarily (oscillate) below the minimum safe level (2v if the lvd is used, or 3v if the lvd is not used), with the reset pin held high. as a conse- v dd reset pin v ihrsmin (reset input high level threshold) 2v
6/11 how to use the st92163 microcontroller reset quence, the internal logic is not guaranteed anymore, and the reset pin cannot be seen at a low logic level to ensure a safe restart. see figure 6 . note: the second case also applies to applications with short power-down/up cycles. figure 5. bad example of power on reset circuit (delay insufficient) v dd reset pin 2v
7/11 how to use the st92163 microcontroller reset figure 6. waveform of power brown-out with rc circuit (bad example ) 4.3 dedicated voltage monitor ic this solution uses a stand-alone power supply monitor with reset output. this kind of device is available from stmicroelectronics in the ts83x product family. it includes a precision voltage reference and a static comparator to issue a reset at a precise threshold independ- ently of how the supply level changes. this solution allows you to get a precise and safe reset function in cases where the application constraints do not allow the use of the lvd of the st92163. v dd reset pin 2v in this example reset never goes logic low (under 0.2*v dd )
8/11 how to use the st92163 microcontroller reset 4.4 basic voltage monitor plus lvd if the lvd is used, it is possible to obtain a more cost-effective solution. a coarse static voltage monitor is sufficient to ensure a reset between 2v and the lvd threshold. independently of the variations on the external monitor threshold, the lvd fixes a precise voltage where the in- ternal reset is released. an implementation example is given below. 4.4.1 schematic this circuitry is composed of 1 pnp transistor, 3 resistors and 1 capacitor (see figure 7 ). figure 7. suggested power on reset circuitry for use with the lvd 4.4.2 static behaviour the c1 capacitor is loaded by the t1 ic current when v dd > v eb x (r2 + r1) / r1 and the reset pin is put at logical 1. for this example: v ddth = 2.60 v (min = 2.2 v ; max = 3.4 v) after reset state, this circuitry is static and consumes about 10 a max. 4.4.3 dynamic behaviour as soon as v dd > v ddth , the capacitor is loaded with the current: ic = hfe x ib = hfe x [((v dd - v eb ) / r2) - (v eb / r1)]. ic is about 300 a (typical, constant if v dd constant). so the time to reach the reset v ih with v dd = 5 v is: t reset = (v ih x c1) / ic. t reset ~ 1 ms. see figure 8 . reset st92163 c1 v dd r1 r2 t1 r1 = 270 kohms, +/- 2 % r2 = 1 mohms, +/- 2 % r3 = 1 mohms, +/- 5 % c1 = 100 nf, accuracy not important t1 = bc857, pnp transistor with: h fe gain approx. 100 v eb = 0.55 v , min = 0.5 v, max = 0.6 v r3
9/11 how to use the st92163 microcontroller reset figure 8. waveform of power on reset (with pnp reset circuit) v dd reset pin 2v
10/11 how to use the st92163 microcontroller reset 4.4.4 power down the c1 capacitor is discharged through both the internal protection diode of the reset pin to the v dd , r3 to the ground in order to be 100% discharged. this discharge is guaranteed slow enough by the v dd capacitors and will cause no damage. for this reason, it is necessary to put a 4.7 f capacitor on v dd to have a maximum current in the diode about 1 ma. r3 allows to have v reset below v dd when v dd is below 2.6v (v ddth ) to guarantee a reset for a slow v dd fall time and a power-up when v dd is not yet at 0v. see figure 9 . figure 9. waveform of power brown-out with pnp reset circuit v dd reset pin 2v discharge through r3, valid reset low level
11/11 how to use the st92163 microcontroller reset "the present note which is for guidance only aims at providing customers with information regarding their products in order for them to save time. as a result, stmicroelectronics shall not be held liable for any direct, indirect or consequential damages with respect to any claims arising from the content of such a note and/or the use made by customers of the information contained herein in connexion with their products." information furnished is believed to be accurate and reliable. however, stmicroelectronics assumes no responsibility for the co nsequences 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 granted by implication or otherwise under any patent or patent rights of stmicroelectronics. specifications mentioned in this publicati on are subject to change without notice. this publication supersedes and replaces all information previously supplied. stmicroelectronics prod ucts are not authorized for use as critical components in life support devices or systems without the express written approval of stmicroele ctronics. the st logo is a registered trademark of stmicroelectronics ? 2001 stmicroelectronics - all rights reserved. purchase of i 2 c components by stmicroelectronics conveys a license under the philips i 2 c patent. rights to use these components in an i 2 c system is granted provided that the system conforms to the i 2 c standard specification as defined by philips. stmicroelectronics group of companies australia - brazil - china - finland - france - germany - hong kong - india - italy - japan - malaysia - malta - morocco - sin gapore - spain sweden - switzerland - united kingdom - u.s.a. http://www.st.com

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