What is a reset generator?
Submitted By: Craig Steiner FAQ Last Modified: 09/13/06
- Generally, any circuit which generates a RESET pulse for microcontroller/cpu. The datasheets usually give RC circuit as reference, which has many short comings. The RC circuit is the simplest form of a circuit which can generate reset signal. The more accurate and reliable method for generating a reset pulse is to use a reset generator chip, specifically designed for a guaranteed reset signal. A reset generator monitors the supply voltage of the system and generates a reset again if the supply falls down below a specified level (known as brown out detection). Some chips even take an input (named strobe or some other name) from the micro/cpu, which they monitor for a periodic change. If there is not change in the strobe signal (for a predetermined time) then the reset generator assumes that micro/cpu is not working (there may be lot of reasons for this), then reset signal is generated again (this action known as Watchdog timer). Now the RC circuit has the short-coming of brown out detection, watch dog timer. You can't be sure of if an RC will always generate a good reset signal. Thats why, a reset generator chip is recommended.
What will happen when I use an RC (or C only) reset?
Submitted By: Jan Waclawek FAQ Last Modified: 09/13/06
- Maybe everything will work OK.
Maybe the '51 will not start at all.
Maybe the '51 will start, but will behave in an unexpected and unpredictible way.
The main trouble with '51 reset is that it is synchronous, it means, that the clock has to be running (it takes considerable time while a crystal oscillator starts oscillating) and than the pulse has to last at least 24 oscillator clocks (this number might be different for some of the derivatives) to properly initialize all the internal registers, including the program counter. Should a reset pulse be shorter, there is no guarantee the program counter starts from zero, nor the content of other registers will be as stated in the "bible".
The main trouble with RC reset is not that it fails consistently, but that its function relies on the circumstances. If the power supply starts with a fast monotonic (no glitch) ramp, the reset pulse from a RC circuit might be OK. But, some power supplies ramp too slow, other produce glitches upon startup, all this resulting in inappropriate reset pulse; and this behaviour changes with different load on them, or with the moment when you actually switch it on (in respect with the momentary voltage of the AC mains). And, if already powered up, and a sudden glitch in the power occurs, RC resets tend to not react on it at all.
It is much more difficult to achieve a consistent behaviour of a power supply under varying load etc., than to use a proper reset IC.
Unpredictible behaviour is the most confusing for the newbies, so, newbies, please, please, do avoid the use of RC (or C only) reset, please use a proper reset IC.
It has to be said, that there are '51 derivatives around with integrated power-on reset (POR) and brown-out detection/reset (BOD) circuits; however, even those tend to have problems when subject to "nonstandard" power. The cause would be poorer control on the exact voltages/thresholds in a digital circuit compared to analog circuits (the dedicated reset ICs are mostly pure analog). One of the recent examples is the LPC9xx series internal POR circuit, which fails if it is not discharged down to below 0.2V - which, again, would require a special design of power supply. So even in the case when the datasheet says "has POR and BOD", if the datasheet fails to explain exactly the whereabouts of both, or there are doubts on how it exactly works, it is better to stand on the safe side and use an external reset IC.
What reset ICs are good for '51s?
Submitted By: Jan Waclawek FAQ Last Modified: 09/13/06
- As '51 and most of its derivatives (check the datasheet of your particular derivative) has a less-common positive (active high) reset, the choice of reset IC's is a little bit limited.
- threshold voltage and hysteresis, with respect to power supply (with tolerances) and the '51 operational voltage range
- length of reset pulse or adjustability of it (polarity is given)
- power consumption of the reset IC itself
- need for a simultaneous reset of other polarity (for surrounding circuitry)
- ability to keep the reset active even at low ("fading-out") supply voltage
- ability to monitor additional voltages
- reset input (e.g. pushbutton or chaning from other reset source)
- integrated watchdog, with or without merging its output with the reset itself
- integrated low-voltage (early powerdown warning) detection circuitry
- integrated battery-backuped circuitry (mainly RAM) chip select protection and supply switchover
- if the particular '51 derivative uses the reset pin also as an output (e.g. for internal watchdog), it has to be ensured there will be no conflict with the supervisor/reset IC
- (MAX)810 - 3 pins, simple
- (TL)7705 - 8 pins, both polarity open collector output, pushbutton input, adjustable pulse length, too many external components
- (MAX)1232 - 8 pins, pushbutton input, adjustable integrated WD (which cannot be deactivated), positive output is push-pull (negative is OC) - Kai Klaas's trick: add a 4k7 pullup to maintain reset during low supply voltage
- (MAX)707, (MAX)813 - well-featured 8-leggers with powerfail monitor; the latter contains also a watchdog
- (ADM)69x - with all the bells and whistles
Things to be considered - some of them might be of interest other not, depending on application:
Some of the popular IC's are listed below. As all of them are heavily second-sourced, prefixes are given in brackets. Check out also this thread for further discussion.
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