I have it only tied to switch via a pull-up resistor on a pic. I'd like to not perform any hardware debouncing to save on costs, but I'm wondering if this could lead to a higher fail rate of something internal to the MCU of which I'm unaware, like how the power-on-reset circuit works.
Electronic – debounce MCU reset pin
debouncepicreset
Related Solutions
IIRC the cap is wired to /RTS not DTR.
When /RTS is high there is 0V across the cap. When /RTS goes low the cap can not change voltage instantaneously so the reset line goes low. The cap charges through the 10K resistor. Once the reset line is above the reset threshold the uC program starts.
@LouisDavis referenced this page, which states
The ATmega168 is reset by pulsing its reset pin to GND. ...by setting the DTR line to LOW ... the reset pin gets sucked to LOW until the capacitor is charged through the internal pull up resistor and R1 - which resets the chip.
Simulated:
Auto-Reset
VDTR 1 0 PWL(0 5 0.5m 5 0.000500004 0 1m 0)
Vcc 3 0 5
C1 1 2 100n
R1 3 2 10k
.control
delete all
tran 10n 5m
plot v(1) v(2)
.endc
.END
V(1) is DTR or /RTS and V(2) is /RESET
You can use 7805's this way, but as they only replace one resistor (and require 2 additional decoupling capacitors each!) I would never do it this way. My suggestions:
- replace the two 'sensor' 7805's by resistor / zener-diode combinations. Use 4v7 zeners.
- add two 100nF capacitors at the input and utput of the remaining 7805
- add a suppressor diode over the relays coil!!
- I am not sure a jellybean 7805 is a good idea in an automotive environment, there can be nasty spikes on the power line.
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Best Answer
It is not possible to physically damage an MCU from not debouncing the RESET input (assuming valid signal levels but invalid timing). It is possible to not reset it properly, which could lead to loss of data, EEPROM corruption, improper or undefined operation (heater stuck on, perhaps) and so on.
A pushbutton switch is also a kind of open invitation for ESD to come into the system via the user's fat electrically charged finger (and similarly disposed body), and it is possible to fry the input or cause potentially destructive latch-up of the chip. Microchip (PIC) products seem to be particularly sensitive as per datasheet warnings, but a series resistor and shunt capacitor is virtually always a good idea.
Since most reset inputs will have Schmitt trigger operation, this has the happy side-effect of guaranteeing proper minimum reset pulse width if you pick the part values sensibly.
Added: Since you've mentioned it's a PIC18F, here are a couple notes from a typical PIC18F datasheet.
Minimum reset (/MCLR) pulse width is 2usec:
Warning about ESD on /MCLR input:
In general, I would recommend using an external supervisory chip to guarantee proper reset unless you can convince yourself that the built-in circuit is totally bulletproof.