Reset circuitry revisited
23 July 2010
Embedded Adventures website reader Karl-Wilhelm Wacker hailing from Easton, PA, USA writes:
"The best circuit I have found for resetting PIC chips and allowing the ICSP function is as follows:
You may not think the diode necessary, but a quick low going glitch on the +5 line will not discharge the .1uF cap without it.
I found this out about 30+ years ago on a circuit for a dot matrix printer that would lockup on power supply spikes.
If you use a reset chip, the 10K resistor to +5 and the .1uF cap drop out, but keep the diode - it protects the reset chip from the programming voltage.
Note that the 10K resistor from MCLR and the diode from the reset chip output to VCC both limit the current during programming and limit the voltage on the reset chip output to one diode drop above VCC so that the reset chip is not damaged by the programming voltage on the MCLR pin."
Thanks for your feedback Karl. In embedded electronics there are often a number of different ways to achive the same objectives but with a different balance of priorities, so it's great to learn from the experience of others.