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Thread: 16f84 Question
www.piclist.com/techref/microchip/devices.htm?key=16F
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face BY : Trevor Page email (remove spam text)



As for damaging the MCLR input by grounding it directly, I quote from the
datasheet: "Voltage spikes below VSS at the MCLR pin, inducing currents
greater than 80 mA, may cause latch-up. Thus, a series resistor of 50-100ohm
should be used when applying a "low" level to the MCLR pin rather than
pulling this pin directly to VSS".

Okay, in practice you wouldn't want to tie MCLR directly to ground anyway. I
guess they mean if you're pulling it low using a TTL signal, or with a
bipolar transistor / FET. I suppose this advice would apply to any input pin
in general (especially those which can be configured as outputs, of course).


I guess that if you're driving the pin with a TTL signal and/or you've got
some inductance in the circuit (PCB track etc) then MCLR would quite likely
go below Vss. But I wonder if this is really necessary if you're, say,
pulling it low with a simple FET and the track from the FET source to the
PIC's Vss pin is very short?

Having just brushed up on my knowledge of the MCLR input, I'm now worried
that I should have implemented an RC reset in my last PIC -based product
rather than connecting it straight to Vdd: there is a lot of capacitance on
the 5V rail in that circuit. Am I likely to run into trouble in practice?
Comments?

Never stop learning...

Trev


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