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PIC16F84 low power mode?

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MikeMl

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Using a normally-open momentary push-button switch, I would like to power-up a 16F84, cause it's program to execute once (where the execution time of the program will be likely be longer than the button is held in), and then have it shut itself off. I will add a FET wired across the push-button controlled by a port-pin to de-power after the program executes.

I'm thinking that having it completely shut off results in lower long-term battery consumption than any sleep mode?

I'm guessing that an appropriate capacitor on the MCLR pin will cause it to gracefully boot-up? How long after the MCLR pin goes high is it ready to start executing the program?
 

MikeMl

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I already have some 16F84s. Probably 3ea AA or AAA. One of the required outputs is a small solenoid, but the thing may sit for months at a time without being turned on.
 

Mr RB

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You can do it with a small SCR and 2 resistors;
PIC switching itself OFF

C106 and C103 SCR's can be bought for a few cents off the surplus suppliers. C106 are common enough you may even have in your junkbox.
 

kpatz

New Member
I've done something similar using a logic-level N-MOSFET. Pressing a button puts a gate voltage on the MOSFET, turning it on and supplying power to the PIC (you could also wire the button across the source/drain of the MOSFET to turn on the PIC). The PIC then sets an output pin high which is also connected to the MOSFET gate (through a diode and resistor) to keep it on. When the PIC wants to turn the power off, it sets the output pin low, turning off the MOSFET and the power.

Current drain in power-off mode is below the minimum my multimeter can read, so it's less than 0.1µA.

The PIC16F84 doesn't need a RC circuit on the reset pin for power-up, unless the rise time of Vdd is too slow. Just tie MCLR to Vdd through a 10k or so resistor.

Another benefit to using a MOSFET-based power control over sleep mode is you can also power down any supporting circuitry in addition to the PIC. That is, LED drivers, the current-hogging 7805 regulator, any other ICs/circuits you have connected to the PIC, etc.
 
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MikeMl

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I've done something similar using a logic-level N-MOSFET. ...
Did you put the N-Fet in the PIC's/other circuitry's negative return wire like RB's SCR? I have been thinking about a similar scheme, but it requires a P-Fet in the high side ahead of the voltage regulator.
 
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Mr RB

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Yeah I want to see the FET schematic... It's a tricky thing to do because when the FET turns off it's gate is still connected to the PIC pin which will inevitably float towards the opposite PSU terminal as the caps discharge, which is of course what turns the FET on.

You really need to run the FET in reverse so the gate and source can both reference to the PIC, but when you run the FET reversed it's body diode conducts so it never turns off the power.
 
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