Power-On Reset & Brown-Out Reset

[hantto]

To use these abilities does one only have to enable their bit when programming the PIC or is there some other that has to be done in the software?

If BOD is enabled then it's no idea to also enable POR because the BOD does the same?

To take advantage of the POR you have to tie the MCLR pin to V+ through a resistor (says the datasheet) is this also the case with BOD? If it is, what value would be satisfactory?

And one more. The power up timer. What purpose does it have? Yes it keeps the PIC reset an additional 72ms, for what? Is it enabled through the configuration bits only, or is there more to it? Does the PWRT apply to both POR and BOD?

Thanks for your time!

 

[Oznog]

POR is not an option, it means Power On Reset. It refers to the reset from powering up the chip versus such things as software issuing the "RESET" command. Obviously POR must happen. You probably mean the Power On Reset Timer option, right?

PWRT and BOD are two totally different things. It's usually good to enable both.

MCLR should always be hooked to Vdd through a resistor. I don't know of any relation to PWRT's specific needs.

PWRT is based on the experience that the first few ms after hooking up power are absolute crap because the power switch bounces. Such short power-outs can corrupt the startup, registers can be fubar-ed, and the part will fail to start.

 

[Nigel Goodwin]

To use these abilities does one only have to enable their bit when programming the PIC or is there some other that has to be done in the software?

You just enable them with the config fuses.

If BOD is enabled then it's no idea to also enable POR because the BOD does the same?

Not quite the same, BOD (Brown Out Detect) will cause a reset for a smaller supply drop if set.

To take advantage of the POR you have to tie the MCLR pin to V+ through a resistor (says the datasheet) is this also the case with BOD? If it is, what value would be satisfactory?

You don't need a resistor, unless you want a manual reset switch - simply connect MCLR directly to +ve.

And one more. The power up timer. What purpose does it have? Yes it keeps the PIC reset an additional 72ms, for what? Is it enabled through the configuration bits only, or is there more to it? Does the PWRT apply to both POR and BOD?

It gives a little extra time for other connected circuits to stabilize, again there's nothing required except enabling it.

These options are there in case you need them, you probably won't in most cases.

 

[hantto]

Thanks for those answers.

So with Brown-Out you also need to connect MCLR to V+? (and enable the MCLR pin)

 

[Nigel Goodwin]

Thanks for those answers.

So with Brown-Out you also need to connect MCLR to V+? (and enable the MCLR pin)

If you're using a PIC with MCLR able to be used as an input (like the 16F628) you can define it how you like, if MCLR is defined as an input it's connected internally to +ve.

 

[hantto]

Good point, thanks for the help