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Setting of Configuration bits for PIC18F26K20. ("HFOFST ON" and "PWRT OFF")

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treez

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Setting of Configuration bits for PIC18F26K20. ("HFOFST ON" and "PWRT OFF")

Hello,
Our remote software engineer has written C code for our PIC18F26K20 controlled offline LED driver. Some of the PCBs work and some don’t.
We noticed that in his configuration bits he has chosen to disable the Power Up Timer [“PWRT OFF”], and also he has chosen “HFOFST ON” (this means that the HFINTOSC starts clocking the CPU without waiting for the oscillator to stabilise).
These seem like unnecessary risks to take? Surely there is no benefit in these selections, but only an increased chance of things going wrong?
 

Re: Setting of Configuration bits for PIC18F26K20. ("HFOFST ON" and "PWRT OFF")

It really depends on the application. Under some circumstances you would give priority to getting the MCU running as fast as possible, in others, particularly timing critical applications, you would want reliability even if it meant a delayed start. Being fair to Microchip, even an UNstable oscillator is pretty accurate, it is the lock detect signal from the PLL but being unlocked just means the frequency may be a little off.

If you can afford to wait a short time (<100mS in total) from power up to operation I would suggest using it and the power up timer as well.

Do bear in mind that if the IC is reset or browns out with power still present that delay could allow other devices in the circuit to lose control.

Brian.
 
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