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'[PIC]: PIC powered on through port diodes'
2002\09\13@120014 by Alan Gorham

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Hello

I know that PICs can be powered on through their port diodes.
I have a circuit that uses the USART on a 16F874 to connect to a PC com port.
The PIC has the WDT enabled and so if a WDT reset occurs while the comms cable
is connected, the PIC is powered up through the port diode while resetting.

This seems to cause screwy behaviour after the reset and I'm wondering if there is a way around this
problem without electrically isolating the PIC USART pins when resetting?

Thanks

Alan
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2002\09\13@170256 by M. Adam Davis

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The WDT doesn't turn power off to the PIC, so the power source that is
powering the pic should still be powering the pic when the WDT times
out.  The serial port shouldn't be powering it at all, especially since
you are likely to be using a rs-232 level shifter (such as the max232,
or a couple of transisters) and your pic pin shouldn't be seeing 12v.

How is your project powered?  What circuitry lies between your PC the
the PIC?

-Adam

Alan Gorham wrote:

{Quote hidden}

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'[PIC]: PIC powered on through port diodes'
2002\10\01@135808 by Alan Gorham
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Hi Adam

Sorry for not replying sooner - our mail has been down and I'm wading through
thousands of messages.
I'm using a MAX 202 twixt PC and PIC and with the PIC circuit powered down
I can see 1.73V at the UART Rx pin of the PIC and 1.68V at the Tx pin.
According to the F87x datasheet, the voltage must rise from Vss to achieve a
clean reset
and I think this may be the problem.

Alan


The WDT doesn't turn power off to the PIC, so the power source that is
powering the pic should still be powering the pic when the WDT times
out.  The serial port shouldn't be powering it at all, especially since
you are likely to be using a rs-232 level shifter (such as the max232,
or a couple of transisters) and your pic pin shouldn't be seeing 12v.

How is your project powered?  What circuitry lies between your PC the
the PIC?

-Adam

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