serial data keeps chip on

[l_gaminde]

Im using 16f84, a's and 16f628, I have serial data to pin b2 from another device when I power down the pic side there is still 1.2 volts on the power rail and its coming from the serial device through the pic pin b2 this keeps the pic just on so that it starts where it left off on power down not from the start of the program what state should b2 be put in to stop this leaking through to the power side I pull the pic and only b2 shows this serial data the other pins do not show serial data just 1.2 volts. I use b2 to write and read data to this serial device.

Larry

[Bruce]

Hard-wire a small telecom type relay into the circuit where it breaks the connection when the PIC powers down.

[l_gaminde]

Bruce that a good Idea!! Im still wondering why this would happen these things are made to handle this I would think its something internal in the chip they should not bleed into the power rail !!

[Bruce]

The external power feeds through the I/O-pins internal protection diodes.

Look at the port pin block diagrams and you'll see why. You might also try to enable BOR, but I'm not sure that's the best route. Of course, there's always the option of powering your external device from the same power supply. When it's off, it's all off.

Check the attached graphic. With the PIC powered off, and +V applied to the input pin, it goes right through the high-side diode to VDD.

[l_gaminde]

ok
so would a resistor from ground to MCLR work? once power is off and there is only 1.2 volts pulling this down would reset the chip. It doesn't seem to be running just doesn't start in the same place on power up. I also tried a .1 and .01 cap in the serial line from the device this worked great for blocking the dc but could not comm correctly either will be looking more into this also.

[l_gaminde]

Oh and trade places with the pretty lady I would rather see her face than yours!!

[Bruce]

Do you have brown out reset enabled?

Sorry. Can't change places with her. That's the only pic I have of us both that she would let me use...:)

[l_gaminde]

Is this what you mean about brownout reset below ? I see nothing about brownout reset other than a circuit for MCLR called EXTERNAL POWER-ON RESET CIRCUIT
(FOR SLOW VDD POWER-UP)

@ DEVICE PIC16F84,HS_OSC,WDT_ON,PWRT_ON,PROTECT_ON

[mackrackit]

The 628 has it.