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Unstable PIC output - Pins sensitive on touch

Q

quiksilv

Jan 1, 1970
0
Hi,

I have a simple LED flasher programmed on a PIC16F877A, but the output
of the pin (RC4) is always high unless I say put my finger on any one of
the other pins, then the LED flashes as expected.

I presume this is capacitance related but what is happening and how can
I rectify it ?

The circuit is extremely simple, two 15pF caps on a 4Mhz crystal, an LED
and 1k resistor on an output pin RC4.

Thanks !
 
Q

quiksilv

Jan 1, 1970
0
quiksilv said:
Hi,

I have a simple LED flasher programmed on a PIC16F877A, but the output
of the pin (RC4) is always high unless I say put my finger on any one of
the other pins, then the LED flashes as expected.

I presume this is capacitance related but what is happening and how can
I rectify it ?

The circuit is extremely simple, two 15pF caps on a 4Mhz crystal, an LED
and 1k resistor on an output pin RC4.

Thanks !

Nevermind, /MCLR had no pullup resistor... durr :)
 
Q

quiksilv

Jan 1, 1970
0
quiksilv said:
Nevermind, /MCLR had no pullup resistor... durr :)

Well it seems now its working but whenever i wave my fingers/hand close
to the micro (touch some pins or even touch the DIP casing) it still
goes wierd and keeps the LED on ! If I do this enough the PIC seems to
stop completely and only re-programming it will fix it.

This doesnt happen when I ground or vdd myself... what on earth is going
on ?
 
Q

quiksilv

Jan 1, 1970
0
quiksilv said:
Well it seems now its working but whenever i wave my fingers/hand close
to the micro (touch some pins or even touch the DIP casing) it still
goes wierd and keeps the LED on ! If I do this enough the PIC seems to
stop completely and only re-programming it will fix it.

This doesnt happen when I ground or vdd myself... what on earth is going
on ?

Ok I solved the problem by disable low voltage programming in the
config... seems to play havoc with the thing !
 
quiksilv said:
Hi,

I have a simple LED flasher programmed on a PIC16F877A, but the output
of the pin (RC4) is always high unless I say put my finger on any one of
the other pins, then the LED flashes as expected.

I presume this is capacitance related but what is happening and how can
I rectify it ?

The circuit is extremely simple, two 15pF caps on a 4Mhz crystal, an LED
and 1k resistor on an output pin RC4.

Thanks !

Is the earth-return connected?
 
D

Dingo

Jan 1, 1970
0
Have you made all the unused pins outputs or tied them high?
 
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