Unstable PIC output - Pins sensitive on touch

[quiksilv]

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 !

 

[quiksilv]

quiksilv wrote:

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

 

[quiksilv]

quiksilv wrote:

quiksilv wrote:

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

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 ?

 

[Dingo]

Have you made all the unused pins outputs or tied them high?

"quiksilv" <quiksilv@optushome.com.au> wrote in message
news:41736a2e$0$2216$afc38c87@news.optusnet.com.au...

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 !

 

[quiksilv]

quiksilv wrote:

quiksilv wrote:

quiksilv wrote:

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


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 !

 

[robin.pain@tesco.net]

quiksilv <quiksilv@optushome.com.au> wrote in message news:<41736a2e$0$2216$afc38c87@news.optusnet.com.au>...

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?