Not all port A pins can drive both Vdd and Vss (RA4/T0Clk) but 2 should be
able to.  Possibly the pin is damaged from sourcing too much current at one
point in time? (random speculation).  If you were using RA4, you'd expect to
see no voltage swing.  Oscillation between Vss and high impediance (no
output) shouldn't show up on your meter.  When you attach the pullup, you'll
expect to read Vdd when the pin is in high impedance, and Vss when sinking.

Aaron Lahman


-----Original Message-----
From: pic microcontroller discussion list
[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Tim Webb
Sent: Tuesday, January 29, 2002 11:55 AM
To: PICLIST@MITVMA.MIT.EDU
Subject: [PIC]: Do I need pull-up resistors on PortA of a 16F877 ?


Looking in the 16F877 Data Sheet, I don't see any internal pull-up resistors
on any ports except port B has a weak pull-up.

Does this mean that I have to add external pull-up resistors to get +5V on
Port A pins?

When I connect my scope to PortA,2 which should show a pulse signal, I see
the signal resting at Gnd.
When I add a 2.2k Ohm resistor from PortA,2 to Vcc, I see my +5V to Gnd
swing.

This is disturbing...

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