I have corrected the typo, although that would not have stopped something
from toggling. I checked my circuit, and it is basically the same as yours.
The only difference is that I hardwired ~MCLR to +5 volts. The rise time is
good, and I have tried triggering that pin separately, but nothing happens
on the board. I've tried several 16C74B chips, and I have tried erasing
them as long as 40 minutes. My programmer, a TopMax, shows the erase to be
ok, and shows no problem with the programming. But, now matter what I do,
the only "activity" that  I see on the chip is the oscillator input signal
on pin 13, which is a very precise 0 to 5 volt 5 MHz signal derived from a
TCXO. Pin 14 shows a triangular waveform going from 0 to about 1.8 volts.
(I currently have the configuration set for XT crystal. I've tried HS and
RC. The best waveform on pin 14 was with RC selected, as I recall, but I
believe I should be using either HS or XT since it is a single-ended clock
signal derived from a discrete temperature compersated, ovenized crystal
oscillator. I am at a total loss to figure out what I am doing wrong. The
circuit is one I designed and layed out. It is very simple, with pins 1,
11, and 32 tied to +5V (eventually +3.3) and 12 and 31 tied to ground. 1
and 20 have 10K pullup resistors. Most pins are unused except for the ones
that I am using for the SPI bus (which is strictly outgoing, so it only
requires 3 wires, to program a National Semiconductor synthesizer chip
(LMX2306). I use pins 17, 18, and 24 for this, for latch, clock, and data,
respectively. I am beginning to think that I have bad 16C74B chips, but I
find it very unlikely that I have 2 bad chips. Also, the programmer
indicates no problem.

I disable watchdog timer, enable power up timer, and disable brownout
feature. (Although I have tried other combinations, to no avail.)

Any any additional thoughts or suggestions would be appreciated.

Ed

At 10:27 PM 8/5/2001 -0700, you wrote:
>Re:
>
>>In an effort to try to debug the program that I am having trouble with, I
>>have stripped it down to the very basic essentials. It is now a program
>
>etc.
>
>Again, you have a typo:
>
>>         while(1)  // loop forever
>>         {
>>                 RC6=1;
>>                 DelayMs(2) ;
>>                 if (RA4 = 1)
>>                 {
>
>..
>
>should be if (RA4 == 1)
>
>Anyway, your _existing_ program toggles RD7 and RD6 every 92us on a
>PIC16C77 @ 20MHz.
>
>If you are running with a _crystal_, then there are a variety of
>reasons why it might not start, though frankly I've never had any
>problems with PICs and xtals.  If you're running with an external
>oscillator-in-a-can (like what's on the PICDEM-2 board), that
>eliminates yet more variables. The schematic in our Assembly Guide
>AG-1 available on our website may be useful to you ...
>
>One problem I've had with windowed parts is that they behave very
>strangely if they are nut fully UV erased prior to (re-)programming.
>
>You  don't mention what your "board" is ...
>
>--
>
>  ______________________________________
>Andrew E. Kalman, Ph.D.
>
>
>Salvo(TM), The RTOS that runs in tiny places(TM)
>Pumpkin, Inc.
>750 Naples Street
>San Francisco, CA 94112
>tel: (415) 584-6360
>fax: (415) 585-7948
>web: http://www.pumpkininc.com
>email: aek@pumpkininc.com
>
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