16c74b programming problem

Started by childofbrimstone July 17, 2004
Hi, everyone,

I can't program either of two 16c74b chips I just got from Digikey. I
am using a Picstart plus. I have no trouble burning the same code
(with the required modifications in the header file of course), but no
luck with the '74b. The programming window reads success, which
usually indicates that the program was successfully burned into the
chip. Yet when I connected an LED to an 1/0 pin addressed in the
program, the LED stayed unlit. I can't find the pinout diagram for the
'74b in the old Microchip datasheet that I have, so I am using the
16c74 pinout diagram. I believe both chips have the same pinout
diagram. I don't know what else to do. If anybody can help, I'd
greatly appreciate it. Thank you COB



On Sun, 18 Jul 2004 02:25:02 -0000, childofbrimstone wrote:
>Hi, everyone,
>
>I can't program either of two 16c74b chips I just got from Digikey. I
>am using a Picstart plus. I have no trouble burning the same code
>(with the required modifications in the header file of course), but no
>luck with the '74b. The programming window reads success, which
>usually indicates that the program was successfully burned into the
>chip. Yet when I connected an LED to an 1/0 pin addressed in the
>program, the LED stayed unlit. I can't find the pinout diagram for the
>'74b in the old Microchip datasheet that I have, so I am using the
>16c74 pinout diagram. I believe both chips have the same pinout
>diagram. I don't know what else to do. If anybody can help, I'd
>greatly appreciate it. Thank you

If the chips verify OK with the Picstart plus you can be pretty sure
the code was programmed into the chip correctly. I'm also sure there is
no difference in pinout between the two devices.

A couple areas that can bite you when changing from an older die
revision to a newer one...

1. Are you sure the config fuses are the same between chip versions?
Microchip has changed them from time to time between revisions of chips
so make sure you compare the 16C74 and 16C74b datasheets. I ran into
this with PIC16C65 to 16C65B (or 16C65C) some time ago. A design that
worked for years suddenly stopped working and it was because they now
used previously "unimplemented" bits if the config register. Once
fixed, the old code worked flawlessly.

2. Check for any other functional changes made to the chips, especially
any errata that may have been corrected (or added) in the new chips.
These fixes can sometimes break "good" code.

3. Die shrinks can sometimes cause problems that were not critical in
earlier chips to result in failures with the new chips. In particular
reset circuits can sometimes be affected by the die shrink as well as
causing the new chip to be more sensitive to power supply noise and the
like.

Those are the kind of problems I'd look for -- subtle differences in
the chip due to die shrink can make what worked before stop working.
While it's possible you have a programming problem, the fact the chip
programs and verifies correctly makes me think that's not likely the
cause of your problem.

Matt Pobursky
Maximum Performance Systems