Unwanted pulse on PortC.1 on 16F876

[Darrel Taylor]

Hi Muttley,

It's hard to tell if it's a single pass program, or if the statements were extracted from other subroutines. but, assuming it's single pass (as shown).

When a PIC first powers up, the state of the output latches are undefined. So when you set a pin to output, without setting it's state first, it could go either high or low. Leaving RC1 in the wrong state during the shiftout to the first DPOT.

After both of the shiftout's execute, things would get back "in sync" again. But then the programs finished at that point. (still assuming single pass)

Maybe try setting the pins LOW instead of OUTPUT at the beginning.<br><br>

[Muttley]

Darrel
It seems my efforts to keep the information as brief as possible while containing the relevant parts has lead to some confusion. The three lines of code that cause the problem are in fact part of a subroutine that is called whenever the user presses a button (monitored elsewhere in the prog). It is not just a single pass, sorry.
At the start of the program code I use the statement TRISC=%11110000 that I hope is setting the ports I am using for the DS1267's to outputs.
The devilish pulse occurs at EVERY pass of the subroutine.
Hope this clarifies.
Muttley

[Darrel Taylor]

Maybe you can clarify something else.

You say that...
When RC0 goes high a pulse appears on RC1 that is sufficiently long enough ...

Are we talking a few nanoseconds, or several hundred microseconds? Are you looking at a scope?

I'm also a bit confused with this part

The last three lines seem to put the unwanted pulse on PORTC.1
However,

high rst2
SHIFTOUT dq,clk,1,[%0\1,freqwiper,pwwiper]
low rst2

Since "rst2" is PORTC.1, you will obviously get a pulse there on every pass. But the original question mentioned that When RC0 goes high a pulse appears on RC1. Is it the exact same time?<br><br>

[Acetronics2]

My question reaches Darrel's idea ...

could it be a capacitive coupling between your lines ( Pcb, test board ??? ), or is it a software problem ???

one thing to try is to turn RST1 low when turning RST2 high ( at the same time ...) and the opposite.

and one further step, one !!!

Alain

[Muttley]

Thanks guys so far.
It's not a capacive coupling problem, pretty sure of that
The pulses on the 2nd DS1267 (RC1) are around 1mS long and the desired pulse on RC0 is about 3mS long. The pulse on RC1 occurs during the last third of the RC0 pulse, which is really weird.
As if that wasn't enough, when I deliberately try to address the 2nd DS1267, i.e make RC1 go high the pulse is only 1mS long in contrast to the 3mS when I am addressing RC0. Yet the two subroutines do exactly the same thing??
I've attached the entire relevant code for you experts. It's not that long and as I'm not that experienced, isn't very heavy.
It's got to be a stupid coding error but I'm damned if I can spot it.
BTW, whoever asked, no there are no pull-up resistors on the lines between PIC and DS1267. All the literature I've read suggests I don't need it and the PIC ports I'm using are TTL compatable, I think.

[Bruce]

How does this version work?

[Muttley]

Hi Bruce,
I think I can see what you're thinking
I'll give it a go and might even add a PAUSEUS in the other subroutines if it doesn't work as they are repeated over and over, compared to the setpotszero subroutine that is only called at start up.
Won't be for a day or so as a tad busy earning a crust.
Thanks for advice and I'll report back.
If there are any other suggestions, I'll still be happy to read them.
Muttley