What is the main difference Bet. 16f877 vs 16f877A

[mackrackit]

Try CMCON=7 instead of 1.

Do you have a pull-up of around 470 from MCLR to VDD?

[ledsignmen]

Dave,
Thanks for the input.
When I said that the lower 3 bits of CMCON were set = 1
I SHOULD have said that Each of the lower 3 bits is set to 1
which makes CMCON = %00000111 which = 7.

I do have a 1K pullup to +5 on MCLR.

I have also discovered that if I set the config bits for the 16F877A
to turn the Brownout reset = OFF the code I have been running in
the 16F877 now also runs perfectly in the 16F877A.

What is it about pulling PortA.7 to ground with a pushbutton switch that
could lead to a system reset ??? And why would turning the brownout
reset OFF correct that problem ???

Thanks,
Dan

[mackrackit]

Sounds like a power supply problem or maybe something causing a short when the button is pushed.

The data sheet states when VDD drops to 4V a you have a brown-out.
Are both VDDs powered? Check the voltage on VDD when the button is pushed.

Two points to clarify.
Why the .1 cap to ground and not a pull-up of say 10k?
PORTA.7??? pin 7 is PORTA.5.

[ledsignmen]

Dave,

Sorry about the mistakes in my typing . . .
I have TRIED a .1 cap to ground on PortA.5 because some comments in this thread indicated that might be of benefit. And it sort of makes sense as a method for smoothing out the switch bounce noise generated by a push button switch.

And there IS a 1K pullup on PortA.5.

It was my confusion that typed PortA.7 in the previous text.
I SHOULD have typed PortA.5 and got that mixed up with the pin number which resulted in my typing PortA.7.

This whole thing is about PortA.5 which is also Pin 7.

I do have +5 to both VDD pins and the 5 volt source is a 150 watt switching supply.

I have had a friend of mine look at VDD and Master Clear with his scope and both of these are steady as a rock when the push button is pressed and the whole thing RESETS.
Pressing the pushbutton to pull PortA.5 to ground does create a little noise from the switch bounce but I cannot see that any amount of noise on an input should result in a reset ??? Do you think ??

Thanks again,
Dan

[skimask]

Time to show us the schematic of the whole thing I suppose...

[mackrackit]

I assume this is all on a test bench. I have had noise problems with switches when the device in the field next to a plasma cnc. Switch in question would start the torch.

Tell us a little more about the project along with the schematic.

[mister_e]

to answer the original question What is the main difference Bet. 16f877 vs 16f877A

The answer is 1588F03