12F629 reset latching problem

test153 · Joined: 09 Feb 2009 Posts: 28

As I understood the 12F629 has some kind of reset latching problem and the supply voltage need to be clean.

Is having a decoupling capacitor (0.1uF) near the PIC all that is needed to fix this reset latching problem or do I need more capacitors? Also could someone explain in more detail what the reset latching problem is?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The person who made that statement did not provide a link to a post
on the Microchip forum or to an Errata document. Until they do that,
I just assume it's a generic power-on reset problem.

The person suggests using a voltage regulator. I always use a regulator.
The person suggests using a decoupling cap on Vdd. I always do that.

The original poster gives a list of his config settings. He does not have
Brownout enabled. That could be a problem, if a wall transformer is
used. I suggest that the BROWNOUT fuse should be used.

Ttelmah · Guest

Look at document 80125G at MicroChip.
Really the key is good supply design. Capacitors only reduce the likelyhood, with some types of supply (you need decoupling close to the PIC anyway).
Ideally use a supply regulator that can be held in shutdown, till the incoming supply reaches a critical threshold, and reset the regulator when the supply falls back below this threshold.

Best Wishes

test153 · Joined: 09 Feb 2009 Posts: 28

The reason why I asked was that the pic is doing strange things, I'm using delay_us to blink some LEDs and when I connect the pic directly to 5V the LEDs begin to flash instead of being illuminated, also the soft rs232 doesn't work (although I can say for sure if this is related to the LED flashing problem). So I thought that it might have something to do with the power because if I use the VCC output from the programmer the pic runs without any issues. The programmer has two capacitors with I think helps to stabilize the power. I was under the impression that one decoupling cap is enough, though I seem to be wrong.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Post a schematic and a photo of your board.

Or

Post a very small test program that shows the LED problem.
Describe the circuit for the LEDs in detail. Describe the power
supply circuit for the PIC and LEDs in detail.

test153 · Joined: 09 Feb 2009 Posts: 28

Here is the schematic

And here is the picture of the board - perhaps not the best image but the only one I have on such a short notice.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The power for this board appears to come from the PC's serial port.
I suggest that you don't do this, during the initial development. Use a
bench power supply, or use a wall-transformer with a +5v regulator.

test153 · Joined: 09 Feb 2009 Posts: 28

I know it's a bad idea to draw current from the serial port - so I do use a bench power supply, and that where the problem comes in. As I said before if I use the power from the serial port trough the pic programmer (with by the way also acts as a interface) every thing works great. So do you think I need to add another capacitor or a voltage regulator?

Anyhow when I get this problem sorted out I was thinking of connecting the pic to a FTDI chip and get the power from the USB-bus.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Do you have the BROWNOUT fuse enabled ? If not, then enable it.

test153 · Joined: 09 Feb 2009 Posts: 28

I tried with BROWNOUT as well as with PUT and it makes no difference. Any other suggestions?

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

Remove most of the LEDs. Try to make the board work with a simple
program that flashes one LED on/off. Simplify the program and the
board, until you get something that works reliably.

Ttelmah · Guest

One thing I notice. I can see lots of LEDs on the ends of wires, and the board, but no sign of any current limiting resistors for the LEDs. They could be hidden under the heatshrink at the LED's, but if not, then this is the problem...

Best Wishes

test153 · Joined: 09 Feb 2009 Posts: 28

Well I added the current limiting resistors, 56ohm (the reason why I didn't put them in in the first place was that I got the wrong info for them from the site I bought it on). Anyhow I doesn't work any better...

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

I think your 56-ohm is a bit aggressive. In the link you provide they use 180-ohm, how about at least following that (which in my opinion is still aggressive, since each PIC pin could be sourcing/sinking two LEDs simultaneously). I do not think you would need that much current anyway, so why not start with around 500-ohms (470 or 510).

test153 · Joined: 09 Feb 2009 Posts: 28

I finally solved the problem. When I connected the curcuit to the external power I didn't connect the VPP line (which handles the RS232 communication), so GP3 was a 0 - and the program was designed to fire an ISR on 0's (a RS232 signal is 1 on start). Thanks for the help on the way.