Development board problems [was : Circuit Capacitance?]

[Joel Rainville]

I am having what seems to be a strange capacitance effect (strange to me at least) on a circuit I built, a modified version of Nigel's 877 devboard.

When I power it without a PIC inserted everything is fine except for the IDC connector for PORTA (see pictures). Checking with my multimeter, I read voltages up to 1.5V on pin 3 (pin 1 is at bottom left and is GND, pin 2 is Vdd, port pins are then in descending order CCW). But, the 1.5V reads for a fraction of a second, then it quickly drops to half that to about .75V and slowly decreases. I have the same phenomenon on pins 4 & 5, but with lower voltages, starting from 0.35V.

There are no shorts. Is this "circuit capacitance"? What can I do to get rid of this?

 

[Nigel Goodwin]

No, it's not stray capacitance, I suspect it's more probably a function of your meter?, or leakage across where you've cut the tracks?.

Of the three pins you mention, pins 3, 4 don't appear to be connected to the PIC anywhere?.

 

[Styx]

or an intermitten short that is aggrivated with the probe in place - have you got rid of all copper barbs from the veraboard?

 

[Joel Rainville]

Of the three pins you mention, pins 3, 4 don't appear to be connected to the PIC anywhere?.

Not directly, they just reach across the IDC connector to pins 9 & 10, to make routing to the E/AN connector easier. I did this to get all 8 analog inputs on PORTE connector.

I'll check for shorts AGAIN :lol:

 

[Joel Rainville]

Good, I went and cleaned up all suspicious specks of copper, and the weird behaviour is gone. Everything's fine without a PIC.

Now, when I insert my PIC, and power it up, voltages on MCLR and Vdd are perfect and the crystal oscillates. But, I read about 1V on *all* PORT pins. Is that some sort of "severe error" report from the PIC?

The code on the PIC just drives the whole PORTB high. The same PIC inserted in a solderless breadboard and properly powered works as intended with 5V across PORTB, all other port pins low, so there's obviously a bad connection somewhere...

Is the 1V on all pins a clue to an obvious cause?

 

[Styx]

Good, I went and cleaned up all suspicious specks of copper, and the weird behaviour is gone. Everything's fine without a PIC.

He shoots he scores.. Wait England cant play Football,
He knocks it for 6...

 

[Joel Rainville]

He knocks it for 6...

Is that cricket? Bowling? Polo? :lol:

He shoots he scores...

We're finally gonna hear that again in about a month up here in Canada. Professional hockey was on lockout for the last 16 months!...

LeClair... to Lemieux... to Crosby... SCOOOOOOOOOOORES :lol:

Now, what about my PIC? :lol:

 

[Gorgon]

Good, I went and cleaned up all suspicious specks of copper, and the weird behaviour is gone. Everything's fine without a PIC.

Now, when I insert my PIC, and power it up, voltages on MCLR and Vdd are perfect and the crystal oscillates. But, I read about 1V on *all* PORT pins. Is that some sort of "severe error" report from the PIC?

The code on the PIC just drives the whole PORTB high. The same PIC inserted in a solderless breadboard and properly powered works as intended with 5V across PORTB, all other port pins low, so there's obviously a bad connection somewhere...

Is the 1V on all pins a clue to an obvious cause?

Hi,
If you measure 1V it indicates more low outputs than high outputs shorted together. The PIC will sink and source about 25mA with a voltage drop. If you connect one high and one low output together you'll measure 2.5V. The PIC will also heat up, at least after some time.

Insert your PIC but leave all B-port pins outside the connector. If you now measure 5V on them you will know that the protoboard is not good.

The first thing I would have done is to measure (Low ohm, no power) all pins on the PIC connector agains Gnd and Vdd. If that's ok, all pins against each other, to look for short circuits. If thats ok, the circuit should work!

Just do it systematically and you'll find the culprit(s).

TOK

 

[Joel Rainville]

So, to verify your theory, I could decrease the number of HIGH pins on PORTB and see a gradually decreasing value on other pins. Do I get this right?

 

[Joel Rainville]

With all PORTB pins LOW, I still read ~1V on all pins.

New development : while I can't find a short, I've just found out something interesting.

When the PIC is in the socket, there is slight continuity between MCLR and all port pins when the negative test lead is on MCLR, not the other way around. I tried it directly on the PIC, out of the socket, and it doesn't show any continuity at all. Testing the empty socket doesn't show continuity either.

Again, the same pic powered outside the board works fine, so the board is the problem.

There are no shorts betweem pins, neither between Vdd and any pin as far as my DMM can tell...

I am clueless.

 

[bryan1]

Hiya Joel,
Eh mate I've been developing my own development board based on a 40 pin pic. Below is a screen dump of the pcb. Now I have used sprint layout to do the board and rather than use a max232 I have implented Mike K8lh's level inverter using a couple of mosfets. If your interested I can send you the sprint layout files so you can etch your own. I've based the circuit on Nigel's board and I will give Nigel all the credit for the idea.

cheers Bryan

 

[Joel Rainville]

Hey Bryan,

Nice looking PCB layout!

Thanks for the offer, but I don't give up that easy I really wanna use my version, and make it work on stripboard. I plan on using stripboard quite a lot for prototyping since I found a good cheap source...

 

[Gorgon]

With all PORTB pins LOW, I still read ~1V on all pins.

New development : while I can't find a short, I've just found out something interesting.

When the PIC is in the socket, there is slight continuity between MCLR and all port pins when the negative test lead is on MCLR, not the other way around. I tried it directly on the PIC, out of the socket, and it doesn't show any continuity at all. Testing the empty socket doesn't show continuity either.

Again, the same pic powered outside the board works fine, so the board is the problem.

There are no shorts betweem pins, neither between Vdd and any pin as far as my DMM can tell...

I am clueless.

Hi
As far as I can see the MCLR input is pulled up with 1k. Do you measure 1V at the MCLR pin too? You are sure that the oscillator starts up? Have you tried to pull MCLR to GND for a momentary reset?

You will normally see a oneway connection between port pins and Vdd. Thats the input protection diode. Depending on the polarity of your meter. The reason for a connection between MCLR and the other pins is the 1k resistor.

Have you checked that the GND and Vdd is continous for the whole board?

TOK

 

[Joel Rainville]

The reason for a connection between MCLR and the other pins is the 1k resistor.

Ah, that makes sense.

Have you checked that the GND and Vdd is continous for the whole board?

Yeah. Everything looks good.

It has to be a short somewhere. A big ugly obvious short, too obvious for me to find. I haven't touched the board since yesterday. The occasional break away from it is what saves it from ending in the microwave on HI. :twisted:

I will find it, even if it takes a month. Thanks for your help!

 

[Joel Rainville]

It's a short. I can't find it but it's a short. :roll:

I've disconnected Vdd and GND from all IDC connectors and it works. My LED board shines with joy.

Again, thanks for your help.