PK-70 5V DC power usage

[rsg]

Hi,

I have a new custom board that I just received, and it has some problems. When plugging it into the internal 40-pin connector of my PK-70, the latter appears to be frozen. Specifically, at most only a single garbage character is emitted over the serial port, and the a user LED on the side of the box fails to blink - as it should if my program is running.

I looked at the voltages that are available on the 40-pin connector, both with and without the custom card, and this is what I found:

Case 1: PK-70 alone:

3.3V at J3-1, measured 3.36V
5V at J3-37, measured 5.24V
VRAW at J3-39, measured 8.12V

Case 2: PK-70, with custom card

3.3V at J3-1, measured 3.40V
5V at J3-37, measured 0.880V
VRAW at J3-39, measured 7.80V

Seems pretty clear the 5V (which isn't actually used on the board, but simply routed to external pins, currently with no connections) is likely being pulled to ground.

But my question is: why does the processor not run? I don't have the PK-70 schematics, but I would think the processor doesn't depend on the 5V, given the 3.3V that is available. So, is it possible that the processor is actually running, and the lack of 5V explains the serial port and LED issues? Not sure if this actually matters, but it might help my understanding of the situation.

The good news is that no damage appears to have happened to the PK-70!

Thanks,
-Bob

[lgitlitz]

If possible I would look at all the voltage rails on an oscilloscope. There is a single switcher with two outputs that generates the 5V and 3.3V rails. I would think that if the 5V is tied to ground it will have some effect on the 3.3V rail as well. Look to see how much noise is on this rail with and without your custom blade board.

[rsg]

Hi Larry,

Interesting...

Actually, I did use a scope when making these measurements, and noise-wise, I didn't see anything that jumped out at me, either with or without the custom board. The only significant things I saw was the obvious 5V supply measuring at 0.88V, and the slight drop in the VRAW voltage. So I think the 3.3 V rail is fine, but I will look more carefully later, and report back. Does that make any sense?

FWIW - I looked at the schematics of the custom board, and the only thing on the 5V is a capacitor and an ESD diode; maybe that diode is in backwards... I'll check in a few hours...

Thanks,
-Bob

[lgitlitz]

You should also make sure that the address and data lines brought out to the 40 pin header are not being pulled in any direction. This would prevent the processor from running since it will not be able to talk to the flash or SDRAM.

[rsg]

Seems the diode was indeed bad, thus messing up the 5V supply.

Now another issue, but seems a new topic is wise...

Thanks!