STM32VL Discovery Board electrical failure: Output pins max. 2.5V

marcusrydh · ‎2012-02-23

Posted on February 24, 2012 at 04:24

Hi,

I recently noticed that my STM32VL Discovery board, while powered from the USB port would only output 2.5V on any of the pins. I checked the 5V and 3.3V outputs on the board, which only delivered approx. 4.5V and just under 3V respectively. I expected that the ''5V'' pin would not reach a full 5V while being powered from the USB port because of the Schottky diode (LDO regulator?), but I would have expected the 3.3V regulator to be up to spec. I'm curious if others have seen this with their boards. In any case, this doesn't explain why I'm only seeing 2.5V on the output pins.

They are connected to high impedance loads at the moment (I'm trying to communicate with a 5V 16x1 LCD display by using GPIOA in open-drain mode with 10K pull ups) but even when I connect low-current external circuits, none of the pins show any more than about 2.5-2.6V regardless of push-pull or open-drain configuration. I'm at a loss as to why this is, I would have expected a component failure to disable the board entirely. Could this be a software issue? I can't imagine it's current draw related since I haven't powered anything substantial with the board. Could this be the result of accidental short circuiting between an output pin and an adjacent ground pin? Any insight into this would be helpful.

Thanks

ingwmeier · ‎2012-02-23

Posted on February 24, 2012 at 07:41

There is another Diode (D1 see schematic) in series of the 3.3V Regulator, if you just use the Discovery board, you can short circuit this. I observed around 3V, 2.5V seems very low.

Werner

marcusrydh · ‎2012-02-24

Posted on February 24, 2012 at 17:45

I've been able to determine that the 3.3V regulator appears to be working fine (I read the output at 3.2V which I'm willing to accept), with a drop across diode D1 of .35V. Still, I cant understand why the GPIO outputs are limited to 2.5V. At the Discovery-side STM32F100, there is just under 3V present at Vdd pins, and I thought outputs should be more or less ''rail to rail''. No peripherals are activated apart from the GPIO pins, which are definitely set to output mode (both OD and PP). I noticed a while ago that the blue LED on the board was slightly dimmer than I remember it being after I first bought the board. Is there a possibility that the GPIO circuitry was damaged, but not completely disabled by some kind of overcurrent?

marcusrydh · ‎2012-02-24

Posted on February 24, 2012 at 17:46

Also, more importantly, with outputs set to OD mode with external pullups and connected to the 5V rail (4.5V in my case) should the outputs not swing from 0V to 5V?!

marcusrydh · ‎2012-03-07

Posted on March 08, 2012 at 00:51

If anyone is interested, the solution to this problem was to buy a new discovery board. I didn't find any information as to what could have gone wrong, so there was only one option left.