Fried mbed after pressing reset button

Added by krekr about 11 years ago

Hey all,

I recently bought a LAOS-board from Jaap, and after some soldering I was ready to perform all the tests documented [[]] and [[]]. Voltages were okay, plugging in the mbed gave no trouble.

When proceeding to the "TestFirmware" step, with the mbed in the board, I loaded the firmware (which was already on there from an earlier test btw), opened the serial connection and pressed the reset button to get the mbed to read it. Directly after pressing the reset button, a short but loud "pang!" came from the mbed, with a little puff of magic smoke. Now there is a tiny hole in the LPC1768, and the mbed seems short-circuited when power is supplied to GND and Vin. That's €50 of smoke.

I'm eager to try again, but don't want to fry another €50. Can you guys help me figure out what the hell happened here?
Some pointers:

- 24v was supplied to Vmot with a wall-wart
- The same 24v was supplied to the 7805 (I know it would get hot, but I was monitoring that.)
- USB was feeding 5v to the mbed directly (even before the smoke)
- All 5v / 3v3 / 24v nets were measuring okay with / without the mbed
- There are no (visible) defects to the board, components or soldering (could someone please check the photo's?)
- There is a connection between the ground of the polulu's and the rest of the board (
- The polulu's, opto's and SD-card were plugged in at the time. I don't have the LCD-kit, so all the CAN-stuff was left out. Also, no motors were connected.
- The board was on a clean, non-metal surface.
- I left out R10 and R12 on advice from Jaap and after reading about them
- Vsensors and Vendstops were both jumpered to 5v

Some observations with my logic thinking attached:
- The testfirmware was already running in the mbed from an earlier test.
So, if some output got pulled high/low by the reset, you'd expect it to be high the moment (or shortly after) the mbed gets powered, as it'll start running the firmware. Right?

- Both the 7805 and USB were feeding 5v to the mbed
If there was a conflict there, this would have had consequences the moment the board is powered on, right?

- The "pang" happened directly when pressing reset
Like the reset button was somehow tied to some self-destruct mechanism ;)

- The polulu's or opto's might be faulty.
You'd expect the mbed to blow up earlier. The SD-card works fine.

- The mbed might be faulty
It wouldn't have functioned before

- Rework the mbed with a new LPC1768
The chip probably has some bootloader that I can't get on there. Also, other stuff on the board is probably fried.

I'm lost here. I followed every step carefully. I definitively don't want to waste another €50 on a new mbed and also seeing that go up in smoke.
Thanks in advance for anyone looking into this.

P.S. on the photo's, R10 and R12 look connected, but they're cut loose.


Replies (2)

RE: Fried mbed after pressing reset button - Added by DaveStyles about 11 years ago

drop me a line if you didn't get my email about some mbed support.
(email in my profile)
Dave Styles