Using one discovery board as a signal generator and another as input capture

kevinhuang · ‎2013-11-22

Posted on November 22, 2013 at 17:57

Hello, I've been attempting to set up two discovery boards. One as a signal generator that generates a 1 Hz pulse (50% duty cycle), and the other to accept that signal as input capture. I can't seem to get the input capture board to generate interrupts on the 1 Hz pulse though.

I'm getting stuck because it seems like both boards are configured correctly. I confirmed that the 1 Hz pulse is correct on an oscilloscope. For the input capture board, if I take a wire and plug one end onto a 3.3 V output pin, then tap the other end on the input capture pin, I can see that the interrupt is generated.

Despite working when tested separately, when I connect their grounds and then their respective pins together, it fails to generate any interrupts. An interesting symptom that I noticed is that when I measure the resistance between the input capture pin and its respective ground, I only get 10 Ohms, which strikes me as a red flag.

Anyone have an idea what might be causing this?

frankmeyer9 · ‎2013-11-22

Posted on November 22, 2013 at 19:54

Could be a difference in potential between the two boards, i.e. ground potentials are significantly different. A solution would be to operate both boards from one power supply. But that rules out USB as supply, and could make debugging troublesome.

I remember setups including a PC for debugging, a scope for measuring, and a mains supplied debuggee, which went up in smoke for that very reason ...

Mark Edwards · ‎2013-11-22

Posted on November 23, 2013 at 00:47

What pins are you connecting? As some pins are used by other devices on the board and don’t behave as you would expect (from someone who had to remove the LCD and MEMS to get them to behave properly).

kevinhuang · ‎2013-11-22

Posted on November 23, 2013 at 01:00

On the board generating the signal, I'm using PE9. On the input capture board, I'm using PE0.

kevinhuang · ‎2013-11-22

Posted on November 23, 2013 at 01:00

On the board generating the signal, I'm using PE9. On the input capture board, I'm using PE0.

Mark Edwards · ‎2013-11-22

Posted on November 23, 2013 at 01:18

''when I measure the resistance between the input capture pin and its respective ground, I only get 10 Ohms, which strikes me as a red flag. ''

-So effectively tied to ground.

What type of discovery board?

Using a STM32F429 Discovery I am getting 1M7 to ground at PE0 (on my chopped board), but the only possible connections seem to be NBL0 (SDRAM) which seems unlikely.

kevinhuang · ‎2013-11-22

Posted on November 23, 2013 at 01:48

I'm using an stm32f3.

Based on Tede's response, I checked the schematics and indeed, the MEMs is also tied to that particular pin. I used a heat gun to remove it and now input capture seems to be working fine. Thank you for your help!

frankmeyer9 · ‎2013-11-23

Posted on November 23, 2013 at 16:35

Based on Tede's response, I checked the schematics and indeed, the MEMs is also tied to that particular pin. I used a heat gun to remove it and now input capture seems to be working fine.

Isn't that called ''due diligence'' ?