[CLOSED] Q2687 stillborn on board

Legacy Legacy AirPrime modules
1

Hi

We have some new prototypes of a board we’re developing. We can move the Q2687 from a known working board to a new board, and it doesn’t appear to “chirp” on its COM1 Tx. It doesn’t respond to AT commands either (I can see them being received on the test points). I’ve probed around on the test points and appear to see the following (looking at the CPU “upside-down”):

*
               *
               *        <- 3.6V – VBATT I presume
               *

               *
            *           <- SIM Power?
               *        <- SIM Power?
            *           <- SIM ck/data
               *        <- 1.8V

            *           <- SIMPRES?

            *           <- 32.8kHz squarewave, briefly (I think this is used by the program as a GPIO)

            *

               *
            *           <- 3.6V
               *        <- COM1 Rx (to CPU)  2.8V when idle
            *           <- COM1 Tx (from CPU)  2.8V when idle
               *
            *
               *        <- 1.8V
2

Other power supplies appear to be correct. What other critical signals should we check for that would prevent the CPU from trying to boot or download?

3

https://forum.sierrawireless.com/t/problems-booting-the-board/3926/4

https://forum.sierrawireless.com/t/faq-forum-wiki/3491/1

4

Turned out the RESET signal was being held at 0.8V - so it looked high, but wasn’t!

It was interesting to note that the chirps on COM1 Tx (“RxD” I think…) appear irrespective of the state of the handshaking signals. So it seems that as long as the supplies are there, and RESET is 1.8V, it should start chirping…