I'm still working with a client's hardware and a SIM340 GPRS modem. It's mostly working but we're having two problems:

1) The PWRKEY line (used to turn the modem on/off by bringing the signal momentarily low) will turn the modem on but it won't turn it off; however, as far as we can tell the EXACT same circuit on the modem's evaluation board WILL turn it on AND off (as it should).

2) The microcontroller is controlling the DTR line going to the modem. It starts out at 0V (correct) and I raise that line and I measure the line at 2.7V (rather than 3V). I then bring it low again and it goes back to 0V (correct). When I then raise the line again, the measured voltage goes to 0.7V (rather than 3V or even 2.7V). All subsequent toggles go between 0V and 0.7V. If the modem is removed from its socket then the microcontroller successfully can alternate the DTR line between 0V and 3V as would be expected.

I think these two issues might be related.

Without getting into the actual schematics (which are quite trivial and appear to be the same between the evaluation board and the client's board) and without getting into the depths of the modem's manual, does anyone have any ideas what could cause the symptoms in #2? It seems to me like something is grounding or somehow trying to control DTR other than the microcontroller when the modem is inserted. The DTR line is labeled as an input to the modem so I wouldn't expect the modem to assert any control over it, yet the weird voltage only presents itself when the modem is connected.

Any thoughts, anyone? We're kind of pulling our hair out here.

Regards,
Craig Steiner