OsmocomBB

FPC interface pin 22 on the GTM900-B module is defined by Huawei as UART_DTR (input to the modem, output from external host), but physically inside the module it is connected to Calypso GPIO3. This Calypso GPIO pin powers up as an input, and Huawei's official firmware leaves it as such, as needed for the intended logical function of DTR input to the modem. However, the current OBB port to GTM900-B configures this GPIO3 pin as output, as a result of steve-m having blindly copied my GPIO config code which was (and still is) correct for the Openmoko gta0x target, but is not correct for GTM900-B.

If the current OBB code is loaded and run on a GTM900-B module that is wired up externally per Huawei's official documentation, with PFC interface pin 22 connected to the DTR output of a CP2105, FT2232x or RS-232 receiver chip, the result will be a driver conflict, with the Calypso chip's GPIO output and the external chip's DTR output fighting on the same wire. It is my understanding that such driver fighting can cause hardware damage in one or both of the chips, hence this bug should be considered critical.

The proposed patch attached to this bug report is one possible way to fix this bug, out of an infinite number of possibilities. Also my experiments on GTM900-B modules using my MMTB1 adapter indicate that this GPIO3/DTR line appears to be outfitted with an internal pull-down resistor inside the module, hence the Calypso input won't float if the DTR pin on the FPC interface is simply left unconnected.