design breakout board for GTM900-B
Let's build a small 2-layer PCB design as a break-out board for the calypso/iota/rita based GTM900-B module.
It should have the following key features:
- on-board power supply for (5V?) input
- over-voltage + polarity protection
- 2.5mm jack for the UART (+ TVS array)
- expose all pins of the module on 2.54mm header
- LED for power and modem (is there one?)
- power button and jumper in parallel (auto-start)
- reset button
- SIM card slot (2FF) with TVS array
One option would also be to directly put a CP2102 or even CP2105 on the board to attach to one (or both) UARTs. This way we'd also have the option to control reset and/or power via GPIOS or serial status lines, like tsaitgaist used to do..
The shortest 40pos 0.5mm pitch FPC available from Digikey seems to be https://www.digikey.com/product-detail/en/molex/0151660425/WM16424-ND/3281306 at 29.97mm.
GTM900 exposes no LED at all, so the breakout only will get a power-on LED.
Should the modem reside on the break-out PCB like on the mPCIe adapter, e.g. with hex spacer, or will this board be much smaller for only carrying the additional functionality and not the modem itself?
- Priority changed from Normal to Low
the modem should be mounted with spacers on the board. I typically want to have a mechanically stable configuration, without any pigtails or other fragile constructs hanging off it. That's why we have the SMA sockets on the mpcie breakout board, and why I believe we should have a similar construct here.
GTM900 exposes no LED at all,
this might be not true, there are two pins with funny names and ambiguous descriptions, so likely one of them (or both?) might drive a LED (plus aditional driver):
Pin 13 is VDD (output), and its function "signal indication of normally started GTM900". The other is pin 32, LPG (output), "mainly controls the status of the indicator".