I try to communicate with the SIM800C with a 3,3V FTDI converter, but most of the time the module does not respond on the UART interface. On my board
the pin called “link GND” in the picture below is called “boot”. When controlling it from a microcontroller I get a stable communication when I set the “boot” (link)
pin to low for about 1s then the communication works stable; the other pins SPK, MIC, DTR and RING are not connected.
Do you know if any of the other pins should be connected and what the “link” (boot) pin is for?
Also, when using a 5V/2A socket power supply instead of the lab power supply the SIM800 module does now work anymore. In between the power supply and the SIM800 module I am using a LM2596 voltage regulator to generate 4V. I also tried a goldcap with 0.47F on the SIM800 side, but it did not fix the problem.