SIM card detection issues with SIM900/SIM800

I noticed that at some point when working with SIM900 chip from SIMCOM, the SIM card was not being detected any longer. The SIM card was never removed from the tray during my tinkering. I suspect the frequent connections/disconnections to the USB port for serial monitoring/debugging had something to do with it.

Error detection

A simple run of the AT command AT+CCID or AT+CPIN? will give an undesired error result.

Solution

A soft reset using the AT command AT+CFUN=1 solved the issue. What this command does is reset the GSM chip to full functionality.

P.S. The AT command AT+CFUN=0 will not work. This command essentially shuts down the GSM chip.