hi
I have a problem in integrating a 1.8V GSM modem with nRF52832 through UART. nRF52832 supplied with 3.3V and there is a voltage divider that covert 3.3V to 1.8V in the Tx pin . Rx pin is directly connected to the GSM. When i send an AT command from nRF52832, its showing a COMMUNICATION ERROR in the uart_event_handler and app error handler will reset the chip.But when i use 3.3V modem i am getting the correct 'OK, reply . So is there any problem using 1.8V in the UART Rx Pin?
Yes there is a problem using 1.8V logic when Vdd is 3.3V. A input high have to be at least 0.7*Vdd, see here. With 3.3V the minimum input high voltage is 2.31V which is a higher than 1.8V.
You can create a logic level converter in the low (1.8V) to high (3.3V) voltage direction using a mosfet transistor and two pull-up resistors (f.ex. 10KOhm). Here is the schematic (from sparkfuns logic level converter):
LV should be 1.8V, Hv should be 3.3V, TX_LV should be GSM TX, TX_HV should be nRF RX.
