The digital I/O pins are driven from the module's internal LDO, fixed at 1.8 V. No external I/O supply is required from the host.
All standard digital pins (UART, status indicators, coexistence interface, and RF control interface) are referenced to the internal 1.8 V LDO output. The host PCB must use 1.8 V logic levels on these pins. See Recommended operating conditions for the I/O voltage tolerance.
The SIM interface pins follow the SIM_VOUT supply rather than the internal 1.8 V LDO, so the module supports both 1.8 V (Class C) and 3.0 V (Class B) UICC cards transparently. SIM_VOUT is auto-selected by the modem firmware based on the inserted UICC class.