(TECH) Is it safe to connect a 3.3V device to the GSM+BLE?

what is the IO (SDA,SCL,INT1,INT2) voltage level for GSM+BLE? Is it safe to connect a 3.3V device to the GSM+BLE without using a level converter?
I notice the GSM breakout boaard is using 1.8V & 2.8V

Hi there:
Watching the schematics, i´d bet on the breakout module voltages; they seem to be physically attached to the GSM+BLE module itself throught the flex cable.
3,3 volts are only used for main vcc (3,5 with a SIM) and nothing more as far as i’ve been playing with the breakout.

Best regards,

The Xadow Duino module uses a MCU ATmega32u4 which is power by 3.3V.

As shown in the Xadow Duino schmeatic, the ATmega32u4 is connected to the Xadow GSM+BLE module via a FPC ribbon cable without using any level converter.

Link for Xadow Duino schematic:
seeedstudio.com/wiki/images/ … _Duino.rar

Just want to clarify the following:
Is the ATmega32U4 uses 2.8V for its IO? If it is not true, why it can connect to the GSM+BLE directly?

No, and because the rephone tolerates the 3.3V (but it does not like it)