What is F-Bus?
The F-Bus is a full-duplex bus. It uses one pin for transmitting data(Tx) and one pin for receiving data(Rx). It's baudrate is 115200bps, 8 data bits in each character, no parity, one stop bit.
After little online research for the F-Bus I found that the F-Bus is based on UART and all I need is a digital isolation for Rx/Tx lines to solve the difference in voltage issue and isolate the device if it is powered from another source, so for this I used the ADuM1201.
The Power Supply:
For the supply I used a mini step down converter and set it to value between 3.7V and 3.8V, the USB is enough to power the device so there's no need for an external one (the device could consume up to 1W).
The phone's battery has four pads (details on 3310 battery types):
- VBATT, battery voltage, rated: 3.7V and its connected to the output of step down converter;
- BSI, battery size indication and it depends on the battery type, basically this pin is a pull-down resistor of a specific value (I used 73k7, 3 smd resistors in series);
- BTEMP, battery temperature, it is also a pull-down resistor (I used a 47k);
- GND, the ground.