Siemens SXG 75 Service Repair Documentation page 64

The MSM6250 interfaces to the fuel gauge are:
• IRDA_RXD and IRDA_TXD from UART 3. These lines can be disconnected from the fuel gauge
interface via a tri-state buffer.
• IRDA_EN. The tri-state buffer is disabled when IRDA_EN is low. It is enabled when high.
The fuel gauge interface is open-drain at the battery pack. It has a pull-up (R1106) to VREG_MSMP.
Since the single-wire interface is operated in an open-drain environment, an open-drain driver circuit
(V2100) has been added to the IRDA_TXD interface of the fuel gauge. The fuel gauge interface is
shown below:
Fuel gauge interface to UART
Data is passed from the TXD line to the fuel gauge as follows:
The FUEL_GAUGE line is pulled to VREG_MSMP by R1106.
When IRDA_TXD is taken low, the collector of V2101(b) is pulled high, which causes V2101(a) to
pull FUEL_GAUGE low. This data is read by the fuel gauge. The IRDA_RXD line goes low at the
same time, but this is ignored by the MSM6250.
Data is passed from the fuel gauge to the RXD line as follows:
The battery fuel gauge has an internal pulldown transistor. This pulls FUEL_GAUGE low, which in
turn takes IRDA_RXD low to the MSM6250.
Technical Documentation
TD_Repair_L3_SXG75_R1.0.pdf
Company Confidential
2006©BenQ
Release 1.0
01/2006
Page 64 of 73