The PCA9540B is a 1-of-2 bidirectional translating multiplexer, controlled via the I²C-bus.
The SCL/SDA upstream pair fans out to two SCx/SDx downstream pairs, or channels.
Only one SCx/SDx channel is selected at a time, determined by the contents of the
programmable control register.
A power-on reset function puts the registers in their default state and initializes the I²C-bus
state machine with no channels selected.
The pass gates of the multiplexer are constructed such that the VDD pin can be used to
limit the maximum high voltage that is passed by the PCA9540B. This allows the use of
different bus voltages on each SCx/SDx pair, so that 1.8 V, 2.5 V or 3.3 V parts can
communicate with 5 V parts without any additional protection. External pull-up resistors
can pull the bus up to the desired voltage level for this channel. All I/O pins are 5 V