
XBUS asynchronous / synchronous serial interface
UM0404
DocID13284 Rev 2
Figure 108. Asynchronous 9-bit data frames
When the last stop bit has been received, the content of the receive shift register is
transferred to the receive data buffer register XS1RBUF. Simultaneously, the receive
interrupt request flag is set after the 9th sample in the last stop bit time slot (as
programmed), regardless whether valid stop bit have been received or not. The receive
circuit then waits for the next start bit (1-to-0 transition) at the receive data input pin.
The receiver input pin RXD1/P8.6 must be configured for input, using direction control
register DP8.6 = ‘0’.
Asynchronous reception is stopped by clearing bit S1REN. A currently received frame is
completed including the generation of the receive interrupt request and an error interrupt
request, if appropriate. Start bit that follow this frame will not be recognized.
Note:
In wake-up mode received frames are only transferred to the receive buffer register, if the
9th bit (the wake-up bit) is ‘1’. If this bit is ‘0’, no receive interrupt request will be activated
and no data will be transferred.
11.2 Synchronous
operation
Synchronous mode supports half-duplex communication, basically for simple I/O expansion
via shift registers. Data is transmitted and received via pin RXD1/P8.6, while pin TXD1/P8.7
outputs the shift clock. These signals are alternate functions of Port8 pins. Synchronous
mode is selected with S1M = ‘000b’.
8 data bits are transmitted or received synchronous to a shift clock generated by the internal
Baud rate generator. The shift clock is only active as long as data bits are transmitted or
received.
2nd
Stop
bit
Start
bit
D0
(LSB)
D1
D2
D3
D4
D5
D6
9th
bit
(1st)
Stop
bit
D7
• Data bit D8
• Parity
• Wake-up bit