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SERIAL COMMUNICATIONS UNIT
Register Name:
Register Mnemonic:
Register Function:
15
Bit
Mnemonic
TB7:0
Transmit
Data Field
NOTE: Reserved register bits are shown with gray shading. Reserved bits must be written
to a logic zero to ensure compatibility with future Intel products.
Figure 11-9. Serial Transmit Buffer Register (SxTBUF)
11.2.1 Baud Rates
The baud rate generator is composed of a 15-bit counter register (BxCNT) and a 15-bit compare
register (BxCMP). BxCNT (Figure 11-10) is a free-running counter that is incremented by the
baud timebase clock. The baud timebase clock can be either the internal CPU clock or an external
clock applied to the BCLK pin. BxCMP (Figure 11-11) is programmed by the user to determine
the baud rate. The most-significant bit of BxCMP (ICLK) selects which source is used as the baud
timebase clock.
BxCNT is incremented by the baud timebase clock and compared to BxCMP. When BxCNT and
BxCMP are equal, the baud rate generator outputs a pulse and resets BxCNT. This pulse train is
the actual baud clock used by the RX and TX machines. The baud clock is eight times the baud
rate in the asynchronous modes because of the sampling requirements. The baud clock equals the
baud rate in the synchronous mode.
11-10
Serial Transmit Buffer Register
SxTBUF
Bytes are written to SxTBUF to be transmitted.
Reset
Bit Name
State
0
T
T
T
T
B
B
B
B
6
5
4
7
Function
Data byte to be transmitted.
0
T
T
T
T
B
B
B
B
3
2
1
0
A1291-0A
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