VRS51L1050
RB8, and to set RI, will be generated if, and only if, the
following conditions are met at the time the final shift
pulse is generated:
UART Operation in Mode 3
In Mode 3, 11 bits are transmitted (through TXD) or
received (through RXD). The transactions are
composed of: a start bit (low), 8 data bits (LSB first), a
programmable 9th data bit and one stop bit (high).
o
o
Either SM2 = 0 or the received stop bit = 1
RI = 0
If both conditions are met, the stop bit enters RB8, the
8 data bits go into SBUF and RI is activated. If one of
these conditions is not met, the received frame is
completely lost. At this time, whether the above
conditions are met or not, the unit returns to searching
for a 1 to 0 transition in RXD.
Mode 3 is identical to Mode 2 in all respects but one:
the baud rate. Either Timer 1 or Timer 2 generates the
baud rate in Mode 3.
FIGURE 20: SERIAL PORT MODE 3 BLOCK DIAGRAM
Internal Bus
1
Write to
SBUF
UART Operation in Mode 2
Timer 1
Overflow
In Mode 2 a total of 11 bits are transmitted (through
TXD) or received (through RXD). The transactions are
composed of: a start bit (low), 8 data bits (LSB first), a
programmable 9th data bit and one stop bit (high).
For transmission, the 9th data bit comes from the TB8
bit of SCON. For example, the parity bit P in the PSW
could be moved into TB8. For reception, the 9th data bit
is automatically written into RB8 of the SCON register.
Q
S
D
SBUF
TXD
CLK
Timer 2
Overflow
ZERO DETECTOR
÷2
0
1
Shift
Start
Data
SMOD
0
0
1
TX Control Unit
TCLK
TX Clock
÷16
Send
÷16
TI
1
RCLK
Serial Port
Interrupt
RI
SAMPLE
RX Clock
Start
Load
SBUF
RX Control Unit
1-0 Transition
Detector
In Mode 2, the baud rate is programmable to either
1/32 or 1/64 the oscillator frequency.
SHIFT
Bit
Detector
9-Bit Shift Register
Shift
RXD
FIGURE 19: SERIAL PORT MODE 2 BLOCK DIAGRAM
LOAD SBUF
Internal Bus
1
Write to
SBUF
SBUF
READ SBUF
Internal Bus
Q
S
D
SBUF
Fosc/2
÷2
TXD
CLK
ZERO DETECTOR
0
1
Shift
Data
Stop
Start
SMOD
TX Control Unit
TX Clock
Send
÷16
TI
÷16
Serial Port
Interrupt
Sample
RI
RX Clock
Control
Load
SBUF
RX Control Unit
1-0 Transition
Detector
Start
SHIFT
Bit
Detector
9-Bit Shift Register
Shift
RXD
LOAD SBUF
SBUF
READ SBUF
Internal Bus
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