C8051F124-GQR Datasheet, PDF (288/350 Pages) Silicon Laboratories – Mixed Signal ISP Flash MCU Family

C8051F120/1/2/3/4/5/6/7
C8051F130/1/2/3
21.1. UART0 Operational Modes
UART0 provides four operating modes (one synchronous and three asynchronous) selected by setting
configuration bits in the SCON0 register. These four modes offer different baud rates and communication
protocols. The four modes are summarized in Table 21.1.
Mode
0
1
2
3
Synchronization
Synchronous
Asynchronous
Asynchronous
Asynchronous
Table 21.1. UART0 Modes
Baud Clock
SYSCLK / 12
Timer 1, 2, 3, or 4 Overflow
SYSCLK / 32 or SYSCLK / 64
Timer 1, 2, 3, or 4 Overflow
Data Bits
8
8
9
9
Start/Stop Bits
None
1 Start, 1 Stop
1 Start, 1 Stop
1 Start, 1 Stop
21.1.1. Mode 0: Synchronous Mode
Mode 0 provides synchronous, half-duplex communication. Serial data is transmitted and received on the
RX0 pin. The TX0 pin provides the shift clock for both transmit and receive. The MCU must be the master
since it generates the shift clock for transmission in both directions (see the interconnect diagram in
Figure 21.3).
Data transmission begins when an instruction writes a data byte to the SBUF0 register. Eight data bits are
transferred LSB first (see the timing diagram in Figure 21.2), and the TI0 Transmit Interrupt Flag
(SCON0.1) is set at the end of the eighth bit time. Data reception begins when the REN0 Receive Enable
bit (SCON0.4) is set to logic 1 and the RI0 Receive Interrupt Flag (SCON0.0) is cleared. One cycle after
the eighth bit is shifted in, the RI0 flag is set and reception stops until software clears the RI0 bit. An inter-
rupt will occur if enabled when either TI0 or RI0 are set.
The Mode 0 baud rate is SYSCLK / 12. RX0 is forced to open-drain in Mode 0, and an external pullup will
typically be required.
RX (data out)
TX (clk out)
MODE 0 TRANSMIT
D0
D1
D2
D3
D4
D5
D6
D7
RX (data in)
MODE 0 RECEIVE
D0
D1
D2
D3
D4
D5
D6
D7
TX (clk out)
Figure 21.2. UART0 Mode 0 Timing Diagram
TX
C8051Fxxx
RX
CLK
DATA
Shift
Reg.
8 Extra Outputs
Figure 21.3. UART0 Mode 0 Interconnect
288
Rev. 1.4