Datasheet
71M6543F/71M6543G Data Sheet
36 v2
• UART0 RX: Serial input data are applied at this pin. Conforming to RS-232 standard, the bytes are
input LSB first.
• UART0 TX: This pin is used to output the serial data. The bytes are output LSB first.
The 71M6543 has several UART-related registers for the control and buffering of serial data.
A single SFR register serves as both the transmit buffer and receive buffer (S0BUF, SFR 0x99 for UART0
and S1BUF, SFR 0x9C for UART1). When written by the MPU, SxBUF acts as the transmit buffer, and
when read by the MPU, it acts as the receive buffer. Writing data to the transmit buffer starts the
transmission by the associated UART. Received data are available by reading from the receive buffer.
Both UARTs can simultaneously transmit and receive data.
WDCON[7] (SFR 0xD8) selects whether timer 1 or the internal baud rate generator is used. All UART
transfers are programmable for parity enable, parity, 2 stop bits/1 stop bit and XON/XOFF options for
variable communication baud rates from 300 to 38400 bps. Table 16 shows how the baud rates are
calculated. Table 17 shows the selectable UART operation modes.
Table 16: Baud Rate Generation
Using Timer 1
(WDCON[7] = 0)
Using Internal Baud Rate Generator
(WDCON[7] = 1)
UART0
2
smod
* f
CKMPU
/ (384 * (256-TH1))
2
smod
* f
CKMPU
/(64 * (2
10
-S0REL))
UART1
N/A
f
CKMPU
/(32 * (2
10
-
S1REL
))
S0REL and S1REL are 10-bit values derived by combining bits from the respective timer reload registers.
(S0RELL, S0RELH, S1RELL, S1RELH are SFR 0xAA, SFR 0xBA, SFR 0x9D and SFR 0xBB, respectively) SMOD
is the SMOD bit in the SFR PCON register (SFR 0x87). TH1 (SFR 0x8D) is the high byte of timer 1.
Table 17: UART Modes
UART 0 UART 1
Mode 0
N/A
Start bit, 8 data bits, parity, stop bit, variable
baud rate (internal baud rate generator)
Mode 1
Start bit, 8 data bits, stop bit, variable
baud rate (internal baud rate generator
or timer 1)
Start bit, 8 data bits, stop bit, variable baud
rate (internal baud rate generator)
Mode 2
Start bit, 8 data bits, parity, stop bit,
fixed baud rate 1/32 or 1/64 of f
CKMPU
N/A
Mode 3
Start bit, 8 data bits, parity, stop bit,
variable baud rate (internal baud rate
generator or timer 1)
N/A
Parity of serial data is available through the P flag of the accumulator. 7-bit serial modes with
parity, such as those used by the FLAG protocol, can be simulated by setting and reading bit 7 of
8-bit output data. 7-bit serial modes without parity can be simulated by setting bit 7 to a constant 1.
8-bit serial modes with parity can be simulated by setting and reading the 9
th
bit, using the control
bits TB80 (S0CON[3]) and TB81 (S1CON[3]) in the S0CON (SFR 0x98) and S1CON (SFR 0x9B) registers
for transmit and RB81 (S1CON[2]) for receive operations.
All supported operation modes use oversampling for the incoming bit stream when receiving data. Each
bit is sampled three times at the projected middle of the bit duration. This technique allows for deviations
of the received baud rate from nominal of up to 3.5%.
The feature of receiving 9 bits (Mode 3 for UART0, Mode A for UART1) can be used as handshake signals
for inter-processor communication in multi-processor systems. In this case, the slave processors have bit
SM20 (S0CON[5]) for UART0, or SM21 (S1CON[5] for UART1, set to 1. When the master processor outputs
the slave’s address, it sets the 9
th
bit to 1, causing a serial port receive interrupt in all the slaves. The slave
processors compare the received byte with their address. If there is a match, the addressed slave clears
SM20 or SM21 and receive the rest of the message. The rest of the slaves ignore the message. After