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18.1.3. Mode 2: 9-Bit UART, Fixed Baud Rate

Mode 2 provides asynchronous, full-duplex communication using a total of eleven bits per data byte: a start bit, 8

data bits (LSB first), a programmable ninth data bit, and a stop bit (see timing diagram in Figure 18.6). On transmit,

the ninth data bit is determined by the value in TB8 (SCON.3). It can be assigned the value of the parity flag P in

the PSW or used in multiprocessor communications. On receive, the ninth data bit goes into RB8 (SCON.2) and the

stop bit is ignored.

Data transmission begins when an instruction writes a data byte to the SBUF register. The TI Transmit Interrupt

Flag (SCON.1) is set at the end of the transmission (the beginning of the stop-bit time). Data reception can begin

any time after the REN Receive Enable bit (SCON.4) is set to logic 1. After the stop bit is received, the data byte

will be loaded into the SBUF receive register if the following conditions are met: RI must be logic 0, and if SM2 is

logic 1, the 9

th

bit must be logic 1.

If these conditions are met, the eight bits of data are stored in SBUF, the ninth bit is stored in RB8 and the RI flag is

set. If these conditions are not met, SBUF and RB8 will not be loaded and the RI flag will not be set. An interrupt

will occur if enabled when either TI or RI are set.

The baud rate in Mode 2 is a direct function of the system clock frequency as follows:

Mode 2 Baud Rate = 2

SMOD

* (SYSCLK / 64).

The SMOD bit (PCON.7) selects whether to divide SYSCLK by 32 or 64. In the formula, 2 is raised to the power

SMOD, resulting in a baud rate of either 1/32 or 1/64 of the system clock frequency. On reset, the SMOD bit is

logic 0, thus selecting the lower speed baud rate by default.

Figure 18.6. UART Modes 2 and 3 Timing Diagram

MARK

SPACE

BIT TIMES

START

BIT

D0

D1

D2

D3

D4

D5

D6

D7

D8

STOP

BIT

BIT SAMPLING

18.1.4. Mode 3: 9-Bit UART, Variable Baud Rate

Mode 3 is the same as Mode 2 in all respects except the baud rate is variable. The baud rate is determined in the

same manner as for Mode 1. Mode 3 operation transmits 11 bits: a start bit, 8 data bits (LSB first), a programmable

ninth data bit, and a stop bit. Timer 1 or Timer 2 overflows generate the baud rate just as with Mode 1. In

summary, Mode 3 transmits using the same protocol as Mode 2 but with Mode 1 baud rate generation.

Rev. 1.7

134