XC886/888CLM
Serial Interfaces
User’s Manual
12-5
V1.3, 2010-02
Serial Interfaces, V 1.0
12.1.1.3
Mode 2, 9-Bit UART, Fixed Baud Rate
In mode 2, the UART behaves as a 9-bit serial port. A start bit (0), 8 data bits plus a
programmable 9th bit and a stop bit (1) are transmitted on TXD or received on RXD. The
9th bit for transmission is taken from TB8 (SCON.3) while for reception, the 9th bit
received is placed in RB8 (SCON.2).
The transmission cycle is activated by a write to SBUF. The data is transferred to the
transmit register and TB8 is copied into the 9th bit position. At phase 1 of the machine
cycle following the next rollover in the divide-by-16 counter, the start bit is copied to TXD
and data is activated one bit time later. One bit time after the data is activated, the data
starts shifting right. For the first shift, a stop bit (1) is shifted in from the left and for
subsequent shifts, zeros are shifted in. When the TB8 bit gets to the output position, the
control block executes one last shift and sets the TI bit.
Reception is started by a high to low transition on RXD (sampled at 16 times the baud
rate). The divide-by-16 counter is then reset and 1111 1111
B
is written to the receive
register. If a valid start bit (0) is then detected (based on two out of three samples), it is
shifted into the register followed by 8 data bits. If the transition is not followed by a valid
start bit, the controller goes back to looking for a high to low transition on RXD. When the
start bit reaches the leftmost position, the control block executes one last shift, then
loads SBUF with the 8 data bits, loads RB8 (SCON.2) with the 9th data bit, and sets the
RI bit, provided RI = 0, and either SM2 = 0 (see
) or the 9th bit = 1. If none
of these conditions is met, the received byte is lost.
The baud rate for the transfer is either
f
PCLK
/64 or
f
PCLK
/32 for UART module, depending
on the setting of the top bit (SMOD) of the PCON (Power Control) register, which acts as
a Double Baud Rate selector. For UART1 module, the baud rate is fixed at
f
PCLK
/64.
12.1.1.4
Mode 3, 9-Bit UART, Variable Baud Rate
Mode 3 is the same as mode 2 in all respects except that the baud rate is variable.
In all modes, transmission is initiated by any instruction that uses SBUF as a destination
register. Reception is initiated in the modes by the incoming start bit if REN = 1.
The serial interface also provides interrupt requests when transmission or reception of
the frames has been completed. The corresponding interrupt request flags are TI or RI,
respectively. If the serial interrupt is not used (i.e., serial interrupt not enabled), TI and
RI can also be used for polling the serial interface.
The associated timings for transmit/receive in modes 2 and 3 are illustrated
in
.
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