C868_02 Datasheet, PDF (144/244 Pages) Infineon Technologies AG – 8 - Bit CMOS Microcontroller

C868
On-Chip Peripheral Components
4.9
Serial Interface
The serial port of the C868 is full duplex, meaning it can transmit and receive
simultaneously. It is also receive-buffered, meaning it can commence reception of a
second byte before a previously received byte has been read from the receive register
(however, if the first byte still hasn’t been read by the time reception of the second byte
is complete, one of the bytes will be lost). The serial port receive and transmit registers
are both accessed at special function register SBUF. Writing to SBUF loads the transmit
register, and reading SBUF accesses a physically separate receive register.
The serial port can operate in 3 asynchronous modes. The baud rate clock for the serial
port is derived from the oscillator frequency (mode 2) or generated either by timer 1 or
by a dedicated baud rate generator (mode 1, 3). Mode 0 is reserved.
Mode 1, 8-Bit UART, Variable Baud Rate:
In mode 1, ten bits are transmitted (through TxD) or received (through RxD). They are a
start bit (0), 8 data bits (LSB first), and a stop bit (1). On receive, the stop bit goes into
RB8 in special function register SCON. The baud rate is variable. (See section 4.9.2 for
more detailed information)
Mode 2, 9-Bit UART, Fixed Baud Rate:
11 bits are transmitted (through TxD) or received (through RxD): a start bit (0), 8 data
bits (LSB first), a programmable 9th bit, and a stop bit (1). On transmit, the 9th data bit
(TB8 in SCON) can be assigned to the value of 0 or 1. Or, for example, the parity bit (P,
in the PSW) could be moved into TB8. On receive, the 9th data bit goes into RB8 in
special function register SCON, while the stop bit is ignored. The baud rate is
programmable to either 1/32 or 1/64 of the oscillator frequency. (See section 4.9.3 for
more detailed information)
Mode 3, 9-Bit UART, Variable Baud Rate:
11 bits are transmitted (through TxD) or received (through RxD): a start bit (0), 8 data
bits (LSB first), a programmable 9th data bit, and a stop bit (1). In fact, mode 3 is the
same as mode 2 in all respects except the baud rate. The baud rate in mode 3 is variable.
(See section 4.9.3 for more detailed information)
In all modes, transmission is initiated by any instruction that uses SBUF as a destination
register. Reception is initiated in the modes by the incomming start bit if REN = 1.
The serial interface also provides interrupt requests when transmission or reception of a
frames have been completed. The corresponding interrupt request flags are TI or RI,
resp. See chapter 7 of this user manual for more details about the interrupt structure. The
interrupt request flags TI and RI can also be used for polling the serial interface, if the
serial interrupt is not to be used (i.e. serial interrupt not enabled).
User’s Manual
4-100
V 0.4, 2002-01