C515A_9708 Datasheet, PDF (92/182 Pages) Siemens Semiconductor Group – 8-Bit CMOS Microcontroller

On-Chip Peripheral Components
C515A
6.3
Serial Interface
The serial port of the C515A 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 4 modes (one synchronous mode, three asynchronous modes). The
baud rate clock for the serial port is derived from the oscillator frequency (mode 0, 2) or generated
either by timer 1 or by a dedicated baud rate generator (mode 1, 3).
Mode 0, Shift Register (Synchronous) Mode:
Serial data enters and exits through RxD. TxD outputs the shift clock. 8 data bits are transmitted/
received: (LSB first). The baud rate is fixed at 1/12 of the oscillator frequency. (See section 6.3.4
for more detailed information)
Mode 1, 8-Bit UART, Variable Baud Rate:
10 bits are transmitted (through TXD) or received (through RXD): 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 6.3.5 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. 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 6.3.6 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 6.3.6 for more
detailed information)
In all four modes, transmission is initiated by any instruction that uses SBUF as a destination
register. Reception is initiated in mode 0 by the condition RI = 0 and REN = 1. Reception is initiated
in the other modes by the incoming start bit if REN = 1. The serial interfaces also provide interrupt
requests when a transmission or a reception of a frame has completed. The corresponding interrupt
request flags for serial interface 0 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 0 if the serial interrupt is not to be used (i.e. serial interrupt 0 not enabled).
Semiconductor Group
6-41
1997-08-01