C501_1 Datasheet, PDF (63/121 Pages) Siemens Semiconductor Group – 8-Bit Single-Chip Microcontroller

On-Chip Peripheral Components
C501
6.3 Serial Interface
The serial port 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):
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 USART, 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 USART, 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 data 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 6.3.6 for more detailed information)
Mode 3, 9-Bit USART, 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 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).
Semiconductor Group
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