
1996 Dec 11
38
Philips Semiconductors
Preliminary specification
Low voltage 16-bit microcontroller
P90CL301BFH (C100)
12 SERIAL INTERFACES
12.1
UART interface
The UART can operate in 4 modes. The baud rate for
receive and transmit can be generated internally or by the
baud rate generator. The UART is full duplex, meaning it
can receive and transmit simultaneously. The receive and
transmit registers are both accessed as a unique register
SBUF. Writing to SBUF loads the transmit register, and
reading from SBUF accesses a physically separate
receive register.
12.1.1
UART
OPERATING MODES
The serial port can operate in one of the four modes:
Mode 0 Serial data enters and exits through RXD. TXD
pin delivers the synchronous shift clock. 8 bits
are transmitted/received (LSB first). When the bit
PCLK3 in the SYSCON register is reset, the baud
rate is equal to
1
6
×
CLK. When the bit PCLK3 in
register SYSCON is set, the baud rate is equal to
1
12
×
CLK. The UART baud rate should not
exceeds 4.5 Mbaud.
Mode 1 10 bits are transmitted (through TXD) or received
(through RXD): a start bit at logic 0, 8 data bits
(LSB first) and a stop bit at logic 1. On receive the
stop bit goes into RB8 in the register SCON.
The baud rate is given by the baud rate generator
output BGCLK0 for the UART0 and BGCLK1 for
the UART1.
Mode 2 11 bits are transmitted (through TXD) or received
(through RXD): a start bit at logic 0, 8 data bits
(LSB first) a programmable 9
th
data bit, and a
stop bit at logic 1. On transmit the 9
th
bit is taken
from the bit TB8 from the SCON register. On
receive the 9
th
bit goes into RB8 of SCON, while
the stop bit is ignored. The baud rate is equal to
1
6
×
CLK. The UART clock should not exceed
4.5 Mbaud.
Mode 3 Same as mode 2 except for the baud rate, which
is given by the baud rate generator output
BGCLK0 for the UART0 and BGCLK1 for the
UART1.
In all four modes, transmission is initiated by any
instruction loading SBUF. In Mode 0, reception is initiated
by the condition RI = 0 and REN = 1. In the remaining
modes reception is initiated by the incoming start bit if
REN = 1.