Handshake Solutions
HT80C51 User Manual
Peripheral Modules – Standard Serial Interface (SIO0)
Page 34 of 132
© Philips Electronics N.V. 2005
PCON
Power Control Register
addr = 87H
reset value =
0xxx0000
bits
7
6
5
4
3
2
1
0
SMOD
GF1
GF0
PD
IDL
bit
symbol
Function
PCON.7
SMOD
1:
0:
Double baud rate
If timer 1 is used to generate the baudrate and the serial interface is
used in modes 1, 2 or 3, then the baudrate is doubled.
The baudrate is not influenced
PCON.6..0
See description of PCON register in CPU section.
5.3.3.
Interrupts
The serial interface has two flags to indicate interrupt conditions: TI for transmit interrupts and RI for
receive interrupts. However, there is only one interrupt output, that has to be shared by these interrupt
sources. So, if any of the flags TI and RI is set, an interrupt request will be generated. The software
has to check, then, which source was the cause.
interrupt
source
Description
interrupt
signal
UART:
TI
RI
Transmit interrupt flag.
Receive interrupt flag.
The logical OR of TI and RI generates the interrupt request
signal.
int4
Note: To comply with the standard specification, the serial interface does not support transmit-
buffering. So the transmit interrupt is issued when the transmission has been completed (instead of as
soon as the contents of SBUF are copied to the transmit shift register).
Interrupt flags are set by hardware and have to be reset by software.
5.3.4.
Operation
The UART function is full duplex, meaning it can transmit and receive simultaneously. It is also re-
ceive-buffered, meaning it can commence reception of a second byte before a previously received
byte has been read from the register. (However, if the first byte still hasn’t been read by the time re-
ception of the second byte is complete, one of the bytes will be lost.) The serial port receive and
transmit registers are both accessed thru special function register SBUF. Writing to SBUF loads the
transmit register, and reading SBUF accesses a physically separate receive register.
5.3.4.1.
Overview operating modes
The serial port can operate in 4 modes:
Mode 0:
Serial data enters at input pin sio_rxd_i and exits through output pin sio_txd_o. Pin
sio_clk_o outputs the shift clock during transmission. 8 bits are transmitted/received
(LSB first). The baud rate is fixed at the clock input sio_clk_i. This mode is restricted to
half duplex operation only.
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