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IS89C51
18
1-800-379-4774 — Integrated Silicon Solution, Inc.
MC016-1C
11/21/98
ISSI
SERIAL INTERFACE
The Serial port is full duplex, which means it can transmit
and receive simultaneously. It is also receive-buffered,
which means it can begin receiving a second byte before
a previously received byte has been read from the receive
register. (However, if the first byte still has not been read
when 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 the following four modes:
Mode 0:
Serial data enters and exits through RXD. TXD outputs the
shift clock. Eight data bits are transmitted/received, with
the LSB first. The baud rate is fixed at 1/12 the oscillator
frequency (see Figure 12).
Mode 1:
Ten bits are transmitted (through TXD) or received (through
RXD): a start bit (0), eight 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
Figure 13).
Mode 2:
Eleven bits are transmitted (through TXD) or received
(through RXD): a start bit (0), eight data bits (LSB first), a
programmable ninth data bit, and a stop bit (1). On transmit,
the ninth data bit (TB8 in SCON) can be assigned the value
of 0 or 1. Or, for example, the parity bit (P, in the PSW) can
be moved into TB8. On receive, the ninth 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 the oscillator frequency (see Figure 14).
Mode 3:
Eleven bits are transmitted (through TXD) or received
(through RXD): a start bit (0), eight data bits (LSB first), a
programmable ninth data bit, and a stop bit (1). In fact,
Mode 3 is the same as Mode 2 in all respects except the
baud rate, which is variable in Mode 3 (see Figure 15).
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.