P83CE558EBBSLASHYYY datasheet(29/72 Pages) PHILIPS

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Philips Semiconductors

Preliminary specification

P83CE558/P80CE558/P89CE558

Single-chip 8-bit microcontroller

1996 Aug 06

6.9

Serial I/O

The P8xCE558 is equipped with two independent serial ports:

SIO0 and SI01. SIO0 is the full duplex UART port, identical to the

PCB80C51 serial port. SIO1 is an I2C-bus serial I/O interface with

byte oriented master and slave functions.

6.9.1

SIO0 (UART)

SIO 0 is a full duplex serial I/O port – it can transmit and receive

simultaneously. This serial port is also receive-buffered. It can

commence reception of a second byte before the previously

received byte has been read from the receive register. If, however,

the first byte has still not been read by the time reception of the

second byte is complete, one of the bytes will be lost. The SIO0

receive and transmit registers are both accessed via the S0BUF

special function register. Writing to S0BUF loads the transmit

receive register. SIO0 can operate in 4 modes:

Mode 0:

Serial data is transmitted and received 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. A write into

S0CON should be avoided during a transmission to

avoid spikes on RXD/TXD.

Mode 1:

10 bits are transmitted via TXD or received through

RXD: a start bit (0), 8 data bits (LSB first), and a

stop bit(1). On receive, the stop bit is put into RB8

(S0CON special function register). The baud rate is

variable.

Mode 2:

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 S0CON) can be

assigned the value of 0 or 1. With nominal software,

TB8 can be the parity bit (P in PSW). During a

receive, the 9th data bit is stored in RB8 (S0CON),

and the stop bit is ignored. The baud rate is

programmable to either 1/32 or 1/64 of the oscillator

frequency.

Mode 3:

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). Mode

3 is the same as Mode 2 except the baud rate which

is variable in Mode 3.

In all four modes, transmission is initiated by any instruction that

writes to the S0BUF function register. Reception is initiated in Mode

0 when RI = 0 and REN = 1. In the other three modes, reception is

initiated by the incoming start bit provided that REN = 1.

Modes 2 and 3 are provided for multiprocessor communications. In

these modes, 9 data bits are received with the 9th bit written to RB8.

The 9th bit is followed by the stop bit. The port can be programmed

so that with receiving the stop bit, the serial port interrupt will be

activated if, and only if RB8 = 1.

This feature is enabled by setting bit SM2 in S0CON. This feature

may be used in multiprocessor systems.

For more information about how to use the UART in combination

with the registers S0CON, PCON, IEN0, S0BUF and Timer register

refer to the 80C51 Data Handbook IC20.

Figure 28. Serial port control (S0CON) register.

S0CON (98H)

SM0

SM1

SM2

REN

TB8

RB8

Table 22.

Description of S0CON bits

SYMBOL

BIT

FUNCTION

SM0

S0CON.7

This bit is used to select the serial port mode. See Table 23.

SM1

S0CON.6

This bit is used to select the serial port mode. See Table 23.

SM2

S0CON.5

Enables the multiprocessor communication feature in modes 2 and 3. In mode 2 or 3, if SM2 is set to 1, then

RI will not be activated if the received 9th data bit (RB8) is 0. In mode 1, if SM2 = 1, then RI will not be activated

if a valid stop bit was not received. In mode 0, SM2 should be 0.

REN

S0CON.4

Enables serial reception. Set by software to enable reception. Clear by software to disable reception.

TB8

S0CON.3

The 9th data bit that will be transmitted in modes 2 and 3. Set or clear by software as desired.

RB8

S0CON.2

In modes 2 and 3, RB8 is the 9th data bit that was received. In mode 1, if SM2 = 0, RB8 is the stop bit that was

received. In mode 0, RB8 is not used.

S0CON.1

The transmit interrupt flag. Set by hardware at the end of the 8th bit time in mode 0, or at the beginning of the

stop bit in the other modes, in any serial transmission. Must be cleared by software.

S0CON.0

The receive interrupt flag. Set by hardware at the end of the 8th bit time in mode 0, or halfway through the stop

bit time in the other modes, in any serial reception (except see SM2). Must be cleared by software.

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