Philips Semiconductors

Preliminary specification

89C535/89C536/89C538

CMOS single-chip 8-bit microcontrollers

with FLASH program memory

1997 Jun 05

12

Serial Interface

The 89C538/536 has a standard 80C51 serial port. This serial port

can operate in 4 modes:

Mode 0: Serial data enters and exits through RxD. TxD outputs the

shift clock. 8 bits are transmitted/received (LSB first). The baud rate

is fixed at 1/12 the oscillator frequency.

Mode 1: 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.

Mode 2: 11 bits are transmitted (throughTxD) or received (through

RxD): 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 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

the oscillator frequency.

Mode 3: 11 bits are transmitted (through TxD) or received (through

jRxD): 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 baud rate. The baud rate in Mode 3 is variable.

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.

Serial Port Control Register

The serial port control and status register is the Special

FunctionRegister SCON, shown in Figure 5. This register contains

not only the mode selection bits, but also the 9th data bit for transmit

and receive (TB8 and RB8), and the serial port interrupt bits (TI and

RI).

Additional details of serial port operation may be found in the 80C51

Family Hardware Description found in the

Philips 80C51–Based

8–Bit Microcontroller Data Handbook, IC20.

SCON Address = 98H

Reset Value = 0000 0000B

SM0

SM1

SM2

REN

TB8

RB8

Tl

Rl

Bit Addressable

Symbol

Function

SM0

Serial Port Mode Bit 0

SM1

Serial Port Mode Bit 1

SM0

SM1

Mode

Description

Baud Rate**

0

0

0

shift register

0

1

1

8-bit UART

variable

1

0

2

9-bit UART

1

1

3

9-bit UART

variable

SM2

Enables the Automatic Address Recognition feature in Modes 2 or 3. If SM2 = 1 then Rl will not be set unless the

received 9th data bit (RB8) is 1, indicating an address, and the received byte is a Given or Broadcast Address.

In Mode 1, if SM2 = 1 then Rl will not be activated unless a valid stop bit was received, and the received byte is a

Given or Broadcast Address. In Mode 0, SM2 should be 0.

REN

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

TB8

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

RB8

In modes 2 and 3, 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.

Tl

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.

Rl

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.

NOTE:

SU00867

Bit:

7654

321

0

Figure 5.

SCON: Serial Port Control Register