P80C32 datasheet(23/62 Pages) PHILIPS

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Philips Semiconductors

Product data

P80C3xX2; P80C5xX2;

P87C5xX2

80C51 8-bit microcontroller family

4K/8K/16K/32K ROM/OTP, low voltage (2.7 to 5.5 V),

low power, high speed (30/33 MHz)

2003 Jan 24

FULL-DUPLEX ENHANCED UART

Standard UART operation

The serial port is full duplex, meaning it can transmit and receive

simultaneously. It is also receive-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 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 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 in

12-clock mode or 1/6 the oscillator frequency in 6-clock

mode.

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 (through TxD) 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

rate is programmable to either 1/32 or 1/64 the oscillator

frequency in 12-clock mode or 1/16 or 1/32 the oscillator

frequency in 6-clock mode.

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). 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.

Multiprocessor Communications

Modes 2 and 3 have a special provision for multiprocessor

communications. In these modes, 9 data bits are received. The 9th

one goes into RB8. Then comes a stop bit. The port can be

programmed such that when the stop bit is received, the serial port

interrupt will be activated only if RB8 = 1. This feature is enabled by

setting bit SM2 in SCON. A way to use this feature in multiprocessor

systems is as follows:

When the master processor wants to transmit a block of data to one

of several slaves, it first sends out an address byte which identifies

the target slave. An address byte differs from a data byte in that the

9th bit is 1 in an address byte and 0 in a data byte. With SM2 = 1, no

slave will be interrupted by a data byte. An address byte, however,

will interrupt all slaves, so that each slave can examine the received

byte and see if it is being addressed. The addressed slave will clear

its SM2 bit and prepare to receive the data bytes that will be coming.

The slaves that weren’t being addressed leave their SM2s set and

go on about their business, ignoring the coming data bytes.

SM2 has no effect in Mode 0, and in Mode 1 can be used to check

the validity of the stop bit. In a Mode 1 reception, if SM2 = 1, the

receive interrupt will not be activated unless a valid stop bit is

received.

Serial Port Control Register

The serial port control and status register is the Special Function

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).

Baud Rates

The baud rate in Mode 0 is fixed: Mode 0 Baud Rate = Oscillator

Frequency / 12 (12-clock mode) or / 6 (6-clock mode). The baud

rate in Mode 2 depends on the value of bit SMOD in Special

Function Register PCON. If SMOD = 0 (which is the value on reset),

and the port pins in 12-clock mode, the baud rate is 1/64 the

oscillator frequency. If SMOD = 1, the baud rate is 1/32 the oscillator

frequency. In 6-clock mode, the baud rate is 1/32 or 1/16 the

oscillator frequency, respectively.

Mode 2 Baud Rate =

2SMOD

(Oscillator Frequency)

Where:

n = 64 in 12-clock mode, 32 in 6-clock mode

The baud rates in Modes 1 and 3 are determined by the Timer 1 or

Timer 2 overflow rate.

Using Timer 1 to Generate Baud Rates

When Timer 1 is used as the baud rate generator (T2CON.RCLK

= 0, T2CON.TCLK = 0), the baud rates in Modes 1 and 3 are

determined by the Timer 1 overflow rate and the value of SMOD as

follows:

Mode 1, 3 Baud Rate =

2SMOD

(Timer 1 Overflow Rate)

Where:

n = 32 in 12-clock mode, 16 in 6-clock mode

The Timer 1 interrupt should be disabled in this application. The

Timer itself can be configured for either “timer” or “counter”

operation, and in any of its 3 running modes. In the most typical

applications, it is configured for “timer” operation, in the auto-reload

mode (high nibble of TMOD = 0010B). In that case the baud rate is

given by the formula:

Mode 1, 3 Baud Rate =

2SMOD

Oscillator Frequency

[256–(TH1)]

Where:

n = 32 in 12-clock mode, 16 in 6-clock mode

One can achieve very low baud rates with Timer 1 by leaving the

Timer 1 interrupt enabled, and configuring the Timer to run as a

16-bit timer (high nibble of TMOD = 0001B), and using the Timer 1

interrupt to do a 16-bit software reload. Figure 13 lists various

commonly used baud rates and how they can be obtained from

Timer 1.

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