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8051 includes an on-chip serial port that can
operate in four modes over a wide range of frequencies. |
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Essential function of serial port is to perform
parallel-to-serial conversion for output data, and serial-to-parallel
conversion for input data. |
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Transmission bit is P3.1 on pin 11 (TXD) and
reception bit is P3.0 on pin 10 (RXD). |
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Features full duplex (simultaneous reception and
transmission). |
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Receive buffering allowing one character to be
received and held in a buffer while a second character is received. If the
CPU reads the first character before the second is fully received, data are
not lost. |
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Two SFRs (SBUF & SCON) provide software
access to serial port. |
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Writing to SBUF loads data to be transmitted and
reading SBUF accesses received data. |
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SCON is a bit addressable register containing
status bits and control bits. Control bits set the operating mode and
status bits indicate the end of a character transmission or reception. The
status bits are tested in software or programmed to cause an interrupt. |
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Serial port frequency of operation (baud rate)
can be fixed or variable. |
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Fixed is derived from on-chip oscillator and
variable is supplied by Timer 1 which must be programmed accordingly. |
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SM0, SM1 : Serial Port Mode bits |
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Mode Baud Rate |
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00 = Mode 0 : Shift register I/O Fixed
(oscillator frequency/12) |
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01 = Mode 1 : 8-bit UART Variable (set
by timer) |
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10 = Mode 2 : 9-bit UART Fixed (osc
frq/32 or osc frq/64 ) |
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11 = Mode 3 : 9-bit UART Variable (set
by timer) |
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- SM2 : Serial
Port Mode bit |
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Mode 0 : Not used. |
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Mode 1 : 1 = Ignore bytes with no stop
bit. |
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Mode 2,3 : 0 = Set receive interrupt (RI)
on all bytes. |
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: 1 = Set RI on bytes where 9th
bit is 1. |
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- REN: Receiver
enable. Must be set to receive characters. |
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TB8: Transmit bit 8. Ninth bit transmitted (in
modes 2 and 3); set/cleared by software. |
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- RB8: Receive
bit 8. Ninth bit received (in modes 2 and 3): |
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Mode 0 : Not used. |
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Mode 1 : Stop bit. |
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Mode 2, 3 : Ninth data bit. |
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- TI: Transmit
interrupt flag. Set at end of character transmission; cleared by software. |
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- RI: Receive
interrupt flag. Set at end of character reception; cleared by software. |
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Full duplex UART (Universal Asynchronous
Receiver /Transmitter is a device that receives and transmits serial data
with each data character preceded by a start bit “0” and followed by a stop
bit “1”). Sometimes a parity bit is
inserted between the last data bit and the stop bit. |
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The essential operation of a UART is to perform
parallel-to-serial conversion for output data, and serial-to-parallel
conversion for input data. |
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10 or 11 bit frames. |
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Interrupt driven. |
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Registers: |
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SCON - Serial port control register. |
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SBUF - Read received data. |
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- Write data to be transmitted. |
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Mode 0: 8-Bit Shift Register Mode. Terms RXD
& TXD are misleading in this mode. RXD line is used for both input and
output. TXD line serves as the clock. |
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Eight bits are transmitted and received with the
LSB first. Baud Rate is 1/12 of on-chip oscillator frequency. |
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Transmission is initiated by any instruction
that writes data to SBUF. Data are shifted out on RXD line with clock
pulses sent out by the TXD line. Each transmitted bit is valid on the RXD
pin for one machine cycle. E.g., MOV SBUF, A |
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Reception is initiated when the receiver enable
bit (REN) is 1 and the receive interrupt bit (RI) is 0. REN is set at the
beginning of the program, and then clear RI to begin a data input
operation. The clocking of data into serial port occurs on the positive edge
of TXD. |
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Mode 1: Serial port operates as an 8-bit UART
with a variable baud rate.10-bits are transmitted on TXD or received on
RXD. Start bit (always 0), 8 data bits (LSB first), and a stop bit (always
1). For a receive operation, the stop bit goes into RB8 in SCON. Baud Rate
Clock is variable using Timer 1 overflow or external count input. |
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Transmission is initiated by writing data to
SBUF, but does not start until the next rollover of the divide-by-16
counter supplying the serial port baud rate. Shifted data are outputted on
the TXD line beginning with the start bit. The transmit interrupt flag (TI)
is set as soon as the stop bit appears on TXD. |
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Reception is initiated by a 1-to-0 transition on
RXD. The divide-by-16 counter is immediately reset to align the counts with
the incoming bit stream. |
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Mode 2: Serial port operates as a 9-bit UART
with a fixed baud rate. 11-bits are transmitted or received. Start bit
(always 0), 8 data bits (LSB first), a programmable 9th bit, and a stop bit
(always 1). |
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On transmission, the 9th bit whatever
has been put in TB8 in SCON (may be a parity bit). |
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On reception, the 9th bit is placed
in RB8 in SCON. |
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Baud Rate is programmable to either 1/32 or 1/64
of the on-chip oscillator frequency. |
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Mode 3: Serial port operates as a 9-bit UART
with a variable baud rate. 11-bits are transmitted or received. Baud Rate
is programmable and provided by the Timer 1 overflow or external input. |
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Receiver Enable Bit (REN): must be set by
software to enable the reception of characters at the beginning of a
program when the serial port, timers, etc. are initialized. The
instructions are |
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SETB REN or MOV SCON, #xxx1xxxxB |
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The 9th Bit: transmitted must be
loaded into TB8 by software and received is placed in RB8. |
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Adding a Parity Bit: is a common use of 9th
bit. E.g., if communication requires 8 data bits plus even parity |
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MOV C, P ; Put even parity bit in C flag |
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MOV TB8, C ; This becomes the 9th data bit in TB8 |
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MOV SBUF, A ; Move 8 bits from ACC to
SBUF |
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E.g., if communication requires 8 data bits plus
odd parity |
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MOV C, P ; Put even parity bit in C flag |
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CPL C ; Convert to odd parity |
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MOV TB8, C ; This becomes the 9th data bit in TB8 MOV
SBUF, A ; Move 8 bits from ACC to SBUF |
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Parity can be used in mode 1 also if the 7 data
bits are used. E.g., 7-bit ASCII code with even parity can be transmitted
as follows: |
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CLR ACC.7 ; Ensure MSB is clear |
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MOV C, P ; Put even parity bit in C flag |
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MOV ACC.7, C ; Copy even parity bit into
MSB
MOV SBUF, A ;
Send character |
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RI & TI in SCON play an important role in
serial communications. Both bits are set by hardware but must be cleared by
software. |
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RI is set at the end of character reception and
indicates “receive buffer full”. |
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This condition is tested in software or
programmed to cause an interrupt. |
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If software wishes to input a character from the
device connected to the serial port, it must wait until RI is set, then
clear RI and read the character from SBUF. |
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WAIT: JNB RI, WAIT ; Check RI
until set |
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CLR RI ; Clear the flag |
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MOV A, SBUF ; Read character |
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TI is set at the end of character transmission
and indicates “transmit buffer empty”. |
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If software wishes to send a character to the
device connected to the serial port, it must wait until TI is set (means
previous character was sent, wait until transmission is finished before
sending the next character), then clear TI and send the character. |
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WAIT: JNB TI, WAIT ; Check TI
until set |
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CLR TI ; Clear the flag |
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MOV SBUF, A ; Send character |
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Serial Communication Modes 2 and 3 allow one
"Master" 8051 to control several “Slaves” 8051. |
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The serial port can be programmed to generate an
interrupt (RI) if the 9th data bit = 1 by setting the SM2 bit in SCON. |
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The TXD outputs of the slaves are tied together
and to the RXD input of the master. The RXD inputs of the slaves are tied
together and to the TXD output of the master. |
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Each slave is assigned an address. Address bytes
transmitted by the master have the 9th bit = 1 & data bytes have it =
0. |
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When the master transmits an address byte, all
the slaves are interrupted. The slaves then check to see if they are being
addressed or not. |
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The addressed slave will clear its SM2 bit and
prepare to receive the data bytes that follows and the slaves that weren’t
addressed leave their SM2 bits set and go about their business, ignoring
the incoming data bytes. They will be interrupted again when the next
address byte is transmitted by the master processor. |
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Baud rate is also affected by a bit in the PCON
register. PCON.7 is SMOD bit. If SMOD = 1, baud rate will be doubled in
modes 1, 2 and 3. |
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Mode 2 baud rate is the 1/64th the
oscillator frequency (SMOD = 0) and can be doubled to 1/32nd the
oscillator frequency (SMOD = 1). |
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PCON is not bit-addressable, setting SMOD
without altering the other bits requires a “read-modify-write” operation as
follows: |
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MOV A, PCON ; Get current value of
PCON |
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SETB ACC.7 ; Set SMOD |
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MOV PCON, A ; Write value back to
PCON |
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Usually the timer is used in auto-reload mode
and TH1 is loaded with a proper reload value. |
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Formula for the baud rate in modes 1 and 3 is |
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Baud Rate = Timer 1 Overflow Rate / 32 |
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e.g., For 1200 baud |
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1200 = Timer 1 Overflow Rate / 32 |
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Timer 1 Overflow Rate = 38400 Hz |
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Timer must overflow at a rate of 38.4 kHz and
the timer is clocked at a rate of 1000 kHz (1 MHz), overflow required every
1000/38.4 = 26.04 clocks, so |
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MOV TH1, # -26 |
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Due to rounding, there is a slight error in the
resulting baud rate. Up to 5% is tolerable using asynchronous
communications. Exact baud rates are possible using an 11.059 MHz crystal (Table
5-3). |
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To initialize the serial port to operate as
an 8-bit UART at 2400 baud. |
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ORG 0000H |
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MOV SCON,#52H ;serial port mode 1 |
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MOV TMOD,#20H ;timer 1, mode 2 |
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MOV TH1, #-13 ;reload count for
2400 baud |
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SETB TR1 ;start timer 1 |
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END |
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Notes