One
of the 8051s many powerful features is its integrated UART, otherwise known as a
serial port. The fact that the 8051 has an integrated serial port means that
you may very easily read and write values to the serial port. If it were not
for the integrated serial port, writing a byte to a serial line would be a
rather tedious process requring turning on and off one of the I/O lines in
rapid succession to properly "clock out" each individual bit,
including start bits, stop bits, and parity bits.
However,
we do not have to do this. Instead, we simply need to configure the serial
ports operation mode and baud rate. Once configured, all we have to do is write
to an SFR to write a value to the serial port or read the same SFR to read a
value from the serial port. The 8051 will automatically let us know when it has
finished sending the character we wrote and will also let us know whenever it
has received a byte so that we can process it. We do not have to worry about
transmission at the bit level--which saves us quite a bit of coding and
processing time.
The
first thing we must do when using the 8051s integrated serial port is,
obviously, configure it. This lets us tell the 8051 how many data bits we want,
the baud rate we will be using, and how the baud rate will be determined.
First,
lets present the "Serial Control" (SCON) SFR and define what each bit
of the SFR represents:
Bit
|
Name
|
Bit Addres
|
Explanation of Function
|
7
|
SM0
|
9Fh
|
Serial port mode bit 0
|
6
|
SM1
|
9Eh
|
Serial port mode bit 1.
|
5
|
SM2
|
9Dh
|
Mutliprocessor
Communications Enable (explained later)
|
4
|
REN
|
9Ch
|
Receiver Enable. This bit
must be set in order to receive characters.
|
3
|
TB8
|
9Bh
|
Transmit bit 8. The 9th bit
to transmit in mode 2 and 3.
|
2
|
RB8
|
9Ah
|
Receive bit 8. The 9th bit
received in mode 2 and 3.
|
1
|
TI
|
99h
|
Transmit Flag. Set when a
byte has been completely transmitted.
|
0
|
RI
|
98h
|
Receive Flag. Set when a
byte has been completely received.
|
Additionally,
it is necessary to define the function of SM0 and SM1 by an additional table:
SM0
|
SM1
|
Serial Mode
|
Explanation
|
Baud Rate
|
0
|
0
|
0
|
8-bit Shift Register
|
Oscillator / 12
|
0
|
1
|
1
|
8-bit UART
|
Set by Timer 1 (*)
|
1
|
0
|
2
|
9-bit UART
|
Oscillator / 64 (*)
|
1
|
1
|
3
|
9-bit UART
|
Set by Timer 1 (*)
|
(*)
Note: The baud rate indicated in this table is doubled if PCON.7 (SMOD) is set.
The SCON
SFR allows us to configure the Serial
Port. Thus, well go
through each bit and review its function.
The
first four bits (bits 4 through 7) are configuration bits. Bits SM0 and SM1 let us set the serial mode to a value between 0 and 3, inclusive.
The four modes are defined in the chart immediately above. As you can see,
selecting the Serial Mode selects the mode of operation (8-bit/9-bit, UART or
Shift Register) and also determines how the baud rate will be calculated. In
modes 0 and 2 the baud rate is fixed based on the oscillators frequency. In
modes 1 and 3 the baud rate is variable based on how often Timer 1 overflows.
Well talk more about the various Serial Modes in a moment.
The next
bit, SM2, is a flag for
"Multiprocessor communication." Generally, whenever a byte has been
received the 8051 will set the "RI" (Receive Interrupt) flag. This
lets the program know that a byte has been received and that it needs to be
processed. However, when SM2 is set the "RI" flag will only be
triggered if the 9th bit received was a "1". That is to say, if SM2
is set and a byte is received whose 9th bit is clear, the RI flag will never be
set. This can be useful in certain advanced serial applications. For now it is
safe to say that you will almost always want to clear this bit so that the flag
is set upon reception of any character.
The next
bit, REN, is
"Receiver Enable." This bit is very straightforward: If you want to
receive data via the serial port, set this bit. You will almost always want to
set this bit.
The last
four bits (bits 0 through 3) are operational bits. They are used when actually
sending and receiving data--they are not used to configure the serial port.
The TB8 bit is used in modes 2 and 3. In modes
2 and 3, a total of nine data bits are transmitted. The first 8 data bits are
the 8 bits of the main value, and the ninth bit is taken from TB8. If TB8 is
set and a value is written to the serial port, the datas bits will be written
to the serial line followed by a "set" ninth bit. If TB8 is clear the
ninth bit will be "clear."
The RB8 also operates in modes 2 and 3 and
functions essentially the same way as TB8, but on the reception side. When a
byte is received in modes 2 or 3, a total of nine bits are received. In this
case, the first eight bits received are the data of the serial byte received
and the value of the ninth bit received will be placed in RB8.
TI means "Transmit Interrupt." When
a program writes a value to the serial port, a certain amount of time will pass
before the individual bits of the byte are "clocked out" the serial
port. If the program were to write another byte to the serial port before the
first byte was completely output, the data being sent would be garbled. Thus,
the 8051 lets the program know that it has "clocked out" the last
byte by setting the TI bit. When the TI bit is set, the program may assume that
the serial port is "free" and ready to send the next byte.
Finally,
the RI bit means "Receive Interrupt."
It funcions similarly to the "TI" bit, but it indicates that a byte
has been received. That is to say, whenever the 8051 has received a complete
byte it will trigger the RI bit to let the program know that it needs to read
the value quickly, before another byte is read.
Once the
Serial Port Mode has been configured, as explained above, the program must
configure the serial ports baud rate. This only applies to Serial Port
modes 1 and 3. The Baud Rate is determined based on the oscillators frequency
when in mode 0 and 2. In mode 0, the baud rate is always the oscillator
frequency divided by 12. This means if youre crystal is 11.059Mhz, mode 0 baud
rate will always be 921,583 baud. In mode 2 the baud rate is always the oscillator
frequency divided by 64, so a 11.059Mhz crystal speed will yield a baud rate of
172,797.
In modes
1 and 3, the baud rate is determined by how frequently timer 1 overflows. The
more frequently timer 1 overflows, the higher the baud rate. There are many
ways one can cause timer 1 to overflow at a rate that determines a baud rate,
but the most common method is to put timer 1 in 8-bit auto-reload mode (timer
mode 2) and set a reload value (TH1) that causes Timer 1 to overflow at a
frequency appropriate to generate a baud rate.
To
determine the value that must be placed in TH1 to generate a given baud rate,
we may use the following equation (assuming PCON.7 is clear).
TH1 = 256 - ((Crystal / 384) / Baud)
If
PCON.7 is set then the baud rate is effectively doubled, thus the equation
becomes:
TH1 = 256 - ((Crystal / 192) / Baud)
For
example, if we have an 11.059Mhz crystal and we want to configure the serial
port to 19,200 baud we try plugging it in the first equation:
TH1 = 256 - ((Crystal / 384) / Baud)
TH1 = 256 - ((11059000 / 384) / 19200 )
TH1 = 256 - ((28,799) / 19200)
TH1 = 256 - 1.5 = 254.5
As
you can see, to obtain 19,200 baud on a 11.059Mhz crystal wed have to set TH1
to 254.5. If we set it to 254 we will have achieved 14,400 baud and if we set
it to 255 we will have achieved 28,800 baud. Thus were stuck...
But not
quite... to achieve 19,200 baud we simply need to set PCON.7 (SMOD). When we do
this we double the baud rate and utilize the second equation mentioned above.
Thus we have:
TH1 = 256 - ((Crystal / 192) / Baud)
TH1 = 256 - ((11059000 / 192) / 19200)
TH1 = 256 - ((57699) / 19200)
TH1 = 256 - 3 = 253
Here
we are able to calculate a nice, even TH1 value. Therefore, to obtain 19,200
baud with an 11.059MHz crystal we must:
1. Configure Serial
Port mode 1 or 3.
2. Configure Timer 1 to timer mode 2 (8-bit auto-reload).
3. Set TH1 to 253 to reflect the correct frequency for 19,200 baud.
4. Set PCON.7 (SMOD) to double the baud rate.
Once the
Serial Port has been propertly configured as
explained above, the serial port is ready to be used to send data and receive
data. If you thought that configuring the serial port was simple, using the
serial port will be a breeze.
To write
a byte to the serial port one must simply write the value to the SBUF (99h) SFR. For example, if you wanted
to send the letter "A" to the serial port, it could be accomplished
as easily as:
MOV SBUF,#A
Upon
execution of the above instruction the 8051 will begin transmitting the
character via the serial port. Obviously transmission is not instantaneous--it
takes a measureable amount of time to transmit. And since the 8051 does not
have a serial output buffer we need to be sure that a character is completely
transmitted before we try to transmit the next character.
The 8051
lets us know when it is done transmitting a character by setting the TI bit in SCON. When this bit is set we
know that the last character has been transmitted and that we may send the next
character, if any.
Consider the following code segment:
CLR TI ;Be sure the bit is initially clear
MOV SBUF,#A ;Send the
letter A to the serial port
JNB TI,$ ;Pause until the
TI bit is set.
The
above three instructions will successfully transmit a character and wait for
the TI bit to be set before continuing. The last instruction says "Jump if
the TI bit is not set to $"--$, in most assemblers, means "the same
address of the current instruction." Thus the 8051 will pause on the JNB
instruction until the TI bit is set by the 8051 upon successful transmission of
the character.
Reading data received by the serial port
is equally easy. To read a byte from the serial port one just needs to read the
value stored in the SBUF (99h) SFR after the 8051 has
automatically set the RI flag in SCON.
For
example, if your program wants to wait for a character to be received and
subsequently read it into the Accumulator, the following code segment may be
used:
JNB RI,$ ;Wait for the 8051 to set the RI flag
MOV A,SBUF ;Read the
character from the serial port
The
first line of the above code segment waits for the 8051 to set the RI flag;
again, the 8051 sets the RI flag automatically when it receives a character via
the serial port. So as long as the bit is not set the program repeats the
"JNB" instruction continuously.
Once the RI bit is set upon character reception the
above condition automatically fails and program flow falls through to the
"MOV" instruction which reads the value.
Assembly Language Program for Serial Communication using 89C15:
INCLUDE 89C51.MC
;---------------------MAX232-------------------------------
; Register definitions.
rBUFFER EQU 00h ; read data register
wBUFFER EQU 01h ; send data register
;----------------------------------------------------------
(0000H):
JMP on_reset
;-----------------------Serial Interrupt-------------------
(0023H):
JNB RI,ser_down
MOV rBUFFER,SBUF ; read data
CLR RI
JMP int_end
ser_down:
MOV SBUF,wBUFFER ; send data
CLR TI
int_end:
RETI
;----------------------------------------------------------
(00FFH):
on_reset:
;Initialize Timer
;Baud rate table
;Fosc = 11.0592 MHz
;Baud rate TH1 TH1(hex) SMOD(PCON.7)
;300 160 A0 0
;1200 232 E8 0
;2400 244 F4 0
;4800 250 FA 0
;9600 253 FD 0
;19200 253 FD 1
;28800 255 FF 0
MOV TH1,#FDH ; baud rate 9600
MOV TMOD,#20H ; timer-1 in 8-bit auto reload
MOV SCON,#50H ; receive enable
CLR PCON.7 ; SMOD = 0
; MOV IE,#90H ; Serial Interrupt 10010000b
ENDLESS:
CALL get_byte
MOV A,rBUFFER
MOV wBUFFER,A
CALL send_byte
JMP ENDLESS
;----------------------------------------------------------
;Without interrupt
;----------------------------------------------------------
send_byte:
MOV SBUF,wBUFFER ; send data
SETB TR1
RS232BACK: JNB SCON.1,RS232BACK ; wait for TI
CLR TF1
CLR SCON.1
RET
;----------------------------------------------------------
get_byte:
SETB TR1
RS232B: JNB SCON.0,RS232B ; wait for RI
MOV rBUFFER,SBUF ; get data
CLR TF1
CLR SCON.0
RET
;-----------------------------------------------------------
