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Serial Port Programming: 8051 Serial Communication
One of the 8051’s many powerful features -integrated UART, known as a serial port to easily read and write values to the serial port instead of 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.
ü Setting the Serial Port Mode configures it by specifying 8051 how many data bits
we want, the baud rate we will be using and how the baud rate will be determined. First, let’s present the "Serial Control" (SCON) SFR and define what each bit of the SFR
Table 5.3 Definition of SCON SFR
Additionally, it is necessary to define the function of SM0 and SM1 by an additional table: Table 5.4 SCON as serial Port
Table 5.4 Modes of SCON
The SCON SFR allows us to configure the Serial Port. The first four bits (bits 4 through 7) are configuration bits:
Bits SM0 and SM1 is to set the serial mode to a value between 0 and 3, inclusive as in table above 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 oscillator’s frequency. In modes 1 and 3 the baud rate is
variable based on how often Timer 1 overflows.
The next bit, SM2, is a flag for " Multiprocessor communication whenever a byte has been received the 8051 will set the "RI" (Receive Interrupt) flag to let 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". if SM2 is set and a byte is received whose 9th bit is clear, the RI flag will never be set .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." is set indicate to data received via the serial port.
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 data’s 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 3and 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 nineth 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, the8051 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 functions similarly to the "TI" bit, but it indicates that a byte has been received. 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.
ü Setting the Serial Port Baud Rate
Once the Serial Port Mode has been configured, the program must configure the serial port’s
baud rate. This only applies to Serial Port modes 1 and 3. The Baud Rate is determined based on the oscillator’s frequency when in mode 0 and 2. In mode 0, the baud rate is always the oscillator frequency divided by 12. This means if you’re crystal is 1.059 Mhz, 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 of172,797.
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 mode2) 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, (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 11.059 Mhz 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
To obtain 19,200 baud on a 11.059Mhz crystal we’d 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.
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
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.
ü Writing to the Serial Port
Once the Serial Port has been properly configured as explained above, the serial port is ready to be used to send data and receive data.
To write a byte to the serial 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 the last character has been transmitted and that 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 RI 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 $"—
$, in most assemblers, means "the same address of the current instruction." Thus the 8051 will pause on the JNB instruction until the TI bit is set by the 8051 upon successful transmission of the character.
ü Reading the Serial Port
Reading data received by the serial port is equally easy. To read a byte from the serial port one just needs to read the value stored in the SBUF (99h) SFR after the 8051 has automatically set the RI flag in SCON.
For example, if your program wants to wait for a character to be received and subsequently read it into the Accumulator, the following code segment may be used:
JNB RI,$;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, the8051 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.
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