Serial Communication using microcontroller 8051

UART (Universal Asynchronous Receiver Transmitter) or USART (Universal Synchronous Asynchronous Receiver Transmitter) are one of the basic interface which you will find in almost all the controllers available in the market till date. This interface provide a cost effective simple and reliable communication between one controller to another controller or between a controller and PC.

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.

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.

Setting the Serial Port Baud Rate

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.

Writing to the Serial Port

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 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, 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.

about 8051 architecture & pinout

The architecture of the 8051 family of microcontrollers is referred to as the MCS-51
architecture, or sometimes simply as MCS-51. The microcontrollers have an 8-bit
data bus. They are capable of addressing 64K of program memory and a separate
64K of data memory. The 8051 has 4K of code memory implemented as on-chip
Read Only Memory (ROM). The 8051 has 128 bytes of internal Random Access
Memory (RAM). The 8051 has two timer/counters, a serial port, 4 general purpose
parallel input/output ports, and interrupt control logic with five sources of interrupts.

Besides internal RAM, the 8051 has various Special Function Registers (SFR), which
are the control and data registers for on-chip facilities. The SFRs also include the
accumulator, the B register, and the Program Status Word (PSW), which contains the
CPU flags. Programming the various internal hardware facilities of the 8051 is
achieved by placing the appropriate control words into the corresponding SFRs. The
8031 is similar to the 8051, except it lacks the on-chip ROM.
As stated, the 8051 can address 64K of external data memory and 64K of external
program memory. These may be separate blocks of memory, so that up to 128K of
memory can be attached to the microcontroller. Separate blocks of code and data
memory are referred to as the Harvard architecture. The 8051 has two separate read
signals, RD# (P3.7) and PSEN#. The first is activated when a byte is to be read from
external data memory, the other, from external program memory. Both of these
signals are so-called active low signals. That is, they are cleared to logic level 0 when activated.
All external code is fetched from external program memory. In addition,
bytes from external program memory may be read by special read instructions such
as the MOVC instruction. There are separate instructions to read from external data
memory, such as the MOVX instruction. That is, the instructions determine which
block of memory is addressed, and the corresponding control signal, either RD# or
PSEN# is activated during the memory read cycle. A single block of memory may be
mapped to act as both data and program memory. This is referred to as the Von
Neumann1 architecture. In order to read from the same block using either the RD#
signal or the PSEN# signal, the two signals are combined with a logic AND operation.
This way, the output of the AND gate is low when either input is low. The advantage
of the Harvard architecture is not simply doubling the memory capacity of the
microcontroller. Separating program and data increases the reliability of the
microcontroller, since there are no instructions to write to the program memory. A
ROM device is ideally suited to serve as program memory. The Harvard architecture
is somewhat awkward in evaluation systems, where code needs to be loaded into
program memory. By adopting the Von Neumann architecture, code may be written
to memory as data bytes, and then executed as program instructions.
The 8052 has 256 bytes of internal RAM and 8K of internal code ROM. The 8051 and
8052 internal ROM cannot be programmed by the user. The user must supply the
program to the manufacturer, and the manufacturer programs the microcontrollers
during production. Due to the setup costs, the factory masked ROM option is not
economical for small quantity productions. The 8751 and 8752 are the Erasable
Programmable Read Only Memory (EPROM) versions of the 8051 and 8052. Many
manufacturers offer the EPROM versions in windowed ceramic and non-windowed
plastic packages. These are user programmable. However, the non-windowed
versions cannot be erased. These are usually referred to as One-Time-
Programmable (OTP) microcontrollers, which are more suitable for experimental work
or for small production runs. The 8951 and 8952 contain FLASH EEPROMs
(Electrically Erasable Programmable Read Only Memory). These chips can be
programmed as the EPROM versions, using a chip programmer. Moreover, the
memory may be erased. Similar to EPROMs, Erasing FLASH memory sets all data
bits (data bytes become FFh). A bit may be cleared (made 0) by programming.
However, a zero bit may not be programmed to a one. This requires erasing the chip.
Some larger FLASH memories are organized in banks or sectors. Rather than
erasing the entire chip, you may erase a given sector and keep the remaining sectors
unchanged.

During the past decade, many manufacturers introduced enhanced members of the
8051 microcontroller. The enhancements include more memory, more ports, analogto-
digital converters, more timers with compare, reload and capture facilities, more
interrupt sources, higher precision multiply and divide units, idle and power down
mode support, watchdog timers, and network communication subsystems. All
microcontroller of the family use the same set of machine instructions, the MCS-51.
The enhanced features are programmed and controlled by additional SFRs. In the
remainder of this chapter, the hardware architecture of the 8051 is presented. The
enhancements brought by the 8052 and 80C515 follow. Some of the more popular
enhanced members of the family are reviewed at the end of Chapter 2. The reader is
referred to the manufacturers' data books for the specifics of other enhanced
members.