One of the microcontroller features making it so powerful is an integrated UART, better known as a serial port. It is a full-duplex port, thus being able to transmit and receive data simultaneously and at different baud rates. Without it, serial data send and receive would be an enormously complicated part of the program in which the pin state is constantly changed and checked at regular intervals. When using UART, all the programmer has to do is to simply select serial port mode and baud rate. When it's done, serial data transmit is nothing but writing to the SBUF register, while data receive represents reading the same register. The microcontroller takes care of not making any error during data transmission.

Serial port must be configured prior to being used. In other words, it is necessary to determine how many bits is contained in one serial “word”, baud rate and synchronization clock source. The whole process is in control of the bits of the SCON register (Serial Control).

Serial Port Control (SCON) Register

• SM0 - Serial port mode bit 0 is used for serial port mode selection.
• SM1 - Serial port mode bit 1.
• SM2 - Serial port mode 2 bit, also known as multiprocessor communication enable bit. When set, it enables multiprocessor communication in mode 2 and 3, and eventually mode 1. It should be cleared in mode 0.
• REN - Reception Enable bit enables serial reception when set. When cleared, serial reception is disabled.
• TB8 - Transmitter bit 8. Since all registers are 8-bit wide, this bit solves the problem of transmiting the 9th bit in modes 2 and 3. It is set to transmit a logic 1 in the 9th bit.
• RB8 - Receiver bit 8 or the 9th bit received in modes 2 and 3. Cleared by hardware if 9th bit received is a logic 0. Set by hardware if 9th bit received is a logic 1.
• TI - Transmit Interrupt flag is automatically set at the moment the last bit of one byte is sent. It's a signal to the processor that the line is available for a new byte transmite. It must be cleared from within the software.
• RI - Receive Interrupt flag is automatically set upon one byte receive. It signals that byte is received and should be read quickly prior to being replaced by a new data. This bit is also cleared from within the software.

As seen, serial port mode is selected by combining the SM0 and SM2 bits:

SM0	SM1	MODE	DESCRIPTION	BAUD RATE
0	0	0	8-bit Shift Register	1/12 the quartz frequency
0	1	1	8-bit UART	Determined by the timer 1
1	0	2	9-bit UART	1/32 the quartz frequency (1/64 the quartz frequency)
1	1	3	9-bit UART	Determined by the timer 1

TRANSMIT - Data transmit is initiated by writing data to the SBUF register. In fact, this process starts after any instruction being performed upon this register. When all 8 bits have been sent, the TI bit of the SCON register is automatically set. RECEIVE - Data receive through the RXD pin starts upon the two following conditions are met: bit REN=1 and RI=0 (both of them are stored in the SCON register). When all 8 bits have been received, the RI bit of the SCON register is automatically set indicating that one byte receive is complete. Since there are no START and STOP bits or any other bit except data sent from the SBUF register in the pulse sequence, this mode is mainly used when the distance between devices is short, noise is minimized and operating speed is of importance. A typical example is I/O port expansion by adding a cheap IC (shift registers 74HC595, 74HC597 and similar).

Mode 1

TRANSMIT - Data transmit is initiated by writing data to the SBUF register. End of data transmission is indicated by setting the TI bit of the SCON register. RECEIVE - The START bit (logic zero (0)) on the RXD pin initiates data receive. The following two conditions must be met: bit REN=1 and bit RI=0. Both of them are stored in the SCON register. The RI bit is automatically set upon data reception is complete. The Baud rate in this mode is determined by the timer 1 overflow.

Mode 2

TRANSMIT - Data transmit is initiated by writing data to the SBUF register. End of data transmission is indicated by setting the TI bit of the SCON register. RECEIVE - The START bit (logic zero (0)) on the RXD pin initiates data receive. The following two conditions must be met: bit REN=1 and bit RI=0. Both of them are stored in the SCON register. The RI bit is automatically set upon data reception is complete.

Mode 3

Mode 3 is the same as Mode 2 in all respects except the baud rate. The baud rate in Mode 3 is variable.  

Baud Rate

Baud Rate is a number of sent/received bits per second. In case the UART is used, baud rate depends on: selected mode, oscillator frequency and in some cases on the state of the SMOD bit of the SCON register. All the necessary formulas are specified in the table:

	BAUD RATE	BITSMOD
Mode 0	Fosc. / 12
Mode 1	1 Fosc. 16 12 (256-TH1)	BitSMOD
Mode 2	Fosc. / 32 Fosc. / 64	1 0
Mode 3	1 Fosc. 16 12 (256-TH1)

Timer 1 as a clock generator

Timer 1 is usually used as a clock generator as it enables various baud rates to be easily set. The whole procedure is simple and is as follows:

• First, enable Timer 1 overflow interrupt.
• Configure Timer T1 to operate in auto-reload mode.
• Depending on needs, select one of the standard values from the table and write it to the TH1 register. That's all.

BAUD RATE	FOSC. (MHZ)					BIT SMOD
	11.0592	12	14.7456	16	20
150	40 h	30 h	00 h			0
300	A0 h	98 h	80 h	75 h	52 h	0
600	D0 h	CC h	C0 h	BB h	A9 h	0
1200	E8 h	E6 h	E0 h	DE h	D5 h	0
2400	F4 h	F3 h	F0 h	EF h	EA h	0
4800		F3 h	EF h	EF h		1
4800	FA h		F8 h		F5 h	0
9600	FD h		FC h			0
9600					F5 h	1
19200	FD h		FC h			1
38400			FE h			1
76800			FF h			1

Multiprocessor Communication

As you may know, additional 9th data bit is a part of message in mode 2 and 3. It can be used for checking data via parity bit. Another useful application of this bit is in communication between two or more microcontrollers, i.e. multiprocessor communication. This feature is enabled by setting the SM2 bit of the SCON register. As a result, after receiving the STOP bit, indicating end of the message, the serial port interrupt will be generated only if the bit RB8 = 1 (the 9th bit). This is how it looks like in practice: Suppose there are several microcontrollers sharing the same interface. Each of them has its own address. An address byte differs from a data byte because it has the 9th bit set (1), while this bit is cleared (0) in a data byte. When the microcontroller A (master) 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 will generate an interrupt in all slaves so that they can examine the received byte and check whether it matches their address. Of course, only one of them will match the address and immediately clear the SM2 bit of the SCON register and prepare to receive the data byte to come. Other slaves not being addressed leave their SM2 bit set ignoring the coming data bytes.