The basics

There are two ways to connect a microcontroller to a computer.

• Using RS232 protocol to the legacy serial port.
• Using a USB to serial convertor.

From microcontroller programming point of view, there is fundamentally no difference. The communication between the computer and micrcontroller is asynchronous and full duplex.

• Asynchronous: There is no fixed clock to synchronize transmission of data; rather a fix bit rate is specified at both transmitter and receiver, termed as Baud Rate. In simple words, it specifies number of bit received/transmitted in 1 second. With that transmitter and receiver get to know time duration for each bit transfer.
• Full DuplexBoth computer and micrcontroller have the hardware capability to transmit and receive at the same time.

The protocol

RS232:It is a legacy standard for serial communication. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pin-out of connectors.

RS232 Signals

There are various signal for handshaking and communication in RS232 protocol. These are hardly used with modern computers, hence we will see Transmit(Tx) and Receive(Rx) Signals only.

Voltage Levels

Table below shows the voltage levels for RS232. They are significantly different from the TTL high(5v) and Low(0v). Hence a line driver like the MAX232 is required as shown in the schematic.

• SBUFF, it is a serial buffer for data. It holds 8 bit data that needs to be transmitted or the data that is received. Since the serial port of 8051 is full duplex, the microcontroller internally manages to transmit and receive data into the buffer register simultaneously.

• 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 transmitting 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 transmit. 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.

The Baud Rate

As discussed earlier, both transmitter and receiver should agree upon a fixed data transfer rate known as baud rate. For the 8051 the Timer 1' is used to generate the baud rate in Auto reload mode. The crystal frequency Fclk is divided by 12 internally which is used to execute instructions also known as Machine Clock. Mclk. The timer again divides the Mclk by 32 and uses it as timer frequecny, say Tclk.

Usually a 11.0592 Mhz crystal oscillator is used to provide clock to 8051. The value seems to be odd but we see how it makes sense. From above discussion

Mclk = Fclk / 12 = 11.0592MHz/12 = 921.6KHz

Tclk = Mclk/32= 921.6KHz/32 = 28.8KHz <br\>

If we look at the standard baud rates used for serial communication shown in the table below, we can observe that all the baud rates are factors or multiples of the Tclk (28.8K)! This results in low error rates and hence is a commonly used crystal with 8051.