Serial Communication of 8051 Microcontroller

There are two ways of computer data exchange: serial communication and parallel communication.

Parallel communication: Let's take 8 bits (one byte) of data as an example. Under parallel communication, 8 bits of data are transmitted at the same time, sent and received at the same time. Therefore, both parties in communication need a common clock signal. Parallel transmission is fast and efficient. However, it requires more data lines and a shorter transmission distance.

Serial communication: Still taking 8-bit data as an example, each bit of data is sent in time-sharing, and the receiver also receives it one bit at a time. Serial communication only requires one data line, which is low-cost. It is suitable for large-scale and long-distance data transmission.

Asynchronous transfer mode: Each transmitted character is wrapped with a start bit and a stop bit, and gaps of varying lengths are allowed between characters.

Synchronous transmission mode: A data block contains many consecutive characters, with no idle time between characters. Synchronous transmission can easily implement the format required by a certain communication protocol.

Baud rate: Here it is defined as the number of "bits" transmitted per second in serial communication. That is, n baud = nb/s. In the RS-232C standard, the baud rate of serial communication is specified as follows: 600, 1200, 2400, 4800, 9600, etc.

Calculation of the baud rate of the serial port:

In mode 0, the baud rate is fixed at: F/12 (F is the crystal frequency)

In mode 2, the baud rate is fixed to: F/32 or F/64 (depending on the SMOD bit of the PCON register, if SMOD = 1, it is F/32; otherwise it is F/64).

Mode 1 and Mode 3: The overflow rate of the T1 timer is used as the baud rate generator, so the calculation formula is:

                                K*F/[32*12*(2^n - X)].

If SMOD = 1, then K = 2, otherwise K = 1, F is the crystal frequency, n is the number of bits in the timer (that is, which working mode the timer is in), and X is the initial value filled by the timer.

In serial communication, both parties are required to have the same baud rate, but sometimes we cannot reach that baud rate accurately. At this time, we need to consider which of the two cases, SMOD = 1 and SMOD = 0, produces a smaller relative error, and we will choose that one.

Relative error = (actual value - theoretical value) / theoretical value * 100%