4 stop bits.The stop bit marks the end of a character transfer, typically a high level, one bit, one half or two bits.Microcontrollers often use a stop bit.The baud rate refers to the rate of serial communication data transmission. Since serial communication transmits data bit by bit, the number of bits per second is defined as the baud rate, that is, 1 baud rate = 1 bit / sec (1 bps).).Serial communication requires two devices for data transfer to have the same baud rate.At the beginning of each data block transfer, one or two sync characters are used as the start flag to keep both the sender and the sender synchronized.Synchronization characters can be selected by the user for a particular 8-bit binary code, but both parties must be consistent.Synchronous communication speed is higher than asynchronous communication, but the premise is that there must be an accurate clock to achieve strict synchronization between the two parties, so the hardware requirements are high.