Factors that determine data transfer speed

　・In Japanese
前提知識
　・Serial communication
　・Termination resistor
　・Shift register

■Factors that determine data transfer speed in serial communication

Serial communication sends data bit by bit using a shift register. Therefore, it is transmitted at the timing when the clock for shift register operation is generated. One of the factors that determine the data transfer speed is the clock frequency.

For example, if the clock frequency is 1MHz, it takes 1µs to clock 1bit. The amount of data that can be transferred in one second is called the baud rate, which is expressed in bps (bits per second). The upper limit of the baud rate is 1Mbps when the clock frequency is 1MHz.



However, the original data itself cannot be transferred at the same speed as the clock frequency. This is because information for synchronizing communication, headers, checksums, etc. are added to the original data, and there are restrictions as explained below.

■Transfer speed restrictions

Theoretically, the transfer speed can be increased by increasing the clock frequency, but in reality it is not so easy. First, a highly accurate clock oscillator is required. In addition, in the case of the asynchronous system, the receiving side requires a clock with a frequency several times higher than that of the transmitting side in order to prevent data loss.

Also, increasing the clock frequency makes it more susceptible to ringing (vibration waveform generated by signal reflection at the end of the transmission line), so it is necessary to design the circuit so that ringing does not occur. (For example, install a terminating resistor or adjust the circuit length to prevent resonance in the target frequency band.)



In addition, if there is noise on the transmission line (such as the effect of the magnetic field generated by the current flowing in the transmission line), it is necessary to take measures such as using a shielded wire. For example, in Ethernet, the communication speed is determined according to the detailed cable standards.

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