USB to serial port - maximum peak serial port baud rate vs maximum continuous communication baud rate

Serial port baud rate is also called bit rate , The common unit of expression is bps（ Bits per second ,1 Second transmissible bit total ）. With 9600 Take baud rate as an example ：1 Second serial port signal line can transmit 9600 individual bit position ,1 The time required for a bit is 1/9600 About seconds 104us. As shown in the figure below ：

USB Serial equipment , It usually supports various standard and non-standard serial port baud rates , from USB Turn the internal or external clock source of the serial port chip to obtain the internal serial port clock first through frequency doubling or frequency division , Then the baud rate of the target serial port is obtained by the internal serial port clock frequency division . according to USB The speed is different ,USB The serial port can be divided into full speed USB To serial port and high speed USB To serial port . Full speed USB The physical layer is 12Mbps, High speed USB The physical layer is 480Mbps. For conventional low baud rate communications , Use full speed USB The serial port can meet the application needs , For high baud rate serial communication and multi serial communication , Use high speed USB The effect of transferring to serial port is better .

Maximum peak serial baud rate Refer to USB To serial chip The highest physical serial port baud rate supported , The total bandwidth of two-way serial communication will not exceed USB Bus rate of , So at full speed USB Take serial port chip as an example ,USB The full speed physical layer is 12Mbps Half duplex , in consideration of USB Agreement and CRC Equal cost , Send and receive two-way use 6Mbps And above speed serial port communication , Serial communication must be discontinuous transmission . There will be time intervals between packets . In order to realize continuous transmission without interval , High speed available USB To serial chip .

Maximum baud rate for continuous communication Refer to USB To serial port chip serial port implementation The highest serial port baud rate supported for continuous non interval transmission , Different from the maximum peak serial port baud rate , This parameter emphasizes the continuity of serial port data communication , For serial port transmission , There is no time interval between the stop bit of serial port data and the start bit of the next serial port data . For serial port reception , It can continuously sample and receive serial port data without word interval , Cache on chip receive FIFO in , Then wait for uploading to USB host . This interval can be obtained by oscilloscope and logic analyzer , Without the aid of hardware instruments , Also can be USB Transfer to serial port equipment for big data or big file transmission test , Get the serial port sending and receiving speed .

In short , If there are speed and time requirements for large amount of data serial communication , It is necessary to synchronously evaluate the maximum peak serial port baud rate supported by the chip and the maximum baud rate of continuous communication .