1. profile
UART hardware is configured as DTE device:
the RTS(Request-to-Send) signal as an output
the CTS(Clear-to-Send) signal is an input
2. one transimit
The active-low RTS and CTS signals work together to control transmission flow.
Hardware flow is typically controlled by the DTE device, which could be considered a ‘master’ device. In the case of a DTE-to-DTE configuration, either device can act as a master.
发送方DTE1,通过自己的RTS引脚，发出发送请求给DTE2：
When one DTE device wishes to transmit data, the DTE device pulls the RTS line low, which signals the slave device, through its CTS input, to begin to monitor its RX input.
接收方RTE2拉低自己的RTS引脚，同意接收数据，于是发送方DTE1开始发送数据给DTE2：
When the slave device is ready to accept the data, it pulls its RTS line low, informing the master, through its CTS line, to begin sending data.
Once the transaction has completed, the master device pulls the RTS line high.
plus:
姑且这么理解：
DTE 数字终端设备，一般指电脑
DCE 数字通讯设备，一般指调制解调器