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Rev. 1.00
472 of 576
January 28, 2022
32-Bit Arm
®
Cortex
®
-M0+ MCU
HT32F54231/HT32F54241/HT32F54243/HT32F54253
22 Universal Synchronous
Asynchronous Receiver T
ransmitter (USART)
Hardware Flow Control
The USART supports the hardware flow control function which is enabled by setting the HFCEN
bit in the USRCR register to 1. It is possible to control the serial data flow between two USART
devices by using the CTS input and the RTS output. The The following figure shows the connection
diagram in this mode. The hardware flow control function is categorized into two types. One is the
RTS flow control function and the other is the CTS flow control function.
TX
RX
USART 1
Transmitter
Receiver
USART 2
Transmitter
Receiver
TX
RX
RTS
CTS
RTS
CTS
Figure 172. Hardware Flow Control between 2 USARTs
RTS Flow Control
In the RTS flow control, the USART RTS pin is active with a logic low state when the receive data
register is empty. It means that the receiver is ready to receive a new data. When the RX FIFO
reaches the trigger level which is specified by configuring the RXTL field in the USRFCR register,
the USART RTS pin is inactive with a logic high state. Figure 215 shows the example of RTS flow
control.
Start Bit
Bit 0 Bit 1 Bit 2 Bit 3 Bit 4
Bit N
Parity Bit
Stop Bit
Start Bit
N = 7 ~ 8
Idle
Bit 0 Bit 1 Bit 2 Bit 3
Bit 4
Bit N
Parity Bit
Stop Bit
N = 7 ~ 8
RXFS[3:0]
3
4
0
1
RTS
Read data until RX FIFO is empty
Reach the RX trigger level
RXTL[1:0] = b10
Figure 173. USART RTS Flow Control
CTS Flow Control
If the hardware flow control function is enabled, the URTXEN bit in the USRCR register is
controlled by the USART CTS input signal. If the USART CTS pin is forced to a logic low state,
the URTXEN bit will automatically be set to 1 to enable the data transmission. However, if the
USART CTS pin is forced to a logic high state, the URTXEN bit will be cleared to 0 and then the
data transmission will also be disabled.