STM32W108HB datasheet(121/278 Pages) STMICROELECTRONICS

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STM32W108HB STM32W108CC STM32W108CB STM32W108CZ

Serial interfaces

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9.6.3

RTS/CTS flow control

RTS/CTS flow control, also called hardware flow control, uses two signals (nRTS and nCTS)

in addition to received and transmitted data (see Figure 14). Flow control is used by a data

receiver to prevent buffer overflow, by signaling an external device when it is and is not

allowed to transmit.

Figure 14.

RTS/CTS flow control connections

The UART RTS/CTS flow control options are selected by the HFCE and AHFCE bits in the

SC1_UARTCR register (see Table 30). Whenever the HFCE bit is set, the UART will not

start transmitting a character unless nCTS is low (asserted). If nCTS transitions to the high

state (deasserts) while a character is being transmitted, transmission of that character

continues until it is complete.

If the AHFCE bit is set, nRTS is controlled automatically by hardware: nRTS is put into the

low state (asserted) when the receive FIFO has room for at least two characters, otherwise

is it in the high state (unasserted). If AHFCE is clear, software controls the nRTS output by

setting or clearing the nRTS bit int the SC1_UARTCR register. Software control of nRTS is

useful if the external serial device cannot stop transmitting characters promptly when nRTS

is set to the high state (deasserted).

No RTS/CTS flow control

Flow control using RTS/CTS with software control of nRTS:

nRTS controlled by nRTS bit in SC1_UARTCR register

Flow control using RTS/CTS with hardware control of nRTS:

nRTS is asserted if room for at least 2 characters in receive FIFO

Other Device

UART Receiver

UART Transmitter

STM32W108

UART Transmitter

UART Receiver

RXD

TXD

nRTS

nCTS

TXD

RXD

nCTS

nRTS

Table 30.

UART RTS/CTS flow control configurations

SC1_UARTCR

Pins used

Operating mode

SC1_UARTxxx(1)

HFCE

AHFCE nRTS

TXD, RXD

0/1

TXD, RXD,

nCTS, nRTS

TXD, RXD,

nCTS, nRTS

The notation xxx means that the corresponding column header below is inserted to form the field name.

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