M451
May. 4, 2018
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UART Auto-Flow Control Function
The UART supports auto-flow control function that uses two signals, nCTS (clear-to-send) and
nRTS (request-to-send), to control the flow of data transfer between the UART and external
devices (e.g. Modem). When auto-flow is enabled, the UART is not allowed to receive data until
the UART asserts nRTS to external device. When the number of bytes stored in the RX FIFO
equals the value of RTSTRGLV (UART_FIFO [19:16]), the nRTS is de-asserted. The UART
sends data out when UART detects nCTS is asserted from external device. If the valid asserted
nCTS is not detected, the UART will not send data out.
APB BUS
RX FIFO
TX FIFO
Flow Control
Serial to Parallel
Serial to Parallel
UART_nCTS
UART_RX
UART_nRTS
UART_TX
Figure 6.13-8 Auto-Flow Control Block Diagram
The Figure 6.13-9 demonstrates the nCTS auto-flow control of UART function mode. User must
set ATOCTSEN (UART_INTEN [13]) to enable nCTS auto-flow control function. The CTSACTLV
(UART_MODEMSTS [8]) can set nCTS pin input active state. The CTSDETF
(UART_MODEMSTS[0]) is set when any state change of nCTS pin input has occurred, and then
TX data will be automatically transmitted from TX FIFO.
