XR16M890
REV. 1.0.0
UART WITH 128-BYTE FIFO AND INTEGRATED LEVEL SHIFTERS
1.13
Auto CTS Flow Control
Automatic CTS flow control is used to prevent data overrun to the remote receiver FIFO. The CTS# input is
monitored to suspend/restart the local transmitter. The auto CTS flow control feature is selected to fit specific
application requirement (see Figure 15 ):
? Enable auto CTS flow control using EFR bit-7.
If needed, the CTS interrupt can be enabled through IER bit-7 (after setting EFR bit-4). The UART issues an
interrupt when the CTS# pin is de-asserted (HIGH): ISR bit-5 will be set to 1, and UART will suspend
transmission as soon as the stop bit of the character in process is shifted out. Transmission is resumed after
the CTS# input is re-asserted (LOW), indicating more data may be sent.
F IGURE 15. A UTO RTS AND CTS F LOW C ONTROL O PERATION
Local UART
UARTA
Remote UART
UARTB
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
Transmitter
Auto CTS
Monitor
RXA
RTSA#
TXA
CTSA#
TXB
CTSB#
RXB
RTSB#
Transmitter
Auto CTS
Monitor
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
Assert RTS# to Begin
RTSA#
1
Transmission
ON
OFF
10
ON
2
7
CTSB#
3
ON
8
OFF
11
ON
TXB
Data Starts
4
6
Suspend
Restart
9
RXA FIFO
INTA
(RXA FIFO
Interrupt)
Receive
Data
RX FIFO
Trigger Level
5
RTS High
Threshold
RTS Low
Threshold
12
RX FIFO
Trigger Level
RTSCTS1
The local UART (UARTA) starts data transfer by asserting RTSA# (1). RTSA# is normally connected to CTSB# (2) of
remote UART (UARTB). CTSB# allows its transmitter to send data (3). TXB data arrives and fills UARTA receive FIFO
(4). When RXA data fills up to its receive FIFO trigger level, UARTA activates its RXA data ready interrupt (5) and con-
tinues to receive and put data into its FIFO. If interrupt service latency is long and data is not being unloaded, UARTA
monitors its receive data fill level to match the upper threshold of RTS delay and de-assert RTSA# (6). CTSB# follows
(7) and request UARTB transmitter to suspend data transfer. UARTB stops or finishes sending the data bits in its trans-
mit shift register (8). When receive FIFO data in UARTA is unloaded to match the lower threshold of RTS delay (9),
UARTA re-asserts RTSA# (10), CTSB# recognizes the change (11) and restarts its transmitter and data flow again until
next receive FIFO trigger (12). This same event applies to the reverse direction when UARTA sends data to UARTB
with RTSB# and CTSA# controlling the data flow.
21
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