XR21B1411IL-0A-EB Datasheet, PDF (8/30 Pages) Exar Corporation – ENHANCED 1-CH FULL-SPEED USB UART

XR21B1411
ENHANCED 1-CH FULL-SPEED USB UART
REV. 1.2.0
1.3.5.1
Automatic RTS/CTS Hardware Flow Control
GPIO5 and GPIO4 of the UART channel can be enabled as the RTS# and CTS# signals for Auto RTS/CTS
flow control when GPIO_MODE[2:0] = ’001’. Automatic RTS flow control is used to prevent data overrun errors
in local RX FIFO by de-asserting the RTS signal to the remote UART. When there is room in the RX FIFO, the
RTS pin will be re-asserted. Automatic CTS flow control is used to prevent data overrun to the remote RX
FIFO. The CTS# input is monitored to suspend/restart the local transmitter (refer to Figure 5):
FIGURE 5. AUTO RTS AND CTS FLOW CONTROL OPERATION
RTSA#
CTSB#
TXB
RXA
Local UART
UARTA
R e c e iv e r F IF O
T rig g e r R e a c h e d
A u to R T S
T rig g e r L e v e l
T ra n s m itte r
A u to C T S
M o n ito r
RXA
R TS A #
TXA
CTSA#
1
2
3
4
ON
ON
6
7
8
5
O FF
O FF
TXB
CTSB#
RXB
RTSB#
9
10
11
R e m o te U A R T
UARTB
T ra n s m itte r
A u to C T S
M o n ito r
R e c e iv e r F IF O
T rig g e r R e a c h e d
A u to R T S
T rig g e r L e v e l
ON
ON
1 ) C O M p o rt o p e n e d , R X F IF O e m p ty , R T S A # o u tp u t is a s s e rte d
2 ) S ig n a l p ro p a g a te d to C T S B # in p u t
3 ) D a ta b y te s e n te r T X F IF O , b e g in tra n s m ittin g o n T X B
4 ) D a ta p ro p a g a te s to R e c e iv in g d e v ic e R X A
5 ) R X F IF O re a c h e s th re s h o ld
6 ) R T S A # d e -a s s e rts
7 ) S ig n a l p ro p a g a te s to C T S B # in p u t
8 ) T ra n s m is s io n s to p s o n T X B
9 ) U S B B u lk -In e m p tie s R X F IF O b e lo w th re s h o ld , R T S A # is a s s e rte d
1 0 ) S ig n a l p ro p a g a te d to C T S B # in p u t
1 1 ) D a ta b y te s re s u m e tra n s m ittin g o n T X B
1.3.5.2
Automatic DTR/DSR Hardware Flow Control
Auto DTR/DSR hardware flow control behaves the same as the Auto RTS/CTS hardware flow control
described above except that it uses the DTR# and DSR# signals. GPIO3 and GPIO2 become DTR# and
DSR#, respectively, when GPIO_MODE[2:0] = ’010’ and FLOW_CONTROL[2:0] = ’001’.
1.3.5.3
Automatic XON/XOFF Software Flow Control
When software flow control is enabled, the B1411 compares the receive data characters with the programmed
Xon or Xoff characters. If the received character matches the programmed Xoff character, the B1411 will halt
transmission as soon as the current character has completed transmission. Data transmission is resumed
when a received character matches the Xon character. Software flow control is enabled when
FLOW_CONTROL[2:0] = ’010’.
1.3.6 Auto RS-485 Half-Duplex Control
The Auto RS-485 Half-Duplex Control feature changes the behavior of the GPIO5/RTS#/RS485 pin when
enabled by the GPIO_MODE register bits 2-0. See ”Section 3.1.1.13, GPIO_MODE Register Description
(Read / Write)” on page 18. The FLOW_CONTROL register must also be set appropriately for use in
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