XR21B1420IL28-0A Datasheet, PDF (13/60 Pages) Exar Corporation – Enhanced 1-Ch Full-Speed USB UART

XR21B1420
Programmable Output Clock
The GPIO6/CLK pin may be enabled as a clock output using the GPIO_MODE register. The OUTCLK register can be used
to program the output frequency of the clock from 24 MHz down to approximately 47 KHz. The duty cycle can also be pro-
grammed from 50/50 to a single low or high going pulse. The default values of zero for both DIV_HI and DIV_LO in the
OUTCLK register will result in a frequency of 24 MHz. For any non-zero values for DIV_HI and DIV_LO, the clock frequency
is determined by the formula:
FREQ = 24 MHz / (DIV_HI + DIV_LO). The duty cycle is determined by the ratio of DIV_HI to DIV_LO.
Flow Control
The XR21B1420 is able to perform both hardware and software flow control. Both hardware and software flow control
modes are configured via the GPIO_MODE and FLOW_CONTROL registers. In both modes, flow control is asserted when
the bytes in the RX FIFO reach the watermark set in the RX_THRESHOLD register.
Hardware flow control can either be RTS/CTS or DTR/DSR controlled. Note that although the default pin configuration for
GPIO5/RTS#/RS485 and GPIO4/CTS# are for RTS output and CTS input respectively, the hardware RTS/CTS flow control
mode must be set in the FLOW_CONTROL register in order to utilize the flow control functionality.
Automatic RTS/CTS Hardware Flow Control
Automatic RTS flow control is used to prevent data overrun errors in the local RX FIFO using the RTS signal to the remote
UART. The RTS signal will be asserted (low) when there are less than 450 bytes in the receive FIFO. When the RX FIFO
reaches the 450 byte threshold, the RTS pin will be de-asserted. The CTS# input is monitored by the remote UART to sus-
pend/restart the local transmitter. Refer to Figure 6. Conversely, when the remote UART reaches its receive FIFO threshold,
its RTS will be de-asserted, and the XR21B1420 CTS input will cause the device to suspend data transmission.
Local UART
UARTA
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
Transmitter
Auto CTS
Monitor
RXA
RTSA#
TXA
CTSA#
RTSA#
1
ON
6
OFF
TXB
CTSB#
RXB
RTSB#
9
Remote UART
UARTB
Transmitter
Auto CTS
Monitor
Receiver FIFO
Trigger Reached
Auto RTS
Trigger Level
ON
CTSB#
2
ON
7
OFF
10
ON
3
TXB
4
RXA
8
11
5
1) COM port opened, RX FIFO empty, RTSA# output is asserted
2) Signal propagated to CTSB# input
3) Data bytes enter TX FIFO, begin transmitting on TXB
4) Data propagates to Receiving device RXA
5) RX FIFO reaches threshold
6) RTSA# de-asserts
7) Signal propagates to CTSB# input
8) Transmission stops on TXB
9) USB Bulk-In empties RX FIFO below threshold, RTSA# is asserted
10) Signal propagated to CTSB# input
11) Data bytes resume transmitting on TXB
© 2014 Exar Corporation
13 / 60
exar.com/XR21B1420
Rev 1A