XR16M554/554D
16
1.62V TO 3.63V QUAD UART WITH 16-BYTE FIFO
REV. 1.0.0
2.10
Receiver
The receiver section contains an 8-bit Receive Shift Register (RSR) and 16 bytes of FIFO which includes a
byte-wide Receive Holding Register (RHR). The RSR uses the 16X clock for timing. It verifies and validates
every bit on the incoming character in the middle of each data bit. On the falling edge of a start or false start bit,
an internal receiver counter starts counting at the 16X clock rate. After 8 clocks the start bit period should be at
the center of the start bit. At this time the start bit is sampled and if it is still LOW it is validated. Evaluating the
start bit in this manner prevents the receiver from assembling a false character. The rest of the data bits and
stop bits are sampled and validated in this same manner to prevent false framing. If there were any error(s),
they are reported in the LSR register bits 2-4. Upon unloading the receive data byte from RHR, the receive
FIFO pointer is bumped and the error tags are immediately updated to reflect the status of the data byte in
RHR register. RHR can generate a receive data ready interrupt upon receiving a character or delay until it
reaches the FIFO trigger level. Furthermore, data delivery to the host is guaranteed by a receive data ready
time-out interrupt when data is not received for 4 word lengths as defined by LCR[1:0] plus 12 bits time. This is
equivalent to 3.7-4.6 character times. The RHR interrupt is enabled by IER bit-0. See Figure 9 and Figure 10 below.
2.10.1
Receive Holding Register (RHR) - Read-Only
The Receive Holding Register is an 8-bit register that holds a receive data byte from the Receive Shift
Register. It provides the receive data interface to the host processor. The RHR register is part of the receive
FIFO of 16 bytes by 11-bit wide, the 3 extra bits are for the 3 error tags to be reported in LSR register. When
the FIFO is enabled by FCR bit-0, the RHR contains the first data character received by the FIFO. After the
RHR is read, the next character byte is loaded into the RHR and the errors associated with the current data
byte are immediately updated in the LSR bits 2-4.
FIGURE 9. RECEIVER OPERATION IN NON-FIFO MODE
Receive Data Shift
Register (RSR)
Receive
Data Byte
and Errors
RHR Interrupt (ISR bit-2)
Receive Data
Holding Register
(RHR)
RXFIFO1
16X Clock
Receive Data Characters
Data Bit
Validation
Error
Tags in
LSR bits
4:2