XR16L2751CM EXAR [Exar Corporation], XR16L2751CM Datasheet - Page 14

• Manufacturer Part Number: XR16L2751CM
• Description: 2.25V TO 5.5V DUART WITH 64-BYTE FIFO AND POWERSAVE
• Manufacturer: EXAR [Exar Corporation]

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The receiver section contains an 8-bit Receive Shift Register (RSR) and 64 bytes of FIFO which includes a
byte-wide Receive Holding Register (RHR). The RSR uses the 16X/8X clock (EMSR bit-7) 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/8X clock rate. After 8 clocks (or 4 if 8X) 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 a logic
0 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.
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 64
bytes by 11-bits 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.
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F
2.12
2.12.1
IGURE
8. T
RECEIVER
Receive Holding Register (RHR) - Read-Only
RANSMITTER
Auto CTS Flow Control (CTS# pin)
16X or 8X Clock
(Xoff1/2 and Xon1/2 Reg.
Auto Software Flow Control
Flow Control Characters
(EMSR bit-7)
O
PERATION IN
Data Byte
Transmit
FIFO
2.25V TO 5.5V DUART WITH 64-BYTE FIFO AND POWERSAVE
AND
Transmit Data Shift Register
F
LOW
Transmit
FIFO
(TSR)
14
C
ONTROL
M
THR Interrupt (ISR bit-1) falls
below the programmed Trigger
Level and then when becomes
empty. FIFO is Enabled by FCR
bit-0=1
ODE
T XF IF O 1
XR16L2751
REV. 1.0.0