xr16l2750im Exar Corporation, xr16l2750im Datasheet - Page 16

• Manufacturer Part Number: xr16l2750im
• Description: High-performance 2.25v - 5.5v Duart
• Manufacturer: Exar Corporation

XR16L2750
2.25V TO 5.5V DUART WITH 64-BYTE FIFO
When software flow control is enabled
characters with the programmed Xon or Xoff-1,2 character value(s). If receive character(s) (RX) match the
programmed values, the 2750 will halt transmission (TX) as soon as the current character has completed
transmission. When a match occurs, the Xoff (if enabled via IER bit-5) flag will be set and the interrupt output
pin will be activated. Following a suspension due to a match of the Xoff character, the 2750 will monitor the
receive data stream for a match to the Xon-1,2 character. If a match is found, the 2750 will resume operation
and clear the flags (ISR bit-4).
Reset initially sets the contents of the Xon/Xoff 8-bit flow control registers to a logic 0. Following reset the user
can write any Xon/Xoff value desired for software flow control. Different conditions can be set to detect Xon/
Xoff characters
selected, the 2750 compares two consecutive receive characters with two software flow control 8-bit values
(Xon1, Xon2, Xoff1, Xoff2) and controls TX transmissions accordingly. Under the above described flow control
mechanisms, flow control characters are not placed (stacked) in the user accessible RX data buffer or FIFO.
In the event that the receive buffer is overfilling and flow control needs to be executed, the 2750 automatically
sends an Xoff message (when enabled) via the serial TX output to the remote modem. The 2750 sends the
Xoff-1,2 characters two-character-times (= time taken to send two characters at the programmed baud rate)
after the receive FIFO crosses the programmed trigger level (for all trigger tables A-D). To clear this condition,
the 2750 will transmit the programmed Xon-1,2 characters as soon as receive FIFO is less than one trigger
level below the programmed trigger level (for Trigger Tables A, B, and C) or when receive FIFO is less than the
trigger level minus the hysteresis value (for Trigger Table D). This hysteresis value is the same as the Auto
RTS Hysteresis value in
* After the trigger level is reached, an xoff character is sent after a short span of time (= time required to send 2 characters);
for example, after 2.083ms has elapsed for 9600 baud and 10-bit word length setting.
A special character detect feature is provided to detect an 8-bit character when bit-5 is set in the Enhanced
Feature Register (EFR). When this character (Xoff2) is detected, it will be placed in the FIFO along with normal
incoming RX data.
The 2750 compares each incoming receive character with Xoff-2 data. If a match exists, the received data will
be transferred to FIFO and ISR bit-4 will be set to indicate detection of special character. Although the Internal
Register Table shows Xon, Xoff Registers with eight bits of character information, the actual number of bits is
dependent on the programmed word length. Line Control Register (LCR) bits 0-1 defines the number of
character bits, i.e., either 5 bits, 6 bits, 7 bits, or 8 bits. The word length selected by LCR bits 0-1 also
determines the number of bits that will be used for the special character comparison. Bit-0 in the Xon, Xoff
Registers corresponds with the LSB bit for the receive character.
The auto RS485 half-duplex direction control changes the behavior of the transmitter when enabled by FCTR
bit-3. By default, it de-asserts RTS# (HIGH) output following the last stop bit of the last character that has been
transmitted. This helps in turning around the transceiver to receive the remote station’s response. When the
host is ready to transmit next polling data packet again, it only has to load data bytes to the transmit FIFO. The
2.16
2.17
2.18
RX T
Auto Xon/Xoff (Software) Flow Control
RIGGER
Special Character Detect
Auto RS485 Half-duplex Control
16
24
28
8
L
(See Table
EVEL
INT P
Table
15) and suspend/resume transmissions. When double 8-bit Xon/Xoff characters are
T
IN
ABLE
14.
A
16
24
28
8
CTIVATION
Table 6
6: A
UTO
(See Table
below explains this when Trigger Table-B (See
X
ON
X
/X
(
OFF
CHARACTERS IN RX FIFO
OFF
15), the 2750 compares one or two sequential receive data
C
16
(S
HARACTER
OFTWARE
16*
24*
28*
8*
(
S
) S
) F
ENT
LOW
)
C
ONTROL
X
(
CHARACTERS IN RX FIFO
ON
C
HARACTER
Table
16
24
0
8
xr
10) is selected.
(
S
) S
ENT
REV. 1.2.1
)