xr20m1280 Exar Corporation, xr20m1280 Datasheet - Page 22

• Manufacturer Part Number: xr20m1280
• Description: I2c/spi Uart With 128-byte Fifo And Integrated Level Shifters
• Manufacturer: Exar Corporation

XR20M1280
I2C/SPI UART WITH 128-BYTE FIFO AND INTEGRATED LEVEL SHIFTERS
When software flow control is enabled
data characters with the programmed Xon or Xoff-1,2 character value(s). If receive character(s) (RX) match the
programmed values, the M1280 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 M1280 will monitor the
receive data stream for a match to the Xon-1,2 character. If a match is found, the M1280 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
are selected, the M1280 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 in the RX FIFO.
In the event that the receive buffer is overfilling and flow control needs to be executed, the M1280
automatically sends an Xoff message (when enabled) via the serial TX output to the remote modem. The
M1280 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. To clear this condition,
the M1280 will transmit the programmed Xon-1,2 characters as soon as receive FIFO is less than one trigger
level below the programmed trigger level.
* 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 M1280 compares each incoming receive character with Xoff-2 data. If a match exists, the received data
will be transferred to the RX 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 feature can be enabled by FCTR bit [3]. The RTS# pin becomes
the half-duplex control output when this feature has been enabled. The RTS# pin is typically connected to both
the Driver Enable (DE) and Receiver Enable (RE) of an RS-485 transceiver. When the Transmitter is idle, the
RTS# pin is de-asserted so that the RS-485 driver is disabled and the RS-485 receiver is enabled. When data
is loaded into the TX FIFO, the RTS# pin is asserted to enable the RS-485 driver and disable the RS-485
receiver. This changes the transmitter empty interrupt to TSR empty instead of THR empty.
1.14
1.15
1.16
RX T
RIGGER
Auto Xon/Xoff (Software) Flow Control
Auto RS485 Half-Duplex Control Operation
Special Character Detect
16
56
60
8
L
EVEL
(See Table
IRQ# P
T
20) and suspend/resume transmissions. When double 8-bit Xon/Xoff characters
IN
ABLE
16
56
60
A
8
CTIVATION
8: A
UTO
(See Table
Table 8
X
ON
PRELIMINARY
/X
X
(
OFF
CHARACTERS IN RX FIFO
OFF
below explains this when Trigger Table-C is selected.
20), the M1280 compares one or two sequential receive
C
22
(S
HARACTER
OFTWARE
16*
56*
60*
8*
(
S
) S
) F
ENT
LOW
)
C
ONTROL
X
(
CHARACTERS IN RX FIFO
ON
C
HARACTER
16
56
0
8
(
S
) S
REV. P1.1.1
ENT
)