XR16L784_08 Datasheet, PDF (14/51 Pages) Exar Corporation – HIGH PERFORMANCE 2.97V TO 5.5V QUAD UART

XR16L784
HIGH PERFORMANCE 2.97V TO 5.5V QUAD UART
REV. 1.2.3
2.11 Auto Xon/Xoff (Software) Flow Control
When software flow control is enabled (See Table 18), the 784 compares one or two sequential receive data
characters with the programmed Xon or Xoff-1,2 character value(s). If receive character(s) (RX) match the
programmed values, the 784 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 784 will monitor the
receive data stream for a match to the Xon-1,2 character. If a match is found, the 784 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, any
desired Xon/Xoff value can be used for software flow control. Different conditions can be set to detect Xon/Xoff
characters (See Table 18) and suspend/resume transmissions. When double 8-bit Xon/Xoff characters are
selected, the 784 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 784 automatically
sends an Xoff message (when enabled) via the serial TX output to the remote modem. The 784 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
784 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/DTR Hysteresis value in Table 17. Table 6 below explains this when Trigger Table-B (See Table 14) is
selected.
TABLE 6: AUTO XON/XOFF (SOFTWARE) FLOW CONTROL
RX TRIGGER LEVEL INT PIN ACTIVATION
8
8
16
16
24
24
28
28
XOFF CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
8*
16*
24*
28*
XON CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
0
8
16
24
* 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.
2.12 Special Character Detect
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 784 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.
14