XR16M564 Datasheet, PDF (20/55 Pages) Exar Corporation – 1.62V TO 3.63V QUAD UART WITH 32-BYTE FIFO

XR16M564/564D
1.62V TO 3.63V QUAD UART WITH 32-BYTE FIFO
REV. 1.0.0
2.14 Auto Xon/Xoff (Software) Flow Control
When software flow control is enabled (See Table 16), the M564 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 M564 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 M564 will monitor the
receive data stream for a match to the Xon-1,2 character. If a match is found, the M564 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 LOW. 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 (See Table 16) and suspend/resume transmissions. When double 8-bit Xon/Xoff characters are
selected, the M564 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 M564 automatically
sends an Xoff message (when enabled) via the serial TX output to the remote modem. The M564 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 M564 will transmit the
programmed Xon-1,2 characters as soon as receive FIFO is less than one trigger level below the programmed
trigger level. Table 8 below explains this.
TABLE 8: AUTO XON/XOFF (SOFTWARE) FLOW CONTROL
RX TRIGGER LEVEL INT PIN ACTIVATION
XOFF CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
XON CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
8
8
8*
0
16
16
16*
8
24
24
24*
16
30
30
30*
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.
2.15 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 M564 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.
20