XR16L570_07 Datasheet, PDF (15/47 Pages) Exar Corporation – SMALLEST 1.62V TO 5.5V UART WITH 16-BYTE FIFO AND POWERSAVE

XR16L570
REV. 1.0.1
SMALLEST 1.62V TO 5.5V UART WITH 16-BYTE FIFO AND POWERSAVE
2.15 Auto Xon/Xoff (Software) Flow Control
When software flow control is enabled (See Table 10), the L570 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 L570 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 L570 will monitor the
receive data stream for a match to the Xon-1,2 character. If a match is found, the L570 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 (See Table 10) and suspend/resume transmissions. When double 8-bit Xon/Xoff characters
are selected, the L570 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 L570 automatically
sends an Xoff message (when enabled) via the serial TX output to the remote modem. The L570 sends the
Xoff character(s) 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 L570 will transmit the
programmed Xon character(s) as soon as receive FIFO is less than one trigger level below the programmed
trigger level (see Table 8). The table below describes this.
TABLE 4: AUTO XON/XOFF (SOFTWARE) FLOW CONTROL
RX TRIGGER LEVEL INT PIN ACTIVATION
1
1
4
4
8
8
14
14
XOFF CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
1*
4*
8*
14*
XON CHARACTER(S) SENT
(CHARACTERS IN RX FIFO)
0
1
4
8
* 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 8-bit word length, no parity and 1 stop bit setting.
2.16 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 L570 compares each incoming receive character with the programmed 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.
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