XR16C2850
Software Flow Control
normal incoming RX data. This condition is selected in
conjunction with EFR bits 0-3. Note that software flow
control should be turned off when using this special
mode by setting EFR bit 0-3 to a logic 0.
When software flow control is enabled, the 2850
compares one or two sequential receive data charac-
ters with the programmed Xon or Xoff-1,2 character
value(s). If receive character(s) (RX) match the pro-
grammed values, the 2850 will halt transmission (TX)
as soon as the current character(s) has completed
transmission. When a match occurs, the receive
ready (if enabled via Xoff IER bit-5) flags will be set
and the interrupt output pin (if receive interrupt is
enabled) will be activated. Following a suspension
due to a match of the Xoff characters values, the 2850
will monitor the receive data stream for a match to the
Xon-1,2 character value(s). If a match is found, the
2850 will resume operation and clear the flags (ISR
bit-4).
The 2850 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 (see Figure 9).
Although the Internal Register Table shows each X-
Register with eight bits of character information, the
actual number of bits is dependent on the pro-
grammed 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 X-registers corresponds with
the LSB bit for the receive character.
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 and suspend/resume transmis-
sions. When double 8-bit Xon/Xoff characters are
selected, the 2850 compares two consecutive receive
characters with two software flow control 8-bit values
(Xon1, Xon2, Xoff1, Xoff2) and controls TX transmis-
sions accordingly. Under the above described flow
control mechanisms, flow control characters are not
placed (stacked) in the user accessible RX data buffer
or FIFO.
Time-out Interrupts
Three special interrupts have been added to monitor
the hardware and software flow control. The interrupts
are enabled by IER bits 5-7. Care must be taken when
handling these interrupts. Following a reset the trans-
mitter interrupt is enabled, the 2850 will issue an
interrupt to indicate that transmit holding register is
empty. This interrupt must be serviced prior to con-
tinuing operations. The LSR register provides the
current singular highest priority interrupt only. It could
be noted that CTS and RTS interrupts have lowest
interrupt priority. A condition can exist where a higher
priority interrupt may mask the lower priority CTS/
RTS interrupt(s). Only after servicing the higher pend-
ing interrupt will the lower priority CTS/ RTS
interrupt(s) be reflected in the status register. Servic-
ing the interrupt without investigating further interrupt
conditions can result in data errors.
In the event that the receive buffer is overfilling and
flow control needs to be executed, the 2850 automati-
cally sends an Xoff message (when enabled) via the
serial TX output to the remote modem. The 2850
sends the Xoff-1,2 characters as soon as received
data passes the programmed trigger level. To clear
this condition, the 2850 will transmit the programmed
Xon-1,2 characters as soon as receive data drops
below the programmed trigger level.
When two interrupt conditions have the same priority,
it is important to service these interrupts correctly.
Receive Data Ready and Receive Time Out have the
same interrupt priority (when enabled by IER bit-0).
The receiver issues an interrupt after the number of
characters have reached the programmed trigger
level. In this case the 2850 FIFO may hold more
characters than the programmed trigger level. Follow-
Special Feature Software Flow Control
A special feature is provided to detect an 8-bit charac-
ter when bit-5 is set in the Enhanced Feature Register
(EFR). When this character is detected, it will be
placed on the user accessible data stack along with
Rev. 1.00P
11
EXAR [ EXAR CORPORATION ]
