XR68C92_05 Datasheet, PDF (11/33 Pages) Exar Corporation – DUAL UNIVERSAL ASYNCHRONOUS RECEIVER AND TRANSMITTER

XR68C92/192
or the FIFO-full status bits can be selected to cause an
interrupt (see MR1A, MR1B bit-6).
In addition to the data byte, three status bits (parity
error, framing error, and received break) are appended
to each data character in the FIFO (overrun is not). By
programming the error-mode control bit (MR1A, MR1B
bit-5), status can be provided for “character” or “block”
modes. In the “character” mode, the status register
(SRA, SRB) is updated on a character-by-character
basis and applies only to the character at the top of the
FIFO. Thus, the status must be read before the char-
acter is read. Reading the character pops the data byte
and its error flags off the FIFO. In the “block” mode, the
status provided in the status register for the parity error,
framing error, and received-break conditions are the
logical OR of these respective bits, for all the data bytes
in the FIFO stack since the last reset error command
(see CRA, CRB bits 7:4) was issued. That is, beginning
immediately after the last reset-error command was
issued, a continuous logical-OR function of corre-
sponding status bits is produced in the status register
as each character enters the FIFO.
The block mode is useful in applications requiring the
exchange of blocks of information where the software
overhead of checking each character's error flags
cannot be tolerated. In this mode, entire messages can
be received and only one data integrity check is per-
formed at the end of each message. Although data
reception in this manner has speed advantages, there
are also disadvantages. If an error occurs within a
message the error will not be recognized until the final
check is performed. Also, there is no indication of
which character(s) is in error within the message.
Reading the status register (SRA, SRB) does not affect
the FIFO. The FIFO is “popped” only when the receive
buffer is read. If the FIFO is full when a new character
is received, that character is held in the receive shift
register until a FIFO position is available. If an addi-
tional character is received while this state exists, the
contents of the FIFO are not affected, but the character
previously in the shift register is lost and the overrun-
error status bit will be set upon receipt of the start bit of
the new overrunning character.
To support flow control, a receiver can automatically
negate and reassert the request-to-send (RTS) output
(RX RTS control - see MR1A, MR1B bit-7). The re-
quest-to-send output (at OP0 or OP1 for channel A or
B respectively) will automatically be negated by the
receiver when a valid start bit is received and the FIFO
stack is full. When a FIFO position becomes available,
the request-to-send output will be reasserted auto-
matically by the receiver. Connecting the request-to-
send output to the clear-to send (CTS) input of a
transmitting device prevents overrun errors in the
receiver. The RTS output must be manually asserted
the first time. Thereafter, the receiver will control the
RTS output.
If the FIFO stack contains characters and the receiver
is then disabled, the characters in the stack can still be
read but no additional characters can be received until
the receiver is again enabled. If the receiver is disabled
while receiving a character, or while there is a charac-
ter in the shift register waiting for a FIFO opening, these
characters are lost. If the receiver is reset, the FIFO
stack and all of the receiver status bits, the correspond-
ing output ports, and the interrupt request are reset. No
additional characters can be received until the receiver
is again enabled.
LOOPBACK MODES
Besides the normal operation mode in which the re-
ceiver and transmitter operate independently, each
XR68C92/192 channel can be configured to operate in
various looping modes (see MR2A, MR2B bits 7:6) that
are useful for local and remote system diagnostic
functions.
AUTOMATIC ECHO MODE
In this mode, the channel automatically retransmits the
received data on a bit-by-bit basis. The local CPU-to-
receiver communication continues normally but the
CPU-to-transmitter link is disabled.
LOCAL LOOPBACK MODE
In this mode, the transmitter output is internally con-
nected to the receiver input. The external TX pin is held
in the mark (high) state in this mode. By sending data
to the transmitter and checking that the data as-
sembled by the receiver is the same data that was sent,
proper channel operation can be assured. In this mode
the CPU-to-transmitter and CPU-to-receiver commu-
nications continue normally.
REMOTE LOOPBACK MODE
In this mode, the channel automatically retransmits the
received data on a bit-by-bit basis. The local CPU-to-
receiver and CPU-to-transmitter links are disabled.
Rev. 1.33
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