Zilog Z16C30 - Manual (Page 82)

5-15
Z16C30 USC ®
U SER ' S M ANUAL Z ILOG
UM97USC0100
5.12 SLAVED MONOSYNC MODE
This mode applies only to the Transmitter. Software selects
it by programming 1100 in the TxMode field of the Channel
Mode Register (CMR11-8), while programming 0100 in the
RxMode field (CMR3-0) to select Monosync mode for the
Receiver.
The mode is intended to implement the X.21 standard and
similar schemes in which character boundaries on TxD
must align with those on RxD. For this to be meaningful,
RxCLK and TxCLK typically come from the same source,
as described in Chapter 4.
Most of the setup and operation in this mode is the same
as in Monosync mode, which was described in an earlier
section. CMR15 determines whether the Transmitter sends
a CRC in an Underrun condition. CMR12 selects whether
sync characters are the same length as data characters,
or are 8 bits long.
CMR13 controls the major operating option in Slaved
Monosync mode. (In regular Monosync mode this bit
controls whether the Transmitter sends a Preamble before
each message; in this mode it can’t send one.)
The Transmitter will not go from an inactive to an active
state while CMR13 is 0. If CMR13 is 1 when the Receiver
signals that it has matched a Sync character, the Transmit-
ter sets the
OnLoop bit in the Channel Command/Status
Register (CCSR7) and becomes active. That is to say, the
Transmitter can go active at any received Sync character,
not just one that makes the Receiver exit from “Hunt mode”.
Once the Transmitter starts, operation is identical with
Monosync mode. The Transmitter sends the Sync charac-
ter from TSR7-0. Then it sends data from the TxFIFO, until
the TxFIFO underruns or until it sends a character marked
as End of Message. Then the Transmitter sends the CRC
if software has programmed that it should do so for this
kind of termination. Finally it sends a Sync character and
checks the CMR13 bit again.
If CMR13 is still 1, the Transmitter waits, sending the
programmed Idle line condition, until the software triggers
it to send another message. If, however, software cleared
CMR13 to 0 during the message just concluded, or if it
does so while the channel is sending the Idle condition, the
Transmitter goes inactive but it leaves OnLoop (CCSR7)
set. In the inactive state it sends continuous ones until
software programs CMR13 back to 1 again, and the
Receiver signals Sync detection.
If all the transmitted and received sync and data charac-
ters are the same length, and the same clock is used for
both the Transmitter and Receiver, this method of starting
transmission assures that transmitted characters start and
end simultaneously with received characters, as required
by X.21.
The USC doesn’t use CMR14 in the TxSubMode field in
Slaved Monosync mode, but Zilog reserves this bit for
future enhancements and software should always pro-
gram it as zero in this mode.
UM009402-0201