Zilog Z16C30 User Manual

Page 155

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Z16C30 USC

SER

ANUAL

ILOG

UM97USC0100

Not using the TCC:
a.

Software doesn’t need to do anything special to the
USC at the start of a frame, other than to initialize its
own frame-length-tracking mechanism.

While writing the frame to the TDR on a 16-bit bus, if
there are two characters left in the frame, software
must write the second-last character to the LSbyte of
the TDR using a byte write operation.

Before writing the last character of the frame to the
LSbyte of the TDR, software should write a Set EOF/
EOM command to the MSbyte of the TCSR.

After the last character goes through the TxFIFO and
out onto the link, the Transmitter finishes the frame,
typically by sending the CRC and closing Flag.

7.16.4 Exiting the ISR

If an IUS bit was set by an interrupt acknowledge cycle or
explicitly by software, then after the ISR has handled one
or more interrupt types as described above, it must clear
the IUS bit that was set. (If nested interrupts were enabled,
it’s a good practice to first disable interrupts again, to avoid
filling the stack with multiple copies of saved registers, in
case another interrupt of the same type happens right after
IUS is cleared. The normal mechanism provided by the
processor for ending an ISR, e.g., an IRET or RETI instruc-
tion, will then re-enable interrupts after saved registers and
such are restored from the stack.)

Software can clear IUS by writing to the MSbyte of the
DCCR, or by writing a “Reset Highest IUS” command to the
MSbyte of the CCAR. The latter method is more general
than the former.

Zilog’s products are not authorized for use as critical compo-
nents in life support devices or systems unless a specific written
agreement pertaining to such intended use is executed between
the customer and Zilog prior to use. Life support devices or
systems are those which are intended for surgical implantation
into the body, or which sustains life whose failure to perform,
when properly used in accordance with instructions for use
provided in the labeling, can be reasonably expected to result in
significant injury to the user.

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Campbell, CA 95008-6600
Telephone (408) 370-8000
Telex 910-338-7621
FAX 408 370-8056
Internet: http://www.zilog.com

© 1997 by Zilog, Inc. All rights reserved. No part of this document
may be copied or reproduced in any form or by any means
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Zilog, Inc. makes no commitment to update or keep current the
information contained in this document.

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