Z80 datasheet(266/330 Pages) ZILOG

266 / 330 page

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7UGT /CPWCN

UM008101-0601

Serial Input/Output

246

insertion of sync characters when there is no data to send. CRC is not

calculated on the automatically inserted sync characters. When the CPU

detects the end of message, a Reset Transmit Underrun/EOM command

can be issued. This action allows CRC to be sent when the transmitter has

no data. In this case, the Z80 SIO sends CRC, followed by sync charac-

ters, to terminate the message.

There is no restriction as to when in the message the Transmit Underrun/

EOM bit can be reset. If Reset is issued after the first data character has

been loaded, the 16-bit CRC is Sent and followed by sync characters the

first time the transmitter has no data to send. Because of the Transmit

Underrun condition, an External/Status interrupt is generated whenever the

Transmit Underrun/EOM bit becomes set.

In the case of sync insertion, an interrupt is generated only after the first

automatically inserted sync character has been loaded. The status indicates

the Transmit Underrun/ EOM bit and the Transmit Buffer Empty bit are set.

In the case of CRC insertion, the Transmit Underrun/EOM bit is set and the

Transmit Buffer Empty bit is reset while CRC is being sent. When CRC has

been completely sent, the Transmit Buffer Empty status bit is set and an

interrupt is generated to indicate to the CPU that another message can begin

(this interrupt occurs because CRC has been sent and sync has been

loaded). If no more messages are to be sent, the program can terminate

transmission by resetting RTS, and disabling the transmitter (WR5, D3).

Pad characters may be sent by setting the Z80 SIO to eight bits/transmit

character and writing FF to the transmitter while sending CRC. Alterna-

tively, the sync characters can be redefined as pad characters during this

time. The following example clarifies this point.

1. The Z80 SIO interrupts with the Transmit Buffer Empty bit set.

2. The CPU recognizes that the last character (ETX) of the message has

already been sent to the Z80 SIO by examining the internal program

status.

Z80 Datasheet(PDF) 266 Page - Zilog, Inc.