ZiLOG Z-80 SIB User Manual page 37

Synchronous Transmission
As
in
asynchronous transmission,
the
TxD output remains
"high" (marking)
until
the
uPD8251 receives
the
first
character from
the
processor which
is
usually
a
sync
character. After
a
command instruction
has set
TxEN and
after clear
to
send
(CTS-)
goes low, the first character
is
serially transmitted.
Data
is
shifted out
on the
falling
edge
of
TxC- and
the same rate as TxC-.
Once transmission has started, synchonronus mode format
requires that the
serial data stream
at
TxD continue
at the
TxC- rate
or
sync will
be lost.
If
a
data character
is
not
provided
by the
processor before
the
uPD8251 transmitter
buffer becomes empty,
the sync
character
(s)
loaded directly
following the mode instruction will
be
automatically inserted
in the
TxD data stream. The sync character{s)
are
inserted
to
fill the line and
maintain synchronization until new data
characters
are available
for
transmission.
If
the
USART
becomes empty, and must send the sync
character
(s)
,
the
TxEMPTY output
is raised
to
signal the processor that the
transmitter buffer
is
empty and sync
characters are being
transmitted,
TxEMPTY
is
automatically reset
by the
next
character from
the
processor.
Synchronous Rece
ive
In
synchronous
rece
ive,
character synchronization can
be
either external
or
internal. If
the internal sync mode has
been selected,
and the
enable hunt
(EH)
bit has been set by
a
command instruction, the receiver goes into the hunt mode
Incoming data
on the RxD input
is
sampled
on the
rising edge
of RxC-, and the receiver buffer
is
compared with
the
first
sync
character after each bit has been loaded until
a
match
is
found
.
If
two sync characters have been programmed
,
the
next received character
is
also compared.
When
the sync
character
(s)
programmed have been detected,
the
USART leaves
the hunt mode and
is in
character synchronization.
At this
time, the syndet (output)
is
set high. Syndet
is
automatically reset by
a
status
read.
29