ZiLOG Z-80 SIB User Manual page 29

After receiving
the
control words,
the
USART
is
ready
to
communicate.
TxRDY
is
raised
to
signal the processor that
the USART
is
ready
to
receive
a
character
for
transmission.
When
the
processor writes
a
character to the USART, TxRDY
is
automatically
reset.
Concurrently,
the
USART may receive serial
data; and after
receiving
an
entire character, the
RxRDY output
is
raised
to
indicate
a
completed character
is
ready
for the
processor.
The processor fetch will
automatically reset
RxRDY.
NOTE:
The 8251 and 9551 USARTs may provide faulty data from
the
receiver buffer
for the
first read after power-on. A
dummy read
is
recommended.
The
USART cannot transmit until the TxEN (Transmitter Enable)
bit has been set by
a
command instruction
and
until the CTS-
(Clear to Send)
input
is a
"zero".
USART Programming
The USART must
be
loaded with
a
group
of two to four control
words provided by
the
processor before data reception
and
transmission can begin,
A
reset (internal
or
external) must
immediately proceed
the
control words which are used
to
program
the
complete operational description
of the
communications interface.
If an
external reset
is
not
available, three successive zeros followed by
a
reset command
instruction can
be
used to
initialize the USART.
TxD
is
held
in the
"marking" state after reset, waiting
for
a
new command
instruction.
-~
There are two control word formats:
1
,
Mode Instruction
2.
Command Instruction
Mode Instruction
This control word specifies
the general characteristics
of
the
interface regarding
the
synchronous
or
asynchronous mode
,
baud rate factor
,
character length
,
par ity
,
and number of
stop bits.
Once the mode instruction has been received, sync
characters
or
command instructions may be inserted depending
on the
mode instruction content.
21