ZiLOG Z-80 SIB User Manual page 56

4,1.2 Enabling USAFTs
Enabling
the
USART requires careful attention.
It should be
enabled before the baud rate generator
is
turned
on
since its
initial state
is
indeterminate and
it
may try
to
transmit
a
character
or
cause
an
interrupt
.
The
process involves four
steps which should proceed
in
the following order;
a.
Reset the USART;
b.
Send
a
mode byte
(see
8251 Product Spec.) to the
control port;
c.
Send
a
command byte to the control port; and
d.
Read one character from the data port
(the
data port
address
is
one less than the control port address
by convention)
.
A USART may
be
reset by sending three zero bytes followed by
a
reset command
(40H)
.
The zero bytes are recommended since
the
USART may have been
in
the
synchronous mode,
and
therefore, would have interpreted the first two characters
as
sync characters. USART application information suggests that
sending three reset commands will cause
a
reset, thus,
enabling
the
USART
to
receive
a
mode byte. Since the MCZ
System reset button
is
connected
to the
USART'
s
external
reset pin, pushing the system reset followed by three
internal reset commands will not cause the USART
to
accept
a
mode byte, but rather cause
the USART to treat the intended
mode byte
as
a
commmand byte.
The next byte sent to the USART must
be
a
mode
byte. The
value 8EH sets the mode
to 1,5
stop bits,
no
parity, eight
bit characters, and
a
16x baud rate factor. The mode byte
must
be
followed
by
a
command
byte. The
value 37H enables
the
USART
for both transmitting and receiving. Request
to
Send
(RTS) (bit
5)
and Data Terminal Ready
(DTR) (bit
1)
should
be turned on
since USARTs and most modems only work
if
Clear to Send
(CTS)
is low, and
one of
these pins will be
connected
to CTS.
One character must
be read from the USART
data port
to
clear the receiver data buffer. The routine
should look
like:
48