ZiLOG Z-80 SIB User Manual page 13

3.0
OPERATION
3.
1
Description
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The Z80-SIB uses four 8251 USART devices
to
implement
the
serial communication channels. Two Z80-CTC devices are used
to
accommodate
Z80
interrupt capability
for
receive and
transmit operation
for
each bi-directional serial channel, A
third ZBO-CTC device
is
provided
to
accommodate programmable
baud rates
for
each serial port from 50 to 9600 baud
derived
from an on-board crystal clock,
the system clock, or an
external clock.
Interrupt operation
is
accommodated
by
including
the two CTC
devices
in the
interrupt daisy chain, and after appropriate
strapping
to
determine clock drivers
for
baud generation, the
interrupt handling CTC devices are loaded with
a
count
of
one.
After USART initialization, the CTC interrupts are
enabled.
(See
Section
4.1 for
a
detailed description of
initialization.)
Consequently, when
the
transmitter buffer
is
empty
or
the receiver buffer
is
full, the
TxRDY
or
RxRDY
lines from the
USART will
go high, causing
the
CTC counter
to
decrement, and thus,
generating
an
interrupt.
The data bus
control PROM detects that
the
interrupt
is
being generated
from the USART interrupt handling CTC devices, lEIl(high) and
IE02(low) and turns the three-state bus buffers towards
the
CPU when Ml-
and
lORQ-
are active. Thus, the vector interrupt
address from
the interrupting CTC channel
is
read by the CPU.
The CTC channel reloads with
a
count
of one,
becoming ready
to
generate another interrupt.
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