ZiLOG Z-80 SIB User Manual page 51

Bit
5=0
(Defined
for
timer mode only.) Prescaler factor
is 16.
Bit 4=1
Timer mode
- positive edge trigger starts
timer operation. Counter
mode
- positive edge decrements
the
down counter.
Bit 4=0
Timer mode - negative edge trigger starts
timer operation.
Counter mode -
negative edge decrements the down counter.
Bit 3=1
Timer mode only
-
external trigger
is
valid
for
starting
timer operation after rising edge of
T of the
machine cycle
following the one that loads the time constant. The
prescaler
is
decremented
two clock cycles later
if the
setup
time
is
met, otherwise, three clock cycles.
Bit 3=0
Timer mode only - timer begins operation
on the
rising edge
of T of the
machine cycle following
the one that loads the
time constant.
Bit 2=1
The time constant data word for the time constant register
will
be the
next word written
to
this channel.
If an
updated
channel control word and time constant data word are written
to
a
channel while
it is
already
in
operation,
the
down
counter will continue decrementing
to zero before the new
time constant
is
loaded into
it.
43