ZiLOG Z8 Technical Manual

Counter/Timers
The counting modes are controlled by bit 00 of
PRE
O
and PRE
1
,
with 00 cleared to 0 for
Single-pass counting mode or set to 1 for
Continuous mode.
The counter/timers can be stopped at any time by
setting the Enable Count bit to 0, and restarted
by setting it back td 1. The counter/timer will
continue its count value at the time it was
stopped. The current value in the counter/timer
(TO or T
1
) can be read at any time without
affecting the counting operation.
New initial values can be written to the prescaler
or the counter/timer registers at any time. These
values will be transferred to their respective
down-counters on the next load operation. If the
counter/timer mode is Continuous, the next load
occurs on the timer clock following an
end-of-count. New initial values should be
written before the desired load operation, since
the prescalers always effectively operate in
Continuous count mode.
The time interval (i) until end-of-count, is given
by the equation
i:txpxv
in which t is 8 divided by XTAL frequency, p is
the prescaler value (1 - 64), and v is the
counter/timer value (1 - 256). It should be
apparent that the prescaler and counter/timer are
true divide-by-n counters.
11.4 TOUT MODES
The Timer Mode register TMR (%F1) (Figure 11-10)
is used in conjunction with the Port 3 Mode
register P3M (%F7) (Figure 11-9) to configure P3
6
for TOUT operation. In order for TOUT to
function, P3
6
must be defined as an output line by
setting P3M bit 05 to O. Output is controlled by
one of the counter/timers (TO or T
1
) or the
internal clock.
The counter/timer to be output is selected by TMR
bits 07 and 06. TO is selected to drive the
TOUT line by setting 07 to 0 and 06 to 1.
Likewise, T1 is selected by setting 07 and 06 to 1
and
0 respectively. The counter/timer TOUT mode
is turned off by setting TMR bits 07 and 06 both
to 0, freeing P3
6
to be a data output line.
T OUT is initialized to a logic 1 whenever the
TMR Load bit (00 for TO or 02 for f
1
)
is set to 1.
R247 P3M
Port 3 Mode Register
(% F7; Write Only)
o P3l = INPUT (TIN) P36 = OUTPUT (TOUT)
1 P3l = DAV2/RDY2 P36 = RDY2IDAV2
Figure 11-9.
Port J Mode Register TOUT Operation
R241 TMR
Timer Mode Register
(% F1; Read/Write)
TOUT MODES I
TOUT OFF = 00
To OUT = 01
Tl OUT = 10
INTERNAL
CLOCK OUT = 11
L
0 = NO FUNCTION
1 = LOAD To
o = NO FUNCTION
1 = LOAD Tl
Figure 11-10. Timer Mode Register TOUT Operation
11-4 3047-010,3047-018

ZiLOG Z8 Specifications

General

Brand	ZiLOG
Model	Z8
Category	Desktop
Language	English