Intel MCS48 User Manual page 225

8155/8156/8155-2/8156-2
TIMER SECTION
The
timer
is
a
14-bit
down-counter
that
counts
the
TIMER
IN
pulses
and
provides either a
square
wave
or pulse
when
terminal count (TC)
is
reached.
The
timer has the I/O
address
XXXXX1
00
for
the
low order
byte
of the register
and
the I/O address
XXXXX101
for
the high order byte
of
the
register.
(See Figure
5).
To
program
the timer, the
COUNT LENGTH REG
is
loaded
first,
one
byte
at
a time,
by selecting the timer
addresses.
Bits
0-13
of
the high order
count
register
will
specify the length
of
the next
count and
bits
14-15
of
the
high order
register
will
specify the timer output
mode
(see Figure
8).
The
value loaded
into
the count length
register
can have any
value from
2H
through
3FFH
in
Bits 0-13.
Note
that
while the counter
is
counting,
you
may
load a
new count and
mode
into
the
count
length
registers.
Before the
new
count and
mode
will
be used by the
counter,
you must
issue a
START command
to the
counter.
This
applies
even though you
may
only
want
to
change
the
count and use the previous
mode.
In
case of an
odd-numbered
count, the
first
half-cycle
of
the
squarewave
output,
which
is
high,
is
one count
longer than the
second
(low) half-cycle,
as
shown
in
Figure
10.
7 6 5
4
3 2
1
M
2
M-i
T13 T12
Tn
Tio
T
9
T
8
JL
TIMER
MODE MSB OF CNT LENGTH
7 6 5
4
3 2
1
T
7
T
6
T
5
T
4
T
3
T
2
Ti
To
LSB
OF CNT LENGTH
Figure
8.
Timer Format
There
are four
modes
to
choose
from:
M2
and
M1
define
the timer
mode,
as
shown
in
Figure
9.
MODE
BITS
TIMER OUT WAVEFORMS:
START
COUNT
TERMINAL
COUNT
TERMINAL
COUNT
SINGLE
SQUARE WAVE
2.
CONTINUOUS
SQUARE
WAVE
3.
SINGLE
PULSE
ON
TERMINAL
COUNT
4.
CONTINUOUS
PULSES
1/—
V
TM
2
TMi
1
Figure
9.
Timer
Modes
Bits
6-7
(TM2 and TMi)
of
command
register
contents
are used to
start
and
stop the counter. There
are four
commands
to
choose
from:
NOP
—
Do
not
affect
counter
operation.
STOP
—
NOP
if
timer has not
started;
stop counting
if
the timer
is
running.
STOP AFTER TC
—
Stop immediately
after
present
TC
is
reached
(
NOP
if
timer
has not
started)
START
—
Load
mode
and
CNT
length
and
start
immediately
after
loading
(if
timer
is
not presently running).
If
timer
is
running,
start
the
new mode
and
CNT
length immediately
after
present
TC
is
reached.
NOTE:
5
AND
4
REFER TO THE NUMBER OF CLOCKS
IN
THAT TIME PERIOD
Figure
10.
Asymmetrical Square-Wave Output
Resulting
from
Count
of
9
The
counter
in
the
8155
is
not
initialized
to
any
particular
mode
or
count
when
hardware
RESET
occurs, but
RESET
does stop the counting. Therefore, counting cannot begin
following
RESET
until
a
START command
is
issued
via
the
C/S
register.
Please note
that
the timer
circuit
on the 8155/8156 chip
is
designed
to
be
a
square-wave
timer, not
an event
counter.
To
achieve
this,
it
counts
down
by twos
twice
in
completing one
cycle.
Thus,
its
registers
do
not con-
tain
values directly representing the
number
of
TIMER
IN
pulses
received.
You cannot
load an
initial
value
of
1
into
the
count
register
and cause
the timer
to
operate, as
its
terminal
count value
is
10
(binary)
or
2 (decimal). (For
the detection
of single pulses,
it is
suggested
that
one
of the
hardware
interrupt
pins
on
the
8085A
be
used.)
After the timer has
started
counting down,
the values
residing
in
the
count
registers
can be used
to
calculate
the actual
number
of
TIMER
IN pulses required
to
com-
plete
the timer cycle
if
desired.
To
obtain the remaining
count,
perform
the following operations
in
order:
1.
Stop
the
count
2.
Read
in
the
16-bit
value from the count
length
registers
3.
Reset the
upper two
mode
bits
4.
Reset the carry
and
rotate right
one
position
all
16
bits
through
carry
5.
If
carry
is
set,
add
1/2 of the
full
original
count
(1/2
full
count
—
1
if
full
count
is
odd).
Note:
If
you
started with an odd
count and you
read the
count
length
register
before the
third
count
pulse occurs,
you
will
not
be
able
to discern
whether one
or
two
counts
has occurred. Regardless
of
this,
the 8155/56 always
counts
out
th e
right
number
of pulses
in
generating the
TIMER
OUT
waveforms.
6-73