Intel MCS48 User Manual page 226

8155/8156/8155-2/8156-2
EXAMPLE
PROGRAM
Following
is
an
actual
sequence
of
program
steps
that adjusts the 8155/56 count
register
contents
to obtain the count, extracted
from
Intel® Application
Note AP38.
"Application
Techniques
for
the
Intel
8085A
Bus."
First
store the value
of
the
full
original
count
in register
HL
of the 8085A.
Then
stop the count
to
avoid
getting
an
incorrect
count
value.
Then sample
the timer-counter, storing the lower-order byte of the current count
register
in
register
C
and
the higher-order count
byte
in
register
B.
Then,
call
the following 8080A/8085A
subroutine:
ADJUST,
78
MOV
A,B
;Load
accumulator
with upper
half
;
of
count.
E63F ANI 3F
;Reset
upper
2
bits
and
clear
carry.
1F
RAR
;Rotate
right
through
carry.
47
MOV
B,A
;Store shifted value back
in
B.
79
MOV
A,C
;Load accumulator
with
lower
half.
1F
RAR
;Rotate
right
through
carry.
4F
MOV
C,A
;Store
lower byte
in
C.
D0
RNC
;lf
in
2nd
half of
count,
return.
;lf
in
1st half,
go
on.
3F
CMC
;Clear carry.
7C
MOV
A,H
;Divide
full
count by
2.
(If
HL
;is
odd, disregard remainder.)
1F
RAR
67
MOV
H,A
7D
MOV
A,L
1F
RAR
6F
MOV
L,A
09
DAD
B
;Double-precision add
HL
and BC.
44
MOV
B,H
;Store results
back
in
BC.
4D
MOV
C,L
C9 RET
;Return.
After executinq
the subroutine.
BC
will
contain the remaining
count
in
the current count
cycle.
6-74