I
PART
PLL SY
ESIZER.
When
the
input frequency
fin
enters the down of 74LS192 the counter
countsdown from
9,8,7
...
1 to 0.
When the counting
proceeds
from
0 to 9,
the
borrow
output
is
delivered. That
is, the
input
frequency
fin
is
divided at
the r:atio of l/10.
The borrow output is
put
into
¡rPD861C
where the
frequency is divided
as
programmed
(codes
from
P'l
-
P6)
and
finally
negative
pulse
"1"
is
Out
out.
87 LSI6ísinvertedbythispulse.
SupposethatO
="O"andÕ ="1".
Thenthelogical
circuitisas
follows:
74L5192
861C
A c D.
tl
¡
A B
C.
D E
o
o
"1"
B
74LS76
Counter for scan
From
the table of true
values for
OR and AND;when,
"1"
and
"0t'
are
respectively
put
in one side
of OB and that of AND, then the
output is
"1"
for OR and
"0"
for
AND
irrespective
of
the input
on the other side. That is, the
kind of code
put
into
the scanning counter has nothing
to
do with
the results in the
above circuit.
.
Therefore, the code of 741S192 is
[1010]
[5]
that of 861C is
[010111]
_
t58l
.
Next,
as 861C
is
"
J
",A74L574
transmits the D input,
therefore
O
-
"1"
and
O=
"0".
Atthat
time, when the content of
74LSl92 becomes 1
(i.e.
OA,
OB, OC
and
OD
are
respectively 1,0,0,
O.),
"0"
is
added to all the diode
OR circuits and, therefore, the D input
of 874LS74 becomes
"O".
The
D input
is transmitted to O
with
the next
Iin
"
--f
".
ln other
words,
"0"
is
transmitted to
O
and the load
signal
is
generated.
The
generation
of load
signal causes the input
[1010]
of Dn
-
Do to be
preset,
therefore
the
resultat correcting
is
g
-
2+
1*Sinsteadof 9- 2*1*0.
Símultaneously,terminal CLRof A74LS74 isclearedwith"O"
added, and corisequently
Õ
becomes
"1".
Then, the D
input
of
874LS74 becomes
"1",
and
O
is
"1"
with the
next fin
"
-l
",
causing the load signal to
go
out.
ln the above condition, however, 74LSl92 doesn't
count
and its content
"5"
remains as it
is.
With
the
next fin
"
J
",74LS192performs
counting:
5+4
*3*2
*1 *0
--9
*8
.-.-:---
That
is, 9
-
2*1
-'@*@*@*@*Ø*O*O
+B+B
""""
Thus, the counting include extra
six
pulses
as shown by encircling
the
numbers.
To summarize the
above
description, when
¡tLI-"
(this
òan be considered asthe
load
signal
from
861C)
comes out at the
OUT
of
861C,
it includes
6
pulses
more than
the ordinary
signal.
Accordingly, the number of
frequency
divisions
by the fixed
codes
is
586
obtained
by
adding
6
and
10 x 58 = 580 that
is
the number of
frequency
divisions by
74LS192 and 861C.
"o"
o
"1"
-o
3j
CI
o
6
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