Philips PM 5390 Service Manual page 69

11-9
(
[
FAIRCHILD ECL
•
11C90
(a)
(b)
Fig.
7.
Pulse
Swallowing Counter and
Equivalent
System
Building Block
The
divide ratios
are summarized
as follows:
U
=
upper (larger) divide ratio of
the prescaler
L
=
lower divide ratio of
the prescaler
S
=
divide
ratio of
the
swallow counter
=
number of times the prescaler
divides by
U
in a
complete program
cycle
M
=
divide ratio of
the
program counter
=
total
number
of prescaler cycles in a
complete
program
cycle
From these definitions,
the
number
of
times the
prescaler div-
ides
by
its
lower
ratio
in
one program
cycle
can be
determined.
M-S = number
of
times the prescaler divides by L
in
a
complete program
cycle
The
number
of
f-j
pulses that
occur
in
each
of
the prescaler
modes
during a
complete program
cycle
can be
stated
as
fol-
lows:
U*S
= number
of
f^
pulses
into
the prescaler
during
its
upper
mode
L(M-S)
= number
of f^
pulses
into
the prescaler
during
its
lower
mode
U*S
+
L(M-S)
—
total
number
of
ft
pulses
into
the pre-
scaler
during a
complete program
cycle
Figure
7b
shows
the pulse
swallowing
programmable
count-
er as a single
functional block,
and
the overall divide ratio
N
can be stated
from
the
above
definitions.
N
=
fi/f3
=
U«S
+
L{M-S)
(5)
Alternatively:
N =
(U-L)S
+
LM
(6)
In
Figure 7a,
the preset data inputs
suggest that
S and
M
are
fine
and course
program
controls respectively.
The
effect of
changing S can
be determined by
letting
S
increase
by
one
and
the subtracting Equation
5.
N'
=
(U-L)
(S+D + LM
=
(U-L)S
+ LM
+
(U-L)
AN =N'-N
=
U-L
And
in
the
general
case:
Equation
7
offers
some
insight into
why
the
most popular
variable
modulus
prescalers
have
divide ratios
such
as 1
O/
1
1
and
5/6.
U
and L
differ
only by
one and changing
S
by a
certain
amount changes
N
by the
same amount. Thus
the
combination
of
the prescaler
and
the
swallow counter actsHke
a single,
very
fast, fully
programmable
divider.
A
similar
analysis
with
M
as the variable
shows
that
the smallest
ad-
justment afforded by the
program counter
is L.
N' = (U-L)S
+
L(M
+
1)
=
(U-L)S
+ LM
4- L
AN - N =
L
And
in
the general case:
AN
=
L(AM)
(8)
Combining Equations 7 and
8
gives
an expression
for the
ef«
fects of
changing
either or
both
S and M.
AN
=
(U-D(AS)
+
L(AM)
(9)
Notice that
if
U
is
1
1
and L
is
10,
Equation
9
is
the
same as
Equation
2 and Equation
6
is
the
same
as Equation
1
,
since
the
swallow counter
of
Figure
7 and
the
first
stage of
Figure
3
are the
same
type of
circuit;
thus S and
K
have the
same
meaning. Using
1
0
for
L also
means
that the
program counter
can be
made
up
of
cascaded decade counters,
with each dec-
ade corresponding
to
a decimal
digit of
the
total
divide ratio
N. This
is
best
shown
by a
numerical example.
To
get
N = 4367
with
U =
1
1
and
L
=
10,
make
S
=
7
and
M
= 436
As
a check, substitute
these values
into
Equation
5.
N =
U*S
+
L (M-S)
=
(11)7
+
10
(436-7)
=
77
+ 4290
=
4367
Thus, the pulse swallowing counter
is
programmed
in
the
same way
as the divider of
Figure
3.
A
practical limitation
on
the pulse
swallowing technique
is
that
M
cannot be
less
than
S;
otherwise the
program
counter
would
reach
maxirnom
count before the
swallow
counter,
and
the
latter
would
not
have
a
chance
to
change
the divide
ratio
of
the prescaler be-
fore being preset.
The
prescaler
would then operate
thes^me
as the fixed
prescaler
of
Figure
4.
Thus
with
decade program-
ming, the practical
minimum
divide ratio for a
pulse
swallow-
ing
counter
is
90.
AN
=
(U-L)
(AS)
(
7
)