D-TYPE
FLIP-FLOP
t = n
)..4
~
c
Figure 3
Ro
D
LO
LO
HI
HI
Q ~
.2.. D So
d ~
t = n
J
Q~
LO
LO
HI
HI
HI
~ CLK
Figure 4
LO
HI
LO
HI
Q
CLR
I
Hl 1
LO ~
PULSE
rL
t = n + 1
K CLK
LO -
HI -
LO -
HI LO
HI HI
Q
Q
I
I
t =- n ot = n
: + 1
NO CHANGE
LO
HI
HI
LO
HI
LO
LO
HI
I
HI
I
LO ~
PULSE
Truth Table valid only
when CLR is HI
?1o
The exclusive OR gate is used
extensively in video games . The truth
table of Figure 2E shows that the
exclusive OR gate has a H 1 output
when anyone but not both of the
inputs is a H 1 . If both inputs ar·e a LO
or a H1 the output is a LO .
Such circuits are used for a variety of
storage , counting and timing oper-
ations .
The two outputs o.LI,.flip flop are Q
and its complement Q . The state of
the Q or normal output tells you the
state of the flip flop . If Q is a 1 the flip
flop is set , if it is a 0 the flip flop is
reset .
Rip Aops
A flip flop is a digital logic device
used to store one bit of binary data . It
has two stable states , one repre-
senting a binary 0 . The flip flop is the
basic logic element used in sequential
logic circuits. The primary character-
istic of a sequential circuit is memory.
D-type flip flop
The D-type flip flop has two
inputs . The D input is where you
apply the bit to be stored . The C or
CLK controls the flip flop . It is used
- to determine whether the input data
CLK
F"JgUre 5
A SIMPLE COUNTER
CLK
QA
QB
~
QD
I
t = n:t = n+1
LO
LO
HI
HI
Truth Table valid only when
and R are both HI
0
J-K MASTER SLAVE
FLIP-FLOP
~ K
c
I
s 0
?1
~J
t = n + 1
Q
I
I
I
I
I
I
I
is recognized or ignored . If the CLK
line is Hi the data on D is stored and
present at the Q output. If C is Lo ,
the D data is ignored and the bit
stored previously is retained . As long
as the CLK input is Hi , the Q output
will follow the D input .
This operation of the D flip flop is
only valid as long as the Preset and
Clear control lines are Hi. A Lo on
the preset causes Q to go Hi and
remain there . A Lo on the clear line
causes Q to go Lo . The Q output will
be the complement of Q . The
present and clear inputs predominate
over all other inputs. (Figure Three) .
J-K flip flop
The J -K flip flop is really two flip
flops in one, where one FF feeds the
other. This arrangement is called a
masterslave flip flop . The master flip
flop is the input circuit where the
input signals are applied . The slave
flip flop in the J -K circuit provides the
output signals and the slave gets its
inputs from the master. A clock pulse
controls both master and slave flip
flops (Figure Four) .
If both the J and K inputs are Lo ,
the clock impulse will have no effect
on the outputs; they will remain in
their previous states. When the J and
the K inputs are at opposite logic
levels , regardless of which input is Hi
and which is Lo , the negative going
edge of the clock signal will transfer
the J input bit to the Q output and
the K bit to the""'O" output . In effect this
input configuration , both Hi, makes
the JK flip flop a divide by 2 counter .
As with the D type flip flop the clear
input must be a Hl. A Lo outear
forces and holds Q Lo and Q , its
complement, H1 .
Counters
There are a wide variety of
counters used in games today .
Basically a counter is noth ing more
than a series of flip flops . A four-bit
counter for instance would have four
JK flip flops in series . The input data
would be fed to the first CLK input
and the Q output of each stage acts
as the clock of each succeeding flip
flop stage . The J and K inputs are at
a H11evel which makes each flip flop
stage a divide by 2 counter . Only the
Q output of each of the four flip -flop
stages are used . Each output will be
one half the output frequency of the
preceding stage (Figure Five) .
There are many other more
complex logic circuits used in games.
A good TTL Data Book will show the
operation of each of the different
types that you may need to know .
The next article will deal these circuits
and some of the more complex
circuits from a troubleshooting stand-
point rather than how they operate .