Avionics News May 2020 - 50

Figure 4: A T-type flip-flop symbol and input/output waveforms

THEORY & PRACTICE
Continued from page 49

behave like an asynchronous R-S latch. Figure 2 shows
the symbol and truth table for a synchronous R-S flipflop.
Unlike the R-S flip-flop, the D-type flip-flop is edgetriggered by a clock. Q will set only if D is high when the
clock signal is transitioning from 0 to 1 in the example
shown. Q will reset only when D is low when the clock
signal is in transition from 0 to 1. This behavior is useful
in creating a delay. Since the D-type flipflop needs to satisfy the clock, a set on the
input is delayed by one clock cycle, before
Q follows suit. Several of these flip-flops
can be connected end to end to create a
long, yet precisely controlled delay. Figure
3 shows the symbol and truth table for the
D-type flip-flop.
Compared to the other circuits described
above, the T-type flip-flop is rather simple.
In fact, it's simple enough to show a
waveform instead of a truth table. Figure
4 shows the inputs and outputs of a T-type
flip-flop. The device is edge-triggered by a
clock and has Q and Q-NOT outputs. The
output of the T-type flip-flop will be exactly
half the frequency of the input. The T-type
flip-flop is useful as binary counters or as
frequency dividers.
50

avionics news

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may

2020

The final flip-flop discussed this month is a real
workhorse. Depending on how it is configured, the J-K
flip-flop can be operated as any of the circuits described
above. Figure 5 shows the symbol and truth table for
a J-K flip-flop. With the J-K, the S and R inputs have
precedence over all other inputs. If the S goes high, Q
will follow, and it won't matter what is happening on the
clock, the J, or the K inputs. In this way, the J-K flip-flop
behaves like an R-S flip-flop. If the S input is low, there
are other options.
With the S input out of the picture, the J input will set
Q to 1 when the clock input transitions from 0 to 1 (edgetriggered). The K input acts like a reset and will set the Q
back to 0 when the clock transitions from 0 to 1. In this
way, the J-K can behave like a D-type flip-flop.
If both J and K are high, the Q output will change states
when the clock transitions from low to high. Like the
T-type flip-flop, while J and K are at a logic 1, the output
will toggle back and forth at half of the clock frequency.
These flip-flops are the devices that make modern
avionics possible. Using them, engineers can design
circuits that do the math necessary to keep our radio
receivers and transmitters stable within a few hertz of
the selected frequency. The circuits allow calculations to
be made in a multitude of different ways. In contrast to
flip-flopping politicians, flip-flopping circuits are a good
thing. This story merely shows the tip of the dynamic
logic iceberg. Still, if a technician takes some time and
commits the truth tables to memory, answering the AET
test questions on the subject should be simple. q

Figure 5: A J-K-type flip-flop symbol and its truth table



Avionics News May 2020

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