Let’s Get Technical
Pattern
sequence of 15 different
frequencies, as indicated in Table 1. An
Exclusive NOR (XNOR)
feedback circuit uses
the last two bits of a
four-bit shift register to
determine the next bit
clocked into the shift
register. Each new bit
clocked into the shift
register causes the four-bit output pattern to
change.
Note the seemingly
random sequence at the
DCBA outputs. It is not
really random, since
the same 15 pattern
sequence will repeat
over and over. This is
because, when the
outputs are 0001, the
next output will be
0000, which is where
we started.
So, the four-bit shift
register provides 15
patterns. A five-bit shift register could
provide 31 patterns. A 16-bit shift
register could provide 65,535
patterns, none of which are the same,
with the entire sequence repeating
every 65,535 patterns. That would
probably be secure enough.
Output
D C B A
Decimal
Value
Frequency (Hz)
Direction of
Change
1
0 0 0 0
0
219
N/A
2
1 0 0 0
8
218
Down
3
1 1 0 0
12
215
Down
4
1 1 1 0
14
204
Down
5
0 1 1 1
7
94
Down
6
1 0 1 1
11
142
Up
7
1 1 0 1
13
179
Up
8
0 1 1 0
6
207
Up
9
0 0 1 1
3
162
Down
10
1 0 0 1
9
195
Up
11
0 1 0 0
4
216
Up
12
1 0 1 0
10
210
Down
13
0 1 0 1
5
187
Down
14
0 0 1 0
2
212
Up
15
0 0 0 1
1
199
Down
Table 1. Output sequence of the frequency hopping circuit with associated output frequencies.
This group of 15 patterns repeats endlessly.
The inverters are used to
provide a little extra current drive to
the second 555 timer circuit. The
diodes allow the resistors to be
connected in parallel whenever the
associated inverter output is high.
With 15 different output patterns,
there will be 15 different combinations of the four frequency resistors,
which — in turn — leads to 15
different timing scenarios for the
second 555 timer.
Look at Figure 2 to see a
student’s breadboard of the frequency
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JANUARY 2005
Circle #47 on the Reader Service Card.
29
