EMT 235 DIGITAL PRINCIPLES II Chapter 1: Digital Design Concepts Part 3
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EMT 235 DIGITAL PRINCIPLES II
Chapter 1:
Digital Design Concepts
Part 3
Timing Response/ Glitches
Glitch
A glitch is any undesired (unwanted)
voltage or current spike (pulse) of
very short duration.
Happens at extremely short duration.
A glitch can be wrongly interpreted as
a valid signal by a logic circuit and
can cause incorrect system operation.
The 74HC138 used as a
3-to-8-line Decoder
conSiDer …
Figure 6.62 Decoder waveforms with output
glitches.
Figure 6.63 Decoder waveform displays showing how transitional input states produce
glitches in the output waveforms.
Figure 6.64 Application of a strobe waveform to eliminate glitches
on decoder outputs.
A Two-phase Clock
Generator
conSiDer …
Figure 7.62 Two-phase clock generator with ideal waveforms
Figure 7.63 Oscilloscope displays for the
circuit in Figure 7–62.
Glitch caused by a “race” problem between
the CLK signal and the Q and Qbar signals
Figure 7.64 Two-phase clock generator using negative edge-triggered
flip-flop to eliminate glitches.
An Asynchronous
Decade Counter
conSiDer …
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved Floyd, Digital Fundamentals, 10th ed
Asynchronous Decade Counter
This counter uses partial decoding to recycle the count sequence to zero after the 1001 state. The flip-flops are trailing-edge triggered, so clocks are derived from the Q outputs. Other truncated sequences can be obtained using a similar technique.
Waveforms are on the following slide…
CLK
K0
J0
Q0
C C C
J1 J2
K1 K2
Q1 Q2
HIGH
C
J3
K3
Q3
CLR
© 2009 Pearson Education, Upper Saddle River, NJ 07458. All Rights Reserved Floyd, Digital Fundamentals, 10th ed
Asynchronous Decade Counter
When Q1 and Q3 are HIGH together, the counter is cleared by a “glitch” on the CLR line.
1 2 3 4 5 6 7 8 9 10
Glitch
Glitch
CLK
Q0
Q1
Q2
Q3
CLR
Glitch
Glitch
Glitch & Hazard
Glitch
A short pulse (like spikes), which may
be produced in a circuit’s output, at a
time that the output should not
change.
Hazard
Exist when a circuit has the possibility
of producing glitches.
Hazards
Can happen in:
Combinational circuits
Asynchronous sequential circuits
=> Will need hazard analysis & elimination
Cannot happen in:
Well-designed synchronous circuits.
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