Transcript Digital Logic Topics - Agricultural engineering

Digital Logic Topics
Brief notes to preface the laboratory exercise
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Brief summary of characteristics
• Digital logic
– Electronic elements for processing digital (numeric)
information
– Normally bi-state, 1 or 0 digital values represented by two
discrete electrical signals
– Our concern is primarily interfacing with logic inputs and
outputs
• Important terms/issues
– Fan-out - the amount of output drive capacity compared to
input drive requirement
– Open collector - an output that is configured as a switch to
reference voltage rather than a voltage level signal
– Propagation delay - the amount of time between a change in
input state and a change in output state.
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Visualization as a comparator
+5
• Comparator
Vin 2
Vout
~
~ Vin 1
+0
• Vout = +5 if Vin 1 > Vin 2 else Vout = 0
• Devices are available designed for the comparison
purpose. They are in general faster and better suited
for this purpose than a conventional op-amp.
• Devices actually used in digital logic are much
simpler internally than an Op Amp
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Truth Tables
• Consider the following and gate
A
C
B
AND
• A truth table “enumerates” the output state for every
possible input state. See Example:
Input
A
0
1
0
1
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B
0
0
1
1
Output
C
0
0
0
1
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Voltage levels
• Common logic types (families)
– TTL
• Transistor-Transistor Logic - Classically has 0 and 5 V nominal
logic levels. lower voltage logic is evolving
• High power consumption, simple, low cost
– CMOS
• Low power consumption. (current flows primarily during
transition and is low when at a particular state)
• Static sensitive
• More complex than TTL
• Fabricated in wider voltage ranges (4000 series)
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Voltage levels
• Noise Margins
•
Comparison among logic
families
Vcc +5
Input
Output
3.4V
VOHTypical
2.7V VOHMinimum
VIHMinimum
2.0V
Noise Margin
VILMaximum
0.5V
5VTTL
5V
CMOS
3.3
LVTTL
2.5V
CMOS
4000
CMOS
VCC
5
5
3.3
2.5
*10
VOH
2.4
4.44
2.4
2.3
9.95
VIH
2
3.5
2
1.7
7
VT
1.5
2.5
1.5
1.2
5
VIL
0.8
1.5
0.8
0.7
3
VOL
0.4
0.5
0.4
0.2
0.05
GND
0
0
0
0
0
0.8V
VOLMaximum
Gnd
*3-18V
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TTL
• Gate Electronic structure
Vcc
Output
Y
Input
A
GND
TTL Bipolar Gate
Vcc
Input
A
Output
Y
GND
TTL Open Collector Gate
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CMOS Logic
• CMOS - Logic
–
–
–
–
• Example gate circuit
Low power
3 - 18 V logic levels
Static Sensitive
Gate Electronic Structure
NOT
Vcc
GND
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