Transcript Design 3 - godinweb

Practical Digital Design
Considerations
Review of Concepts
Created February 2008
©Paul R. Godin
Introduction
◊ There are many considerations that must be
taken into account when designing digital logic
circuits.
◊ We have discussed each of the issues one-byone.
◊ Now that we understand the individual issues, we
can re-visit the concepts discussed in the
practical design section and apply them to circuit
design as a whole.
Specialized Gate Configurations
◊ Specialized applications call for specialized gate
configurations
◊ Open Collector / Open Drain
◊ Tri-State
◊ Totem Pole
Gate-to-Gate Connections
◊ Specific calculations are required for gate-to-gate
connections
◊ Noise margin and interface voltages
◊ Fanout and gate current
◊ Propagation delay
◊ Buffers and drivers
Communication Links and Bus Issues
◊ Bus Communications require special consideration
for design and use
◊ Resistance, Inductance and Capacitance issues
◊ Bus Contention
◊ Noise and Filters
◊ Isolation
Power and Grounds
◊ Appropriate power supplies and grounding must
be considered as part of the digital logic design
◊ Vcc/Vdd, Icc/IT
◊ Ground type
◊ Isolation
◊ Ground Loops
◊ Decoupling
Special Purpose Circuits
◊ Specialized circuits require special considerations
◊ Filters
◊ Power-Up Circuits
◊ Relays
◊ Transistors
High Frequency
◊ High frequencies increase negative effects.
◊ Digital waveform due to RLC
◊ Noise
◊ Grounds and supplies
RLC Effects
Capacitance – Negative Effects
◊ Increases signal delay
◊ Transforms steep edges to soft edges
◊ Adds load to high frequency applications
◊ Increased frequency = more current = more resistive
power loss
◊ Effects increase with frequency (Xc)
Capacitance – Positive Applications
◊ Filters Noise
◊ Decoupling
◊ Passive filter circuits
◊ Used in specialized circuits:
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Reset on power-up (delay)
Time delay circuits
Monostables
Astables
Touch triggering
Dealing with Capacitance
◊ Reduce conductor/bus lengths
◊ Use proper bus/cable construction techniques
◊ dielectric
◊ distance between conductors
◊ Use devices with lower input capacitance for high
frequency applications (TTL better than CMOS)
◊ Use Schmitt-triggered buffers to reconstruct
edges.
Inductance – Negative Effects
◊ EMI/RFI noise
◊ Internal sources (adjacent conductors)
◊ External sources (ambient or transient)
◊ Transient response (voltage spike on a coil)
Inductance – Positive Applications
◊ Relays
◊ Filters
◊ Sound (speakers)
◊ Movement (motors / solenoids)
Dealing with Inductance
◊ Reduce conductor/bus lengths
◊ Increase distance from sources of noise
◊ Shield conductors/circuits
◊ Metallic shields
◊ Circuit board design
◊ Routing of conductor/bus
◊ Specialized solutions:
◊ Add diodes to relay coils
Resistance – Negative Effects
◊ Voltage drops
◊ Power loss
◊ Decreased current with high R
◊ Increased current with low R (fanout limitation)
◊ Timing losses (with ever-present capacitance)
Resistance – Positive Applications
◊ Current limiting for loads
◊ Pull-up / Pull-down
◊ Timing-based circuits
Dealing with Resistance / Loads
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Shorter conductors
Greater cross-sectional area for conductors
Select devices that require less current
Decoupling capacitors on power supply
Use interface circuits such as:
◊ Relays
◊ Transistors or other switching devices
◊ Specialized ICs (drivers/buffers)
Case Studies
Oscilloscope and Grounds: Case 1
◊ The oscilloscope probe is connected to earth
ground. The digital circuit has a common ground.
◊ Can the ground of the scope be connected to the
common ground? Are there any considerations?
Digital Circuit
Oscilloscope
Design Considerations Case 2
1. Contrast the advantages and disadvantages of
each circuit.
2. State what is missing from both circuits
Power Supply
Power Supply
Knowledge Base for Case 2
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Larger conductors have less resistance.
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Noise on a shared connection.
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Resistance on a ground conductor affects signals due to
resistive voltage drops. In copper conductors the length and
cross-sectional area affect resistance.
Switching noise from one system can affect another if they
share a ground connection.
Decoupling capacitors are needed.
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Switching noise affects signals. Use:
0.01 µF between Vcc and Ground at each IC
0.1 µF for every 5 Ics
The power supply should have appropriate filtering
Design Considerations: Case 3
◊ Two digital circuits that utilize different power
supplies need to be interfaced.
◊ Should there be a common ground between the power
supplies?
◊ What are some considerations?
Power
Supply 1
Power
Supply 2
Digital Circuit 1
Digital Circuit 2
END
©Paul R. Godin
prgodin @ gmail.com