practical digital design considerations review of concepts created february 2008 ©paul r. godin
TRANSCRIPT
Introduction
◊ There are many considerations that must be taken into account when designing digital logic circuits.
◊ We have discussed each of the issues one-by-one.
◊ 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
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:◊ 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
◊ 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)
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 CircuitOscilloscope
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
◊ Larger conductors have less resistance.◊ Resistance on a ground conductor affects signals due to
resistive voltage drops. In copper conductors the length and cross-sectional area affect resistance.
◊ Noise on a shared connection.◊ Switching noise from one system can affect another if they
share a ground connection.
◊ Decoupling capacitors are needed.◊ 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?
Digital Circuit 2Digital Circuit 1
Power Supply 1
Power Supply 2