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-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

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:◊ 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)

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 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

END

©Paul R. Godinprgodin @ gmail.com