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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: ◊ ◊ ◊ ◊ ◊ 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 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 ◊ Larger conductors have less resistance. ◊ ◊ Noise on a shared connection. ◊ ◊ 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. ◊ ◊ ◊ ◊ 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