ECE 3110: Introduction to Digital Systems Chapter 6 Combinational Logic Design Practices

1. ECE 3110: Introduction to Digital SystemsChapter 6Combinational Logic Design Practices Circuit Timing

2. Previous… • Drawing Layouts • Flat • Hierarchical • Buses • Signal/bus flags for inter-pages • Complete Schematic Diagrams • IC types • Reference designator (unit number) • Pin numbers • Pinouts for SSI ICs in standard DIP (74 series)

3. Timing Diagrams • A timing diagram illustrates the logical behavior of signals as a function of time. • Causality: which input transitions cause which output transitions. • Different through a circuit paths may have different delays. • A signal timing diagram may contain many different delay specifications. • Delay depends on: Internal circuit structure, Logic Family type, Source Voltage, Temperature

4. Timing Diagram for Data signals and Buses Logic Circuit(Memory) DATAOUT DATA IN WRITE_L t1 Logic Circuit(Counter) STEP[7:0] CLEAR COUNT

5. Propagation Delay • The delay time between input transitions and the output transitions due to the propagation delay of the logic gates. • tp of a signal depends on the signal path inside the logic circuit • For a logic gate tpLH may not equal tpHL, (e.g. in TTL) • tp is specified in the manufacturer data sheets of the IC’s • Example: The delay for 74x00 in nanoseconds for TTL & CMOS Families: LS, HCT,AHCT • To find tp for a signal, add the propagation delays of all gates along the path of the signal

6. Timing specifications • A timing table may specify a range of values for each delay for a device. • Maximum: longest possible delay • Typical: under near-ideal condition • Minimum: smallest. Many manufactures don’t specify this values in most moderate-speed logic families (74LS,74S TTL). Set to zero or 1/4~1/3 of typical delay if not specified.

7. Delays for selected SSI parts

8. Delays of SSI parts • All inputs of an SSI gate have the same propagation delays to the output. • TTL gates usually have different delays for LOW-to-HIGH and HIGH-to-LOW transitions, while CMOS gates usually don’t. • The delay from an input transition to the corresponding output transition depends on the internal path taken by the changing signal.

9. pp. 366 Delays for selected MSI parts

10. Timing analysis • Study logical behavior of SSI/MSI devices • Worst-case delay: • Maximum of tpLH andtpHL for each component • Sum of the worst-case delays through the individual components, independent of the transition direction and other conditions. • Tools • CAD and simulators: Xilinx, MAXPLUS

11. Exercise Example:

12. Next… • Combinational PLDs • Reading Wakerly CH-6.3,6.4