Digital Signals and Switches

1. Digital Signals and Switches Cosc 235: Computer Organization and Digital Electronics

2. voltage time Digital signals • Logically assign “high” or “1” to a voltage above a certain threshold • Logically assign a “low” or “0” to a voltage below a certain threshold • Timing diagram • Change of voltage plotted vs. time Sample clock signal

3. Clock waveform timing • Period tp Length of time from the falling edge of one pulse to the falling edge of the next (or rising edge to rising edge) Unit: seconds • Frequency f How many cycles we have per second Unit: Hertz (cycles per second) • Note f = 1/tp and tp = 1/f

4. Example What is the frequency of a clock waveform whose period is 10 s? f = 1/tp = 1/10 s = 100,000 Hz

5. Switches Change in logic levels is due to switching between voltage levels • Ideal • Manual • Electromechanical relay • Diode • Transistor

6. Ideal switch • Automatic • Instantaneous • R = 0  when switch is closed • R    when switch is open • Uses no power (Unrealizable)

7. Manual switch • Close to ideal, not automatic Signal Switch Output 0 0 (open) 0 0 1 0 1 0 0 1 1 1 Signal Switch Output AND function

8. A X 0 1 1 0 Switches • Automatic switch: relay • Open/Closing are controlled via an electromagnet and solenoid arrangement • Inverting switch X +5v A

9. A B X 0 0 0 0 1 0 1 0 0 1 1 1 Switches • Diode as a switch A +5v B A/B (inputs) high: +5v A/B (inputs) low: ground X

10. RC RL Transistor Switches: RTL inverter +5v A low (=ground): Is transistor on? Most current flows through where? A high: Is transistor on? Most current goes through where? X RB A RL >> RC, RB No RC & RL So X is High Yes Transistor So X is Low Other inputs (series/parallel)

11. Transistor switches: TTL logic +5v X RC T2 T1 Use T1 as 2 diodes RB +5v A High A low (gnd): T1down; T2off, so X is? A High (+5v): T1up; T2on, so X is? Low