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Digital Logic Topics Brief notes to preface the laboratory exercise 3/01/2003 BAE 5413 1 Brief summary of characteristics • Digital logic – Electronic elements for processing digital (numeric) information – Normally bi-state, 1 or 0 digital values represented by two discrete electrical signals – Our concern is primarily interfacing with logic inputs and outputs • Important terms/issues – Fan-out - the amount of output drive capacity compared to input drive requirement – Open collector - an output that is configured as a switch to reference voltage rather than a voltage level signal – Propagation delay - the amount of time between a change in input state and a change in output state. 3/01/2003 BAE 5413 2 Visualization as a comparator +5 • Comparator Vin 2 Vout ~ ~ Vin 1 +0 • Vout = +5 if Vin 1 > Vin 2 else Vout = 0 • Devices are available designed for the comparison purpose. They are in general faster and better suited for this purpose than a conventional op-amp. • Devices actually used in digital logic are much simpler internally than an Op Amp 3/01/2003 BAE 5413 3 Truth Tables • Consider the following and gate A C B AND • A truth table “enumerates” the output state for every possible input state. See Example: Input A 0 1 0 1 3/01/2003 BAE 5413 B 0 0 1 1 Output C 0 0 0 1 4 Voltage levels • Common logic types (families) – TTL • Transistor-Transistor Logic - Classically has 0 and 5 V nominal logic levels. lower voltage logic is evolving • High power consumption, simple, low cost – CMOS • Low power consumption. (current flows primarily during transition and is low when at a particular state) • Static sensitive • More complex than TTL • Fabricated in wider voltage ranges (4000 series) 3/01/2003 BAE 5413 5 Voltage levels • Noise Margins • Comparison among logic families Vcc +5 Input Output 3.4V VOHTypical 2.7V VOHMinimum VIHMinimum 2.0V Noise Margin VILMaximum 0.5V 5VTTL 5V CMOS 3.3 LVTTL 2.5V CMOS 4000 CMOS VCC 5 5 3.3 2.5 *10 VOH 2.4 4.44 2.4 2.3 9.95 VIH 2 3.5 2 1.7 7 VT 1.5 2.5 1.5 1.2 5 VIL 0.8 1.5 0.8 0.7 3 VOL 0.4 0.5 0.4 0.2 0.05 GND 0 0 0 0 0 0.8V VOLMaximum Gnd *3-18V 3/01/2003 BAE 5413 6 TTL • Gate Electronic structure Vcc Output Y Input A GND TTL Bipolar Gate Vcc Input A Output Y GND TTL Open Collector Gate 3/01/2003 BAE 5413 7 CMOS Logic • CMOS - Logic – – – – • Example gate circuit Low power 3 - 18 V logic levels Static Sensitive Gate Electronic Structure NOT Vcc GND 3/01/2003 BAE 5413 8