Digital Logic TopicsTruth TablesSlide 3Slide 4Slide 5Slide 6Slide 7Slide 8TTLCMOS Logic3/28/01 BAE 5413 1Digital Logic TopicsBrief notes to preface the laboratory exercise3/28/01 BAE 5413 2Truth Tables•Consider the following and gate•A truth table “enumerates” the output state for every possible input state. See Example:ANDABC Input OutputA B C0 0 01 0 00 1 01 1 13/28/01 BAE 5413 3•Comparator•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.~Vout~+5+0Vin 1Vin 23/28/01 BAE 5413 4•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.3/28/01 BAE 5413 5•Common logic types (families)–TTL•Transistor-Transistor Logic - Classically has 0 and 5 V nominal logic levels. 3V logic is evolving•Signal levelsVcc +5VOHTypicalVOHMinimumVOLMaximumVILMaximumVIHMinimum3.4V2.7V0.8V2.0V0.5VGndNoise MarginInputOutput3/28/01 BAE 5413 6http://www.interfacebus.com/voltage_threshold.html3/28/01 BAE 5413 7http://www.interfacebus.com/voltage_threshold.html3/28/01 BAE 5413 83/28/01 BAE 5413 9TTLAInputVccYOutputGNDVccGNDAInputYOutputOpen Collector Gate•Gate Electronic structureConventional Bipolar Gate3/28/01 BAE 5413 10CMOS Logic•CMOS - Logic–Low power–3 - 18 V logic levels–Static Sensitive–Gate
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