EE101Lecture17

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Unformatted text preview: Mark Redekopp, All rights reserved Introduction to Digital Logic Lecture 17: Bistables Latches Mark Redekopp, All rights reserved Bistables Cross-Connected NOR gates form a valid bistable (in this case an SR bistable) When Set = 1, output forced to 1 When Reset = 1, output forced to 0 When both are 0, Q at its present value S R Q Q Q Q 1 1 1 1 1 1 0 (illegal) 0 (illegal) R S Q Q Mark Redekopp, All rights reserved Bistable A bistable with active-lo inputs can be built with cross-connected NAND gates S R Q Q 1 1 Q Q 1 1 1 1 1 (illegal) 1 (illegal) = Notice the active-lo inputs (A 0 on S, sets Q=1) To view it as active- lo we can redraw our NAND gates S R Q Q S R Q Q Mark Redekopp, All rights reserved Criteria for a Bistable 1. Able to set (preset) =&gt; Force Q=1 2. Able to reset (clear) =&gt; Force Q=0 3. Able to remember (hold) =&gt; Q = Q Mark Redekopp, All rights reserved Checking for Bistable Validity Need to check the 3 criteria Can it set, reset, remember Method for checking: Find the passive input values of bistable inputs (the values that allow Q to cycle around the cross- connected loop) Those passive input values identify the inactive values of the inputsthus we can know whether the inputs are active hi or low Activate each input separately to find if it is the Set or Reset input Just see what happens to Q when you activate each input separately Mark Redekopp, All rights reserved...
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