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270L13-Sequential+Blocks - Sequential Blocks Goal is to...

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1 1 UM EECS 270 Fall 2009 Sequential Blocks Goal is to create a toolbox of frequently used sequential devices – Registers – Counters Ripple counter Parallel counter – Shift Registers Parallel in shift register Universal shift register – Shift Register Counters Ring counter Johnson counter Linear feedback shift register 2 UM EECS 270 Fall 2009 Registers A collection of two or more D flip-flops with a common clock is called a register Used to store a collection of bits D Q QB D Q QB D Q QB D Q QB CLK Q0 Q1 Q2 Q3 D1 D0 D2 D3 Adding a mux at the input of each D FF allows for loading new value or storing current value D Q QB D Q QB D Q QB D Q QB 1 0 1 0 1 0 1 0 EN CLK Q0 Q1 Q2 Q3 D1 D0 D3 D3
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2 3 UM EECS 270 Fall 2009 Counters Very common FSM block No physical counter can count forever – real counters are modulo counters I.e., they “count” through a finite number of states and then repeat The number of states in a counter’s sequence is called the modulus of the counter Example: A counter that counts the sequence 0, 1, 2, …, 7, 0, 1,… is a modulo 8 counter A ripple counter is an easy counter implementation - State changes ripple through the circuit - Outputs don’t change at the same time! What is the modulus of this counter ? 8 4 UM EECS 270 Fall 2009 Parallel Counter Would like all state variables to run on the same clock signal Examining the binary code yields an easy implementation: - Under what condition does each bit toggle? - When all bits of lesser significance are 1!
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