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LEC-09 - 09 Latches and Flip-flops Latches and Flip-flops...

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09 Latches and Flip-flops 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 1 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 1 Latches and Flip-flops Kenneth Short 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 2 Focus Memory elements can be described behaviorally or instantiated structurally from a device specific library The focus here is on behavioral descriptions of memory elements VHDL descriptions for latches and flip-flops, are presented
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09 Latches and Flip-flops 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 2 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 3 Sequential System A sequential system’s outputs depend not only on its present input values, but also on the past sequence (history) of its input values The history of a sequential system’s input values is characterized by a binary state value stored in its memory elements The output of a sequential system is a function of its present input values and present state 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 4 Synchronous vs. Asynchronous A sequential system may be asynchronous or synchronous The state of an asynchronous sequential system can change value as soon as any of its inputs changes value The state of a synchronous sequential system can change value only in response to a clock pulse. A sequential system’s inputs are sampled only at those times marked by the occurrence of a clock pulse The values of the inputs at each clock pulse define the system’s input sequence The vast majority of sequential systems are synchronous and our interest is limited to systems of that type
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09 Latches and Flip-flops 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 3 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 5 Memory Element A single memory element is, by itself, a simple sequential system that stores 1 bit of information Has two stable states, corresponding to its storing a 0 or a 1 When it stores a 0, it is said to be clear or reset. When it stores a 1, it is said to be set Has an output that can be read to determine its present state Has inputs that allow it to be placed in either of its states 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 6 Synchronous inputs Synchronous inputs, can change the memory element’s state only in response to the occurrence of a clock pulse at the memory element’s clock input Synchronous inputs can change value until the next clock pulse without the memory element’s state changing
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09 Latches and Flip-flops 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 4 3/9/2009 © Copyright 2003, 2006 Kenneth L. Short 7 Asynchronous Inputs A memory element may also have asynchronous inputs Asynchronous inputs allow a memory element to be forced into a desired state independently of it’s clock or synchronous inputs Asynchronous inputs are typically used to place a memory element in a known initial state during a system’s power-on reset. This is necessary because the initial state of a latch or flip-flop after power-on may be either not known or not the desired state.
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