Interesting problem – Latches in series


Problem: 100 latches (either all positive or all negative) are placed in series (figure 1). How many cycles of latency will it introduce?

This figure shows 100 negative level-sensitive latches connected together in a chain
Figure 1 : 100 negative level-sensitive latches in series
As we know, setup check between latches of same polarity (both positive or negative) is zero cycle with half cycle of time borrow allowed as shown in figure 2 below for negative level-sensitive latches:

Setup check between two latches of same polarity is zero cycle with half cycle of time borrow allowed.
Figure 2: Setup check between two negative level-sensitive latches

So, if there are a number of same polarity latches, all will form zero cycle setup check with the next latch; resulting in overall zero cycle phase shift.

As is shown in figure 3, all the latches in series are borrowing time, but allowing any actual phase shift to happen. If we have a design with all latches, there cannot be a next state calculation if all the latches are either positive level-sensitive or negative level-sensitive. In other words, for state-machine implementation, there should not be latches of same polarity in series.

Each latch will form a zero cycle setup check with the following latch, resulting in overall zero cycle phase shift.
Figure 3 : Timing for 100 latches in series


Hope you’ve found this post useful. Let us know what you think in the comments.

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2 comments:

  1. Can you elaborate this concept further as it is somewhat vague to understand this line " with half cycle of time borrow allowed as shown in figure 2 "

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    1. Hi

      I recommend you to go through following post. We can discuss in case of any further queries.

      https://vlsiuniverse.blogspot.com/2016/08/time-borrowing-in-latches.html

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