Suppose we are building circuits using only the following three components:
- inverter: tcd = 0.5ns, tpd = 1.0ns, tr = tf = 0.7ns
- 2-input NAND: tcd = 0.5ns, tpd = 2.0ns, tr = tf = 1.2ns
- 2-input NOR: tcd = 0.5ns, tpd = 2.0ns, tr = tf = 1.2ns
Consider the following circuit constructed from an inverter and four 2-input NOR gates:
- What is tPD for this circuit?
- What is tCD for this circuit?
- What is tPD of the fastest equivalent circuit (i.e., one that implements the same function) built using only the three components listed above?
Solution:
- tPD for the circuit is the maximum cumulative propagation delay considering all paths from any input to any output. In this circuit, the longest path involves three 2-input NAND gates with a cummulative tPD = 6ns.
- tCD for the circuit is the minimum cumulative contamination delay considering all paths from any input to any output. In this circuit, the shortest path involves two 2-input NAND gates with a cumulative tCD = 1ns.
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The most straightforward way to determine the functionality of a circuit is to build a truth table:
A B | OUT
======|=====
0 0 | 1
0 1 | 0
1 0 | 1
1 1 | 0
from which we can see that OUT = not B. We can implement this with a single inverter that has a tPD = 1ns.
why are the rise and fall times then given???
ReplyDeletetrue, tr/tf are not needed, but i believe that filtering the relevant information out of the question is also an important task of the designer/interviewee.
ReplyDeleteWhat is contamination delay (tCD) ?
ReplyDeletehttp://en.wikipedia.org/wiki/Contamination_delay
ReplyDelete