3.7. Types of Functions and Delays
Trivial Functions, one or zero input
- GND, Vcc, Buffer, and Inverter
We use tp to refer to propagation delay through a gate.
Simple Functions contain one sum or one product but may be complemented
- For example:
- Simple functions may be one or two-level depending on the complements.
- To find the delay through a gate you need to refer to the part’s specification from the
manufacturer.
- For example a 74LS02 (NR gate) for R L = 2Kohms and C L = 15 pF:
(a) Maximum propagation delay time Low to High Level output tPLH = 13 ns
(b) Maximum propagation delay time High to Low Level output tPHL = 10 ns
(c) Sometime we use the average worst case propagation delay time
tsu=( tPLH + tPHL )/2
(d) To find the maximum delay through a cascading circuit, tp of each component in the
path must be added to find the total worst case delay through the circuit.
Complex Functions contain multiple levels of sums and/or products
- The delay for each path needs to be calculated by adding the propagation delay, tp, for each
component in the path. For example
If the tp is the time delay for each component then:
* delay from input A to output F is 3*tp.
* delay from input C to output F is 4*tp.
- For a multi-output complex function, at times there is an opportunity to share product terms
among the output’s to lower Literal Count (LC) and, therefore, reducing the number of gates.
A
F
C
