5 Propagation Delay Effects

The treatment of combinational circuits was largely restricted to the logical behaviour. To describe the circuits a truth table is defined or a K-Map is designed or simplified. After that the next step is the implementation of the digital circuit in the preferred structure (e.g. NAND or NOR structure).

After the circuit realization a verification can be done, applying previously defined bit patterns to the circuit inputs and comparing the output values with the truth table.

In an error-free setup the truth table should be confirmed, provided that all signals at the gate outputs are in their final (equilibrium) steady state. A steady state will be reached after all signal waves caused by the input signal changes have completely passed the circuit. However, during this propagation time intermediate values could be produced as output information, that do not correspond to the truth (value) table. Obviously the registration of these values has to be avoided.

A simple nonelectronic example may explain this situation:

A digital thermometer may be used to measure a temperature. The visual indication of new values is done on a connected display always updating information from left to right.

Assuming a change of the indicated value for instance from T = 39ºC to 40ºC, an intermediate value T = 49ºC is produced for a short time. Therefore the indicated measuring sequence will be the following:

39ºC 49ºC 40ºC .

When in an industrial system an alarm is triggered as soon as a maximum limit is exceeded (e.g. 45ºC), this would happen in this case if not special measures would be taken to avoid the faulty intermediate value.

Similar errors occur frequently in analog-digital systems (e.g. during the conversion from analog to digital values) and of course also in purely digital operations.