Gate Delays Until now, we described circuits without any delays (., zero delay). In real circuits, logic gates have delays associated with them. | Gate Delays Until now we described circuits without any delays . zero delay . In real circuits logic gates have delays associated with them. Gate delays allow the Verilog user to specify delays through the logic circuits. Pin-to-pin delays can also be specified in Verilog. They are discussed in Chapter 10 Timing and Delays. Rise Fall and Turn-off Delays There are three types of delays from the inputs to the output of a primitive gate. Rise delay The rise delay is associated with a gate output transition to a 1 from another value. Fall delay The fall delay is associated with a gate output transition to a 0 from another value. Turn-off delay The turn-off delay is associated with a gate output transition to the high impedance value z from another value. If the value changes to x the minimum of the three delays is considered. Three types of delay specifications are allowed. If only one delay is specified this value is used for all transitions. If two delays are specified they refer to the rise and fall delay values. The turn-off delay is the minimum of the two delays. If all three delays are specified they refer to rise fall and turn-off delay values. If no delays are specified the default value is zero. Examples of delay specification are shown in Example 5-10. Example 5-10 Types of Delay Specification Delay of delay_time for all transitions and delay_time a1 out i1 i2 II Rise and Fall Delay Specification. and rise_val fall_val a2 out il i2 II Rise Fall and Turn-off Delay Specification bufifO rise_val fall_val turnoff_val bl out in control Examples of delay specification are shown below. and 5 a1 out il i2 IIDelay of 5 for all transitions and 4 6 a2 out il i2 II Rise 4 Fall 6 bufifO 3 4 5 bl out in control II Rise 3 Fall 4 Turn-off 5 Min Typ Max Values Verilog provides an additional level of control for each type of delay mentioned above. For each type of delay rise fall and turn-off three values min typ and max can be specified. Any one value can .
