Modelling Delays Using Verilog

Verilog can model delay types within its specification for gates and buffers. Parameters that can be modelled are T_rise, T_fall and T_turnoff. To add further detail, each of the three values can have minimum, typical and maximum values.

T_rise, t_fall and t_off

Delay modelling syntax follows a specific discipline;
gate_type #(t_rise, t_fall, t_off) gate_name (paramters);
When specifiying the delays it is not necessary to have all of the delay values specified. However, certain rules are followed.

and #(3) gate1 (out1, in1, in2);
When only 1 delay is specified, the value is used to represent all of the delay types, i.e. in this example, t_rise = t_fall = t_off = 3.

or #(2,3) gate2 (out2, in3, in4);
When two delays are specified, the first value represents the rise time, the second value represents the fall time. Turn off time is presumed to be 0.

buf #(1,2,3) gate3 (out3, enable, in5);
When three delays are specified, the first value represents t_rise, the second value represents t_fall and the last value the turn off time.

Min, typ and max values

The general syntax for min, typ and max delay modelling is;
gate_type #(t_rise_min:t_ris_typ:t_rise_max, t_fall_min:t_fall_typ:t_fall_max, t_off_min:t_off_typ:t_off_max) gate_name (paramteters);

Similar rules apply for th especifying order as above. If only one t_rise value is specified then this value is applied to min, typ and max. If specifying more than one number, then all 3 MUST be scpecified. It is incorrect to specify two values as the compiler does not know which of the parameters the value represents.

An example of specifying two delays;
and #(1:2:3, 4:5:6) gate1 (out1, in1, in2);
This shows all values necessary for rise and fall times and gives values for min, typ and max for both delay types.

Another acceptable alternative would be;
or #(6:3:9, 5) gate2 (out2, in3, in4);
Here, 5 represents min, typ and max for the fall time.

N.B. T_off is only applicable to tri-state logic devices, it does not apply to primitive logic gates because they cannot be turned off.