We will talk of two type of modules, the outer and inner modules, as as analogy the outer module is the E-type and the inner module is the D-type. It might be useful to take a look at the section on modules to understand this.
In an inner module inputs must always be of a net type, since values will be driven into them. In the outer module the input may be a net type or a reg.
In an inner module outputs can be of a net type or a reg. In an outer module the output must be of a net type since values will be driven into them by the inner module.
Inouts must always be of a
net
type.
EXERCISE:
Why? is this the case, the answer follows from the above sections.
When calling a module the width of each port must be the same, eg, a 4-bit register cannot be matched to a 2-bit register.
However, output ports may remain unconnected, by missing out their name in the port list. This would be useful if some outputs were for debugging purposes or if some outputs of a more general module were not required in a particular context. However input ports cannot be omitted for obvious reasons.
For example:
d_ff dff0( , d, reset, clock); // the output (q) has been omitted
// the comma is ESSENTIAL
Ports can be connected by either ordered list or by name. The ordered list method is recommended for the beginner, in this method the port list in the module instantiation is in the same order as in the module definition. See example below.
module d_ff( q, d, reset, clock);
...
endmodule
module e_ff(q, d , enable, reset, clock);
output q;
input d, enable, reset, clock;
wire inD;
...
d_ff dff0(q, inD, reset, clock);
...
endmodule
The second method is by name, when instantiating, the ports in the
definition are accompanied by the corresponding port name in the
instantiation.
EXERCISE
a) Draw a diagram for your own reference illustrating the constraints
on the input, output and inouts.
b) Using the module interfaces for the
d_ff
and the
e_ff
modules above, write code to complete them AND a toggle flip-flop:
t_ff,
based upon a call to the e_ff module. The function of the
toggle flipflop is to either change its output or to hold
its output on each new rising clock edge according to a
control signal:
toggle.
It should also have a synchronous
reset.