In this example, we show a model for a simple 2-bit counter which
counts clock pulses. The component has clock as an input, and
two outputs which represent the LSB and MSB of a two-bit unsigned
number.
There are several constructs that you may not have seen before.
bit'val(count_value mod 2) is a function which returns a value
of type bit. count_value is a natural number (i.e. an integer
greater than, or equal to, zero). count_value mod 2 returns
the LSB value of the counter value, but the LSB value is of type
natural. Since we want the LSB to be of type bit instead, we cast it
by using the 'val (read as "tic val") attribute on the type bit.
Another possibly new concept shown here is generic. A generic
is like a port except it is treated as a constant in the
architecture. Thus, prop_delay is a constant which can be set
by the modeler when the entity count2 is instantiated as a
component. If the modeler decides not to explicitly set the value of
prop_delay, its value defaults to 10 nanoseconds.
