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The MyHDL manual |
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Previous: 3.2.1.1 Template Up: 3.2.1
Combinatorial logic Next: 3.2.2
Sequential logic
3.2.1.2 Example
The following is an example of a combinatorial
multiplexer:
def mux(z, a, b, sel):
""" Multiplexer.
z -- mux output
a, b -- data inputs
sel -- control input: select a if asserted, otherwise b
"""
while 1:
yield a, b, sel
if sel == 1:
z.next = a
else:
z.next = b
Alternatively, it could be written as follows,
using always_comb():
def mux(z, a, b, sel):
def muxlogic():
if sel == 1:
z.next = a
else:
z.next = b
return always_comb(muxlogic)
To verify, let's simulate this logic with some
random patterns. The random module in Python's standard library comes in handy for
such purposes. The function randrange(n)
returns a random natural integer smaller than n. It is used in the
test bench code to produce random input values:
from random import randrange
(z, a, b, sel) = [Signal(0) for i in range(4)]
MUX_1 = mux(z, a, b, sel)
def test():
print "z a b sel"
for i in range(8):
a.next, b.next, sel.next = randrange(8), randrange(8), randrange(2)
yield delay(10)
print "%s %s %s %s" % (z, a, b, sel)
Simulation(MUX_1, test()).run()
Because of the randomness, the simulation output
varies between runs 3.2. One
particular run produced the following output:
% python mux.py
z a b sel
6 6 1 1
7 7 1 1
7 3 7 0
1 2 1 0
7 7 5 1
4 7 4 0
4 0 4 0
3 3 5 1
StopSimulation: No more events
###Ex1
Footnotes
It also possible to have a reproducible random output, by explicitly
providing a seed value. See the
documentation of the random module.
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The MyHDL manual |
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Previous: 3.2.1.1 Template Up: 3.2.1
Combinatorial logic Next: 3.2.2
Sequential logic
Release
0.4, documentation updated on February 4, 2004.
