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56 lines
1.6 KiB
Python
56 lines
1.6 KiB
Python
from myhdl import *
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ACTIVE = 0
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DirType = enum('RIGHT', 'LEFT')
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def jc2(goLeft, goRight, stop, clk, q):
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""" A bi-directional 4-bit Johnson counter with stop control.
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I/O pins:
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--------
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clk : input free-running slow clock
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goLeft : input signal to shift left (active-low switch)
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goRight : input signal to shift right (active-low switch)
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stop : input signal to stop counting (active-low switch)
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q : 4-bit counter output (active-low LEDs; q[0] is right-most)
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Operation:
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---------
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The counter is triggered on the rising edge of the clock (clk).
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A low pulse on the goLeft input will cause the counter to start
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shifting left from its current state. A low pulse on the goRight
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input will cause the counter to start shifting right from its
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current state. A low pulse on the stop input will cause the
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counter to hold its current state until goLeft or goRight is pulsed.
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After power-up, the counter is stopped with all outputs low (LEDs lit).
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"""
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dir = Signal(DirType.LEFT)
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run = Signal(False)
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@always(clk.posedge)
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def logic():
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# direction
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if goRight == ACTIVE:
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dir.next = DirType.RIGHT
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run.next = True
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elif goLeft == ACTIVE:
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dir.next = DirType.LEFT
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run.next = True
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# stop
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if stop == ACTIVE:
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run.next = False
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# counter action
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if run:
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if dir == DirType.LEFT:
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q.next[4:1] = q[3:]
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q.next[0] = not q[3]
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else:
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q.next[3:] = q[4:1]
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q.next[3] = not q[0]
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return logic
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