EE121
John Wakerly
Lecture #12
PLD-based state machine structures
WITH statements
Finite-memory state-machine design
PLD-based state-machine design
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PLD-based
state-machine
timing
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Multiple PLDs
registered
pipelined
registered
pipelined
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Normal vs. pipelined Moore outputs
• Assume 3 state variables, Q1, Q2, Q3.
Z = G(Q1, Q2, Q3) [Moore output]
Z := ?? [equivalent pipelined Moore output]
• Transition equations
Q1 := F1(Q1,Q2,Q3,inputs);
Q2 := F2(Q1,Q2,Q3,inputs);
Q3 := F3(Q1,Q2,Q3,inputs);
• Pipelined output equation:
Z := G( F1(Q1,Q2,Q3,inputs), F2(Q1,Q2,Q3,inputs),
F3(Q1,Q2,Q3,inputs) );
• Can’t do this with Mealy outputs unless
“inputs” are applied one clock tick earlier.
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WITH statements
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Example: Another combination lock
• Unlock after input sequence 0110111
• Also provide “hint” output (1 => correct move).
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WITH recommendations
• If you must use WITH statements, be sure to
assign an output value for all states/transitions.
• Be very careful about registered vs.
combinational assignments.
• It’s usually more convenient to specify output
values using separate equations in the
“equations” section of the program:
UNLK = (S==X0110111) & (X==0);
HINT = (S==ZIP) & (X==0)
# (S==X0) & (X==1)
...
# (S==X0110111) & (X==0);
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Finite-Memory State Machines
• In general, a state machine’s future operation
depends on all the inputs it received since
reset.
• Some state machines depend only on the
inputs received over the last few clock ticks.
• Idea:
– Use a register to keep track of the machine’s input
values over the last few clock ticks.
– Determine the current output as a combinational
function of the past inputs history.
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Example: the combination lock again
This program has a bug
(I can unlock in just six
clock ticks after reset.)
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T-bird tail-lights example
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State
diagram
Inputs:
LEFT, RIGHT, HAZ
Outputs:
Six lamps
(function of state only
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ABEL program
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ABEL “state diagram”
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Output
logic
• Implement this table in another PLD that has
Q2, Q1, Q0 (QSTATE) as inputs.
• Total # of PLD outputs is 3 + 6 = 9.
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Output-coded state assignment
• Each state has a unique encoding using just
the lamp-output bits.
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Sunnyvale Traffic-Light Controller
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Traffic-Light I/O
• Inputs
– NSCAR (north-south car sensors)
– EWCAR (east-west car sensors)
– TMSHORT (short timer -- 5 seconds)
– TMLONG (long timer -- 5 minutes)
• Outputs
– NSRED, NSYELLOW, NSGREEN (lights)
– EWRED, EWYELLOW, EWGREEN (lights)
– TMRESET (timer start)
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State Diagram (ABEL)
• Outputs will be a function of state
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• East-west states are symmetric.
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State encoding
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Output logic
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Output-coded state assignment
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Next Time
• Digital-to-analog conversion
• Analog-to-digital conversion
• Read D-to-A and A-to-D notes
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