Synthesis of Sequential Logic

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ELEN 468
Advanced Logic Design
Lecture 14
Synthesis of Sequential Logic
ELEN 468 Lecture 14
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Synthesis of Sequential Logic: General
Event control of a cyclic behavior must
be synchronized to a single edge of a
single clock
always @ ( posedge clock )
Different behaviors may be
synchronized to



different clocks
or different edges of a clock
but clock periods should be same
ELEN 468 Lecture 14
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Options for Implementing
Sequential Logic
User-defined primitive
Behavior with timing controls
Instantiated library register cell
Instantiated module
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Commonly Synthesized
Sequential Logic
Data register
Transparent latch
Shift register
Binary counter
Finite state machine
Pulse generator
Clock generator
Parallel/serial converter
……
Table 9.2, page 346
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Synthesis of Sequential UDPs
Only one synchronizing signal


Clock level – latch
Clock signal edge – flip-flop
A synthesis tool may have its own
requirement

For example, may constrain the order of
rows – asynchronous first
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Example of Sequential UDP
primitive d_flop( q, clock, d );
clock
output
q;
input
clock, d;
d
q
d_flop
reg
q;
table
// clock d
state
q/next_state
(01) 0
:
?
:
0;
// Parentheses indicate signal transition
(01) 1
:
?
:
1;
// Rising clock edge
(0?) 1
:
1
:
1;
(0?) 0
:
0
:
0;
(?0) ?
:
?
:
-;
// Falling clock edge
?
(??) :
?
:
-;
// Steady clock
endtable
endprimitive
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Synthesis of Latches
Latches are incurred at

Incompletely specified input conditions for
 Continuous assignment -> mux with feedback
 Edge-triggered cyclic behavior -> gated
datapath with register
 Level-sensitive cyclic behavior -> latch

Feedback loop
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Latch Resulted from Unspecified
Input State
module myMux( y, selA, selB, a, b );
input selA, selB, a, b;
output y;
reg y;
always @ ( selA or selB or a or b )
case ( {selA, selB} )
2’b10: y = a;
2’b01: y = b;
endcase
endmodule
b
selA’
selB
selA
selB’
en
y
latch
a
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Latch Resulted from Feedback Loop
module latch1 ( out, in, enable );
input in, enable,
output out;
reg out;
enable
in
always @ ( enable )
begin
if ( enable )
assign out = in;
else
assign out = out;
end
endmodule
ELEN 468 Lecture 14
mux
out
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Synthesis of Edge-triggered Flip-flops
A register variable in a behavior might
be synthesized as a flip-flop if



It is referenced outside the scope of the
behavior
Referenced within the behavior before it is
assigned value
Assigned value in only some branches of
the activity
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Event Control Sensitive to
Multiple Signal Edges
module DReg ( out, in, clock, reset );
input in, clock, reset;
output out;
register out;
always @ ( posedge clock or posedge reset )
begin
if ( reset == 1’b1 ) out = 0;
else out = in;
end
endmodule
• The decoding of signals immediately after the event control tells the
synthesis tool how to distinguish control signals from synchronizing signal
• The control signals must be decoded explicitly in the branches of the if
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Registered Combinational Logic
module reg_and ( y, a, b, c, clk );
input a, b, c, clk;
output y;
reg y;
always @ ( posedge clk )
y = a & b & c;
endmodule
clk
b
ELEN 468 Lecture 14
a
y
c
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Shift Register
module shift4 ( out, in, clock, reset );
input in, clock, reset;
output out;
reg [3:0] data_reg;
assign out = data_reg[0];
Figure 9.10, page 361
always @ ( negedge reset or posedge clock )
begin
if ( reset == 1’b0 ) data_reg = 4’b0;
else data_reg = { in, data_reg[3:1] };
end
endmodule
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Counter
module ripple_counter ( count, clock, toggle, reset );
input clock, toggle, reset; output [3:0] count;
reg [3:0] count; wire c0, c1, c2;
assign c0 = count[0]; assign c1 = count[1]; assign c2 = count[2];
always @ ( posedge reset or posedge clock )
if ( reset == 1’b1 ) count[0] = 1’b0;
else if ( toggle == 1’b1 ) count[0] = ~count[0];
always @ ( posedge reset or negedge c0 )
if ( reset == 1’b1 ) count[1] = 1’b0;
else if ( toggle == 1’b1 ) count[1] = ~count[1];
always @ ( posedge reset or negedge c1 )
if ( reset == 1’b1 ) count[2] = 1’b0;
else if ( toggle == 1’b1 ) count[2] = ~count[2];
always @ ( posedge reset or negedge c2 )
if ( reset == 1’b1 ) count[3] = 1’b0;
else if ( toggle == 1’b1 ) count[3] = ~count[3];
endmodule
ELEN 468 Lecture 14
Fig. 9.14
Page 366
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Synthesis of Explicit Finite State
Machines
A behavior describing the synchronous activity may
contain only one clock-synchronized event control
expression
There is always one and only one explicitly declared
state register
State register must be assigned value as an
aggregate, bit select and part select assignments to
state register is not allowed
Asynchronous control signals must be scalars in the
event control expression of behavior
Value assigned to state register must be constant or
a variable that evaluates to a constant
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Comparison of Explicit and
Implicit FSMs
Explicit FSM
Implicit FSM
Explicit State
Register
State Encoding
Yes
No
Yes
No
Sequence of
States
Specified
Implicit
Sequence Control Explicit assignment
to state register
ELEN 468 Lecture 14
Specified by
procedural flow
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State Encoding Example
#
0
1
2
3
4
5
6
7
Binary
000
001
010
011
100
101
110
111
Same number of bits as binary
Gray
000
001
011
010
110
111
101
100
Johnson
0000
0001
0011
0111
1111
1110
1100
1000
One-hot
00000001
00000010
00000100
00001000
00010000
00100000
01000000
10000000
Two adjacent codes only differ by one bit
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State Encoding
A state machine having N states will require
at least log2N bits register to store the
encoded representation of states
Binary and Gray encoding use the minimum
number of bits for state register
Gray and Johnson code:

Two adjacent codes differ by only one bit
 Reduce simultaneous switching


Reduce crosstalk
Reduce glitch
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One-hot Encoding
Employ one bit register for each state
Less combinational logic to decode
Consume greater area, does not matter for certain
hardware such as FPGA
Easier for design, friendly to incremental change
case and if statement may give different result
for one-hot encoding
Runs faster
‘define state_0 3’b001
‘define state_1 3’b010
‘define state_2 3’b100
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Rules for Implicit State Machines
Synchronizing signals have to be aligned to the same clock edge in
an implicit FSM, the following Verilog code will not synthesize
……
always @ ( posedge clock );
begin
a <= b;
c <= d;
@( negedge clock )
begin
e <= f;
g <= h;
end
end
……
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Resets
Strongly recommended that every
sequential circuit has a reset signal
Avoid uncertain initial states
Specification for output under reset
should be complete, otherwise wasted
logic might be generated
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Gated Clock
Pro: reduce power consumption
Con: unintentional skew
data
clock
Q
flip-flop
clock_enable
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Design Partitions
Partition cells such that connections
between partitions is minimum
1
a
1
b
b
2
3
3
c
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a
2
c
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Example: Sequence Detector
Single bit serial input

Synchronized to falling edge of clock
Single bit output


Assert if two or more successive 0 or 1 at input
Active on rising edge of clock
Clock
Input
Output
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State Transition Diagram
State0
Start state
1/0
0/0
0/1
State1
Input 0
1/1
1/0
0/0
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State2
Input 1
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