Unit v. HDL Synthesis Process

Dr. Sudhir N. Shelke
Ph.D
Principal, Guru Nanak Institute of
Technology, Nagpur.
Dr Sudhir Shelke Page 1 of 51

What is Synthesis?
Synthesis is the process of constructing a gate-level
netlist from a model of a circuit described in VHDL.
A synthesis program generates a RTL netlist (FF’s, ALU,
multiplexer, interconnected by wires). So, the RTL
module builder is necessary and the purpose of this
builder is to build each of the required RTL blocks from
a library of predefined components (user-specified
target technology).
After producing a gate-level netlist, a logic optimizer
reads in this netlist and optimizes the circuit for the
user-specified area and timing constraints.

1. Automatically manages many details of the design process:
• Fewer bugs
• Improves productivity
2. Abstracts the design data (HDL description) from any
particular implementation technology
• Designs can be re-synthesized targeting different chip technologies;
E.g.: first implement in FPGA then later in ASIC
3. In some cases, leads to a more optimal design than could be
achieved by manual means (e.g.: logic optimization)
Why Not Logic Synthesis?
1. May lead to less than optimal designs in some cases
Why Perform Logic Synthesis?

Variety of general and ad-hoc (special case) methods:
1. Instantiation: maintains a library of primitive modules (AND, OR,
etc.) and user defined modules.
2. “Macro expansion”/substitution: a large set of language operators
(+, -, Boolean operators, etc.) and constructs (if-else, case) expand
into special circuits.
3. Inference: special patterns are detected in the language description
and treated specially (e.g.,: inferring memory blocks from variable
declaration and read/write statements, FSM detection.
4. Logic optimization: Boolean operations are grouped and optimized
with logic minimization techniques
5. Structural reorganization: advanced techniques including sharing of
operators, and retiming of circuits (moving FFs), and others

1. A circuit can be described in
any different ways, not all of
which may be synthesizable.
This is due to the fact that
HDL was designed primarily
as simulation language and
not for synthesis.
2. There is no standardized
subset of VHDL for synthesis.
3. There is no direct object in
VHDL that means a latch or
a flip-flop,
4. therefore, each synthesis
system provide different
mechanism to model a flip-
flop or a latch.

In VHDL, a signal, or a variable declared in a process, retains its
value through the entire simulation run, thus inferring memory.
Example :
signal A, B, C, Z : bit;
…….
No _memory : process(A, B, C)
variable temp : bit;
begin
temp := A and B;
Z <= temp or C;
end process;
VHDL semantics says that variable temp retains its value through
the entire simulation run.

Resource sharing is an optimization technique that uses a
single functional block (such as an adder or comparator) to
implement several operators in the HDL code. Use resource
sharing to improve design performance by reducing the gate
count and the routing congestion.
The following operators can be shared either with instances of
the same operator or with the operator on the same line.
*
+ -
>>=<<=
For example, a + operator can be shared with instances of other
+ operators or with - operators.

ONE can implement arithmetic functions (+, -, magnitude
comparators) with gates, Synopsys Design Ware functions, or
Xilinx Design Ware functions.
Resource sharing adds additional logic levels to multiplex the
inputs to implement more than one function.
Since resource sharing allows you to reduce the number of
design resources, the device area required for your design is
also decreased. The area that is used for a shared resource
depends on the type and bit width of the shared operation.

Implementation of resource sharing Implementation of No resource sharing

VHDL is a H/W modeling language used to model digital
circuits
Digital circuits can be either Combinational or
Sequential
Combinational Logic circuits: Implement Boolean functions
whose output is only dependant on the present inputs
Sequential Logic circuits: Implement circuits whose output
depends on the present inputs & the history of the inputs. i.e.
Circuits having storage elements
Introduction
13Dr Sudhir Shelke Page 13 of 51

Synthesis tools translate the VHDL code to a gate level netlist
representing the actual H/W gates [and, or, not, Flip-Flops…et ]
Only a subset of the language is synthesizable
 A model can be either
 Synthesizable: Used for both Simulation & Synthesis
 Non-Synthesizable: Used for Simulation only
Introduction Cont..
14
VHDL
Standard
Synthesizable
VHDL

Combinational
Logic
15
library IEEE;
use IEEE.std_logic_1164.all;
Entity mux_case is
Port(a, b, c, d: in std_logic;
Sel: in std_logic_vector(1 downto 0);
F: out std_logic);
End entity;
Architecture rtl of mux_case is
begin
process (a,b,c,d,sel) is
begin
Case sel is
When "00" => f <= a;
When "01" => f <= b;
When "10" => f <= c;
When "11" => f <= d;
when others => f <= a;
End case;
End process;
End architecture;
Example 1: 4x1 Multiplexer

16
• What is the impact of removing some signals from the sensitivity list as
shown in example 2?
begin
begin
Case sel is
When "00" => f <= a;
When "01" => f <= b;
When "10" => f <= c;
When "11" => f <= d;
End case;
End process;
End architecture;
begin
process (a, sel) is
begin
Case sel is
When "00" => f <= a;
When "01" => f <= b;
When "10" => f <= c;
When "11" => f <= d;
End case;
End process;
End architecture;
Example 2: 4x1 MultiplexerExample 1: 4x1 Multiplexer

17
begin
begin
Case sel is
When "00" => f <= a;
When "01" => f <= b;
When "10" => f <= c;
When "11" => f <= d;
End case;
End process;
End architecture;
begin
process (a, sel) is
begin
Case sel is
When "00" => f <= a;
When "01" => f <= b;
When "10" => f <= c;
When "11" => f <= d;
End case;
End process;
End architecture;
Example 2: 4x1 MultiplexerExample 1: 4x1 Multiplexer
• No Impact on the synthesis results, however we will find that the
simulation results differ
• Synthesis tools don’t use the sensitivity list to determine the logic, but
simulation tools depend on the sensitivity list to execute the process
• Example 2 suffers a problem called “Simulation – Synthesis mismatch”

Combinational Logic
18
• VHDL 2008* introduced the keyword "all" that implicitly adds all read
signals to the sensitivity list to avoid “Simulation Synthesis mismatch”
begin
process (all) is
begin
Case sel is
When "00" => f <= a;
When "01" => f <= b;
When "10" => f <= c;
When "11" => f <= d;
End case;
End process;
End architecture;
Example 3
Golden rule of thumb
• To a oid Si ulatio Sy thesis is at h p o le s he odeli g
Combinational logic, add all read signals to the sensitivity list

Combinational Logic
19
LIBRARY ieee;
USE ieee.std_logic_1164.all;
USE ieee.std_logic_arith.all;
ENTITY add_sub IS
port (a, b : in integer;
result : out integer;
operation: in std_logic);
END ENTITY;
ARCHITECTURE behave OF add_sub IS
BEGIN
process (a, b, operation)
begin
if (operation = '1') then
result <= a + b;
else
result <= a - b;
end if;
end process;
END ARCHITECTURE;
Example 4: Adder-Subtractor

 Consider that someone tries to re-use that code to implement an adder with
an enable  He modifies the add_sub example; removes the else branch &
e a es the ope atio po t to e a le as sho elo , Ho ould these
changes affect the logic?
Combinational Logic
20
LIBRARY ieee;
ENTITY adder IS
enable: in std_logic);
END ENTITY adder;
ARCHITECTURE behave OF adder IS
BEGIN
process (a, b, enable)
begin
if (enable = '1') then
result <= a + b;
end if;
end process;
END ARCHITECTURE;
Example 5:

 This ill i fe a lat h, e ause e did ’t spe ify hat should happe to
esult he e a le is ’t e ual to '1'
 Si ulatio & sy thesis tools ill just keep the alue as is…i.e. It lat hes the last
value
Combinational Logic
21
LIBRARY ieee;
ENTITY adder IS
enable: in std_logic);
END ENTITY adder;
ARCHITECTURE behave OF adder IS
BEGIN
process (a, b, enable)
begin
if (enable = '1') then
result <= a + b;
end if;
end process;
END ARCHITECTURE;
Example 5:

 In the below example, the "11" value of "sel" signal is not listed as a case choice,
hence signal "F" is not assigned a value in this case
 A Latch is inferred in this example  P o a ly that as ’t eeded
Combinational Logic
22
LIBRARY ieee;
ENTITY incomplete_case IS
port(sel : std_logic_vector (1 downto 0);
A, B: std_logic;
F : out std_logic);
END ENTITY;
ARCHITECTURE rtl OF incomplete_case IS
BEGIN
process (sel, A, B)
begin
case (sel) is
when "00" =>
F <= A;
when "01" =>
F <= B;
when "10" =>
F <= A xor B;
when others => null;
end case;
end process;
END ARCHITECTURE;
Example 6:

 Do you think a Latch would be inferred in the below example?
Skills Check
23
LIBRARY ieee;
ENTITY incomplete_assignment IS
port(sel : in std_logic_vector (1 downto 0);
A, B : in std_logic;
O1, O2: out std_logic);
END ENTITY;
ARCHITECTURE rtl OF incomplete_assignment IS
BEGIN
process (sel, A, B) begin
case (sel) is
when "00" =>
O1 <= A;
O2 <= A and B;
when "01" =>
O1 <= B;
O2 <= A xor B;
when "10" =>
O1 <= A xor B;
when "11" =>
O2 <= A or B;
when others =>
O1 <= '0';
O2 <= '0';
end case;
end process;
END ARCHITECTURE;
Example 7:

 Do you think a Latch would be inferred in the below example?
Skills Check (Soln.)
24
LIBRARY ieee;
ENTITY incomplete_assignment IS
port(sel : in std_logic_vector (1 downto 0);
A, B : in std_logic;
O1, O2: out std_logic);
END ENTITY;
ARCHITECTURE rtl OF incomplete_assignment IS
BEGIN
process (sel, A, B) begin
case (sel) is
when "00" =>
O1 <= A;
O2 <= A and B;
when "01" =>
O1 <= B;
O2 <= A xor B;
when "10" =>
O1 <= A xor B;
when "11" =>
O2 <= A or B;
when others =>
O1 <= '0';
O2 <= '0';
end case;
end process;
END ARCHITECTURE;
Example 7:
• Latches are inferred for both signals "O1" & "O2"
• Though the case is complete & no "null" statement is
there, we find that "O1" & "O2" are not assigned in all
case's branches  This is alled I o plete sig al
assig e t

26
Library ieee;
use ieee.std_logic_1164.all;
Entity d_ff is
Port(d, clk, rst : in std_logic;
Q, nQ : out std_logic);
end entity;
Architecture behav of d_ff is
Begin
process(clk)
begin
If (rising_edge(clk)) then
If (rst = '1') then
Q <= '0';
nQ <= '0';
else
Q <= d;
nQ <= not (d);
end if;
end if;
end process;
end behav;
• Let's model the well known D-FF with outputs Q & nQ and see
the synthesis results
Example 8:

27
Library ieee;
Entity d_ff is
Port(d, clk, rst : in std_logic;
end entity;
Begin
process(clk)
begin
If (rst = '1') then
Q <= '0';
nQ <= '1';
else
Q <= d;
nQ <= not (d);
end if;
end if;
end process;
end behav;
Example 8:
Two Flip-Flops ?!
Change the code to have only one Flip-Flop

28
Example 9:
Yep…That's hat e a t!
Library ieee;
Entity d_ff is
Port( d, clk, rst : in std_logic;
end entity;
signal Q_int: std_logic;
Begin
process(clk)
begin
If (rst = '1') then
Q_int <= '0';
else
Q_int <= d;
end if;
end if;
end process;
Q <= Q_int;
nQ <= not (Q_int);
end behav; Dr Sudhir Shelke Page 28 of 51

Sequential Logic
29
VHDL 360 ©
Library ieee;
Entity d_ffs is
Port(d: std_logic_vector (3 downto 0);
clk, rst : in std_logic;
Q, nQ : out std_logic_vector (3 downto 0));
end entity;
Architecture behav of d_ffs is
signal Q_int: std_logic_vector (3 downto 0);
Begin
process(clk)
begin
If (rst = '1') then
Q_int <= (others => '0');
else
Q_int <= d;
end if;
end if;
end process;
Q <= Q_int;
nQ <= not (Q_int);
end behav;
• What about making an array of D-FFs?
Example 10:

7 6 5 4 3 2 1 0
Library ieee;
entity shift_register is
Port ( clk, D, enable : in STD_LOGIC;
Q : out STD_LOGIC);
end entity;
architecture Behavioral of shift_register is
begin
process(clk)
variable reg: std_logic_vector(7 downto 0);
begin
if rising_edge(clk) then
if enable = '1' then
for i in 1 to 7 loop
reg(i-1) := reg(i);
end loop;
reg(7) := d;
end if;
end if;
Q <= reg(0);
end process;
end Behavioral;
Sequential Logic
30VHDL 360 ©
Example 11: 8-bit Shift Register (Shift right)

Assignments under clock edge where the object value needs to be
remembered across multiple process invocations  Flip-Flop
 Signal assignment under clock edge will always infer a Flip-Flop
 Variable assignment under clock edge will infer Flip-Flop only when its value
ought to be remembered across process invocations
Flip-Flop Inference
31
VHDL 360 ©

LIBRARY ieee;
Entity unknown is
port(x: out std_logic;
y: in std_logic_vector(3 downto 0);
c: in integer);
End entity;
Architecture behave of unknown is
Begin
x <= y(c);
End behave;
Deduce what the below code models
Use synthesis tool to validate your answer

Power Estimation and Analysis of FPGA based
System.

 As devices get larger and faster, power
consumption goes up
 First-generation FPGAs had
 Lower performance
 Lower power requirements
 No package power concerns
 Today’s FPGAs ha e
 Much higher performance
 Higher power requirements
 Package power limit concerns
 A System Monitor that provides active monitoring of the die temperature
 Refer to the Virtex-6 User Guide for more information
Performance (MHz)
PMAX
Package Power
Limit
Real World Design
Power Consumption
High Density
Low
Density

High-speed and high-
density designs require
more power, leading to
higher junction
temperatures
Package thermal limits
exist
 125° C for plastic
 150° C for ceramic
Power directly limits
 System performance
 Design density
 Package options
 Device reliability

Estimating power
consumption is a complex
calculation
 Power consumption of an FPGA
is almost exclusively dynamic
 Power consumption is
dependent on design and is
affected by
 Output loading
 System performance
(switching frequency)
 Design density (number of
interconnects)
 Design activity (percent of
interconnects switching)
 Logic block and interconnect
structure
 Supply voltage

Power calculations can be performed at
three distinct phases of the design cycle
 Concept phase: A rough estimate of power can be
calculated based on estimates of logic capacity and
activity rates
 Use the Xilinx Power Estimator spreadsheet
 Design phase: Power can be calculated more
accurately based on detailed information about how
the design is implemented in the FPGA
 Use the XPower Analyzer
 System Integration phase: Power is calculated in a lab
environment
 Use actual instrumentation
Accurate power calculation at an early
stage in the design cycle will result in
fewer problems later

 Accurate activity rates (also known as toggle
rates) are required for meaningful power
calculations
 Clocks and input signals have an absolute
frequency
 Synchronous logic nets use a percentage
activity rate
 100% indicates that a net is expected to change state
on every clock cycle
 Allows you to adjust the primary clock frequency and
see the effect on power consumption
 Can be set globally to an average activity rate on
groups or individual nets
 Logic elements also use a percentage activity
rate
 Based on the activity rate of output signals of the logic
element
 Logic elements have capacitance

 Excel spreadsheets with power
estimation formulas built in
 Enter design data in white boxes
 Power estimates are shown in gray boxes
 Sheets
 Summary (device totals)
 Clock, Logic, I/O, Block RAMs, DSP, MMCM
 GTX, TEMAC, PCIE
 To download go to
http://www.support.xilinx.com ->
Technology Solutions -> Power
 Download the XPE spreadsheet for your device
family
 XPE is not installed with the ISE software
 The Power Solutions page has numerous
resources

Summary and Quiescent power
White boxes allow you to enter
design data
Gray boxes show you the Power
estimates
Tabs at bottom allow you to enter
power information per device
resources (not shown)
Settings reviews device, system,
and environment information
On-Chip Power breaks the
estimated power consumption into
device resources

Summary and Quiescent
power
Power Supply reviews
what power sources will
be necessary
Summary describes
your systems total
power and estimated
junction temperature

I/O power
Clock power
Logic power

Block RAM, DSP, and MMCM power

Graphs

A utility for estimating the power
consumption and junction temperature
of FPGA and CPLD devices
Reads an implemented design (NCD file)
and timing constraint data
You supply activity rates
 Clock frequencies
 Activity rates for nets, logic elements, and output
pins
 Capacitive loading on output pins
 Power supply data and ambient temperature
 Detailed design activity data from simulation (VCD
file)
The XPower Analyzer calculates the total
average power consumption and
generates a report

Expand Implement Design 
Place & Route
Double-click XPower Analyzer
to launch the XPower utility in
interactive mode
Use the Generate Power Data
process to create reports using
VCD files or TCL scripts

Estimated junction temperature
Reporting, settings, and thermal information is all placed in one utility
As you manipulate system characteristics you will update the generated report
Report Navigator allows for quick migration to various reports and functions of
the utility

Produced as a simple text file
File is given .pwr
extension
Report is more detailed
and stored in one text file
Some what-if analysis
information is included
Includes a Power
Improvement Guide

Pipeli
ning
and
Retim
ing
Adding registers along a path
 split combinational logic into multiple
cycles
 increase clock rate
 increase throughput
 increase latency

Pipeli
ning
and
Retim
ing
Delay, d, of slowest combinational stage
determines performance
 clock period = d
Throughput = 1/d : rate at which
outputs are produced
Latency = •d : number of stages *
clock period
Pipelining increases circuit utilization
Registers slow down data, synchronize
data paths
Wave-pipelining
 no pipeline registers - waves of data flow through
circuit
 relies on equal-delay circuit paths - no short paths

Pipeli
ning
and
Retim
ing
Where is the best place to add
registers?
 splitting combinational logic
 overhead of registers (propagation delay and
setup time requirements)
What about cycles in data path?
Example: 16-bit adder, add 8-bits in
each of two cycles

Unit v. HDL Synthesis Process

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Unit v. HDL Synthesis Process