Team ANDAn introduction to FPGAs & VHDL

Stéphane Domas

Introduction

FPGAs• architecture principles,• an example : the spartan 3,• advantages and drawbacks for scientific computing.

VHDL• syntax and semantic,• programming and simulation tools.

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FPGAs

General architecture

• based on SRAM cells (Static RandomAccess Memory),

• composed of logic blocs (CLBs),• mesh to interconnect blocs,• I/O cells,• sometimes specialized blocs : RAM,

clock generation, multipliers, ...• the whole configurable at will.

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FPGAs

Logical blocks

• Architecture :• one or several Look-Up Table (LUT),• multiplexers,• one or several output flip-flops (or latches).

• Functionalities :• LUTs⇒ simple logical functions on few bits,• multiplexers⇒ combine output of LUTs,• latches⇒ 1 bit memories.

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FPGAs

Mesh• Manhattan type (as a grid or a torus),• routing matrix at each cross,

• several channels :• between direct neighbors,• between close neighbors,• sometimes between n-neighbors,• some dedicated to distribute clock signals.• . . .

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FPGAs

Inputs/Outputs

• several types :• power supply,• external clock,• FPGA initialization and programming,• general purpose.

• configurable I/O cells⇒ adaptation to outside environment (e.g.different logic levels)

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FPGAs

abstraction

• state machine :• CLB = vertex with n inputs and m outputs of 1 bit,• interconnection mesh = edges,

• particularities :• considering the physical time to propagate a signal over an edge,

• vertices evolving synchronously or asynchronously⇒ clock signal distributed to all/some CLBs.

• possible to delay an output of a CLB,⇒ store state of several 1 bit signals in a "register" thanks toflip-flops (type D)

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Spartan 3

• the base : CLBs, IO cells,• bonus : RAM and multipliers (18 bits),• DCM = Digital Clock Manager (clock generation, phase

shift, ...)

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Spartan 3

• 4 slices per CLB,• LUTs can be used as RAM or shift-register.

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Spartan 3

version 3A

version 3A-DSP

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For scientific computing

Advantages• natural parallelism,• natural task pipelining,• dynamic and fast reconfiguration,• easy to integrate within an embedded board

Drawbacks• extremely low level programming,• non imperative algorithmic,• binary logic⇒ computation with integer values (no floating point

operations),• low clock frequency (compared to classic processors),• "big" FPGAs costly,• development/production environments not free.

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VHDL : principles

• VHDL = Very High Description Language,• provides an abstraction of the hardware,• based on functional description of blocs with n inputs and

m outputs (called ports),• hierarchical aggregation of blocs,• each bloc contains "instructions" to manipulate values

called signals,• signal = aggregation of n physical signals of 1 bit.

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VHDL : syntax

to create a block

1. description of I/O ports and variables (entity),

2. description of its structure and its code (architecture).

entity clockgen isgeneric ( Tps : Time );port ( phase : out std_logic );

end entity clockgen;architecture rtl of clockgen is

...

end architecture rtl;

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VHDL : syntax

content of architecture

1. sub-blocks declaration,

2. signal declaration,

3. sub-blocks instantiation,

4. code.

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VHDL : syntax

Example :architecture rtl of div_tb is

component clockgengeneric ( Tps : Time );port ( phase : out std_logic );

end component;...signal ext_clk : std_logic;...

beginclkgen0 : clockgen

generic map ( Tps => 5ns )port map ( phase => ext_clk );

...

end architecture rtl;

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VHDL : syntax

the "code"• assignment :signal <= expression

• expression =

• composition of signals using logic, relational or arithmeticoperators,

• cast of types (e.g. std_logic_vector→ signed),• conditional assignment.

valsign <= val(3);

valabs <= "0000" & unsigned(val) when (valsign = ’0’)

else "0000" & unsigned(-val);

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VHDL : syntax

the "code" : two execution contexts

• outside process : concurrent execution⇒ continuous assignments (i.e. follow physical changes).

• within process : sequential execution,⇒ real assignments are effective at the process end.

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VHDL : syntax

example outside processentity setreset is

port(s,r : in bit; q,qb : out bit);end setreset;

architecture rtl of setreset issignal sq,sqb : bit;

beginq <= sq; - assignment 1qb <= sqb; - assignment 2sq <= sqb NOR r; - assignment 3sqb <= sq NOR s; - assignment 4

end architecture rtl;

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VHDL : syntax

process principles :

• sequential execution within the process,• sensibility list = parameters of the process,• any change in the state of a parameter⇒ start the process,• control instructions allowed (if, case, ...),• inner variables with immediate assignment allowed,• a signal can be assigned in a single process,• a signal can be read by several process,

• special case : a process with the clock as a parameter.⇒ the state of all signals are stored in registers, and thus delayed by aclock cycle.

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VHDL : syntax

example of an asynchronous process : subtraction/comparison

signal a : unsigned(3 downto 0);signal b : unsigned(3 downto 0);signal sum : unsigned(3 downto 0);signal cmp : std_logic;...diffcmp : process (a,b)variable s : signed(3 downto 0);begin

s := a - b;sum <= s; - totally uselesscmp <= ’1’;if a < b then

cmp <= ’0’;s := -s;

end if;sum <= s;

end process;

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VHDL : syntax

example of a synchronous process : two cycles delay

signal s : std_logic_vector(2 downto 0);signal sdly : std_logic_vector(2 downto 0);signal r1 : std_logic_vector(2 downto 0);signal r2 : std_logic_vector(2 downto 0);...dly2cy : process (clk)

if rising_edge(clk) thenr1 <= s;r2 <= r1;

end if;end process;sdly <= r2;

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VHDL : syntax

example of mixed process : delay + asynchronous reset

signal s : std_logic_vector(2 downto 0);signal sdly : std_logic_vector(2 downto 0);signal r1 : std_logic_vector(2 downto 0);signal r2 : std_logic_vector(2 downto 0);...dly2cyraz : process (clk,reset)

if reset = ’1’ thenr1 <= "000";r2 <= "000";

elsif rising_edge(clk) thenr1 <= s;r2 <= r1;

end if;end process;sdly <= r2;

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VHDL : coding

Solutions

• "by hand" in text files :• long and extremely tedious,• a lot of syntax mistakes.

• automatic generation (simulink, HDL code, coregen, ...)• design made with drawing boxes,• each box has a fixed functionality or can contain matlab

instructions,

• costly tools,• code need modification to be usable on real FPGAs.

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VHDL : coding

Simulink example

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VHDL : simulation

Solutions

• GPL solution : ghdl and gtkwave• need to write a VHDL code called testbench) that :

I instantiates the design + a clock generator if needed,I defines a process to generate input signals for the design.

• VHDL compilation (via ghdl)⇒ syntax checking,• creation of an executable to simulate the behavior of the design,• this executable produces traces of the signals,• display and analysis of these traces via gtkwave.

• paying solutions (matlab + modelsim)• automatic generation of the testbench,• co-simulation,• integrated trace visualization.

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VHDL : simulation

Visualization example : a divider

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VHDL : execution

Principles

• microcode, called bitstream, generation via a placement/routing tool(ise),

• uploading the bitstream (via USB, PCI, ...),• "execution" starts at the end of the upload.

• reading/writing values through I/O pins :• with external peripherals,• with another FPGA or processor,• with USB/PCI/Ethernet/... controllers.

• on some FPGA boards :• only a part of the bitstream can be uploaded,• bitstream stored in a flash memory and automatically loaded when

powering the board

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