An Introduction to Verilog: Transitioning from VHDL

Tutorial 1

Lesson Plan (Tentative)

• Week 1: Transitioning from VHDL to Verilog, Introduction to Cryptography

• Week 2: A5 Cipher Implementaion, Transitioning from Verilog to Verilog-A

• Week 3: Verilog-A Mixer Analysis

Purpose of HDL Languages

• Simplify design so you can concentrate on the overall picture

• HDL allows description in language rather than schematic, speeding up development time

• Allows reuse of components (standard cells ie libraries)

VHDL/Verilog Differences

• Verilog modelled after C, VHDL is modelled after Ada– VHDL is more strongly typed (ie it checks the typing

more rigorously)– Verilog is case sensitive while VHDL is not

• This means that VHDL has a higher learning curve than Verilog, but with proficiency, offers more flexibility.

• Verilog used extensively in the US while VHDL is used internationally

Verilog Types

• wire, wire[msb:lsb] (single/multiple bit wire)• reg, reg[msb:lsb] (single/multiple bit

register)• integer (integer type, 32 bits)• time (unsigned 64 bit)• real (floating point double)• string

Operators

Arithmetic:• Binary +, -, *, /, % (mod)• Unary +, - (sign)

Relational• Binary <, >, <=, >=

Operators (con’t)

Equivalence Operators• === (equivalence including x and z), ==, !

== (again including x and z), !=

Bit-wise Operators• ~ (not), & (and), | (or), ^ (xor), ~^, ^~ (xnor)

Operators (con’t)

• Shift Operators <<, >>• Cocatenation {a, b, c}• Replication {n{m}} (m n times)• Conditional

cond_exp ? True_exp : false_exp

Built in gates

• Can be used without having to build• and, nand, or, nor, xor, xnor (n-input

gates), used as:and (out, in1, in2, in3, …)

• buf, bufif1, bufif0, not, notif1, notif0 (transmission gates/tristates)

Entity Instantiation• No equivalent of architecture keyword in Verilog (all is

treated as one architecture)

VHDL: entity myentity isport ( a, b : in std_logic;

c : out std_logic);end myentity;Architecture implementation of myentity

--do stuffend implementation;

Verilog: module myentity (a, b, c); input a, b;output c;wire a, b, c;

//do stuffendmodule

Component Instantiation

• Verilog doesn’t require explicit instantiationVHDL: component DFF

port( d : in std_logic;q : out std_logic);

end component;MyDff : DFF

Port map (foo, bar); --implicitPort map (d => foo, q => bar);--

explicitVerilog: dff u0 (foo, bar); //implicit

dff u0 (.foo(d), .bar(q)); //explicit

Combinational Assignment

• Note that gate level primitives in Verilog can be a function or bitwise symbol

VHDL: a <= b;a <= b AND c;a <= b + c;

Verilog: a <= b; //parallel assignment

and(a, b, c); //a is outputa <= b & c; //same as line above{carry, sum} <= b + c; //carry is

//optional

Flow Logic (I): Case• Differentiation in VHDL for select statements and

conditional assignment; none in VerilogVHDL: With std_logic_vector’(A) select

Q <= “1” when “0”<= “0” when “1”<= “0” when others;

• VHDL can also use case statements similar to VerilogVerilog: case (A)

0 : Q <= 1; 1 : Q <= 0; default : Q <= 0;

endcase

Flow Logic (II): If/Then/Else

• Equivalence operators similar to C in VerilogVHDL: Q <= ‘1’ when A = ‘1’ else

‘0’ when B = ‘1’ else‘0’;

Verilog: if (A == 1) beginQ <= 1;

end else if (B == 1) beginQ <= 0;

end else beginQ <= 0;

end

Combinational Processes

VHDL: process (a)begin

b <= a;end process;

Verilog: always @ (a)begin

b <= a;end

Sequential Logic• Example: D Flip Flop: (note edge triggering is in

always statement in Verilog)VHDL: process(clk)

beginif rising_edge(clk)

q <= d;end if;

End process;Verilog:always @ (posedge clk)

beginq <= d;

end

Test Benches

• Useful for simulation and verification• Keyword “initial begin .. end” starts a

testbench– Remember “always begin .. end”

• #delay is used to delay by delay ‘time units’

• $display(“Random text”) displays Random text

• `timescale sets the ‘time unit’ unit

Test Benches

• $monitor (“Clk %b, Reg %d”, clk, reg) displays clock and register variables as binary and decimal respectively

• #delay $finish runs the testbench for delay ‘time units’

4-bit Counter Code//Asynchronous Reset 4-bit//Countermodule counter (clk, reset,

enable, count);input clk, reset, enable;wire clk, reset, enable;

output count;reg[3:0] count;

always @ (reset) //change to //posedge clk for sync reset

beginif (reset == 1) begin

count <= 0;end

end

always @ (posedge clk)begin

if (enable == 1) begincount <= count + 1;

endend

endmodule

Testbench for Counter• Example:`timescale 1ns/1ns

module counter_tb; reg clk, reset, enable;wire [3:0] count;

counter U0 ( .clk (clk), .reset (reset), .enable (enable), .count (count) );

initial beginclk = 0; reset = 0; enable = 0; end

always #5 clk = !clk; //flip clock every 5

ns

initial begin $dumpfile ( "counter.vcd" ); $dumpvars;

end

initial begin$display( "\t\ttime,\tclk,\treset,\tenable,\tcount" ); $monitor( "%d,\t%b,\t%b,\t%b,\t%d" ,$time, clk,reset,enable,count);

end

initial #100 $finish; //ends after 100 ns

endmodule

Introduction to Cryptography

Tutorial 1 Part 2

Cryptography is Everywhere

• Internet (online banking, e-commerce sites)

• Cell phones (GSM and CDMA)• Other wireless devices (PDAs and laptops

with wireless cards)• RFIDs, sensors

Why Hardware Design is important in Cryptography

• Many cryptographic applications are hardware-based (cell phones, RFID, wireless router technology, electronic key dongles)

• Importance of hardware design knowledge to efficiently implement applications in cryptography

Scenario

O

A B

A wants to communicate with B, however O can listen in on the conversation. How to prevent this?

Types of Cryptographic Ciphers• A cipher is a way of making messages

unreadable• Two main categories: public key and

private/symmetric key• Public key uses different keys for encryption and

decryption (which means not having to exchange keys in person), but is computationally expensive

• Symmetric key uses the same key for encryption and decryption, but is relatively cheap in terms of computation power

Types of Cryptographic Ciphers

• Examples of Public Key Cryptography: RSA, Diffie-Hellman key exchange, ECC (Elliptic Curve Cryptography)

• Examples of Symmetric Key Cryptography: RC4 (used in WEP), A5 (used in GSM), DES and Triple-DES (used in banking applications)

Linear Feedback Shift Registers (LFSRs)

• Used in many stream ciphers (a subset of symmetric key ciphers that outputs 1 bit at a time; they’re quick, but less secure than block ciphers)

• Consists of a chain of D Flip Flops and a set of XORs that feed later bits back into the beginning

Example

Practical Example of a cipher: A5

• Used in GSM communications as an encryption scheme

• Consists of 3 LFSRs combined to produce a single stream of bits

• Details were kept secret; people reverse-engineered the cipher to produce the structure

• There have been many attacks since it was reverse engineered and is now considered broken

Diagram of A5-1

Summary

• Cryptography is used in many everyday applications

• Hardware knowledge important to implement efficient ciphers

• LFSRs important to implement stream ciphers

• A5-1 (GSM) an application of LFSRs