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**An Introduction to Verilog: Transitioning from VHDL**

Tutorial 1 An Introduction to Verilog: Transitioning from VHDL

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**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

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**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)

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**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

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**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

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

Relational Binary <, >, <=, >=

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**Operators (con’t) Equivalence Operators**

=== (equivalence including x and z), ==, !== (again including x and z), != Bit-wise Operators ~ (not), & (and), | (or), ^ (xor), ~^, ^~ (xnor)

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**Operators (con’t) Shift Operators <<, >>**

Cocatenation {a, b, c} Replication {n{m}} (m n times) Conditional cond_exp ? True_exp : false_exp

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**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)

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Entity Instantiation No equivalent of architecture keyword in Verilog (all is treated as one architecture) VHDL: entity myentity is port ( a, b : in std_logic; c : out std_logic); end myentity; Architecture implementation of myentity --do stuff end implementation; Verilog: module myentity (a, b, c); input a, b; output c; wire a, b, c; //do stuff endmodule

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**Component Instantiation**

Verilog doesn’t require explicit instantiation VHDL: component DFF port( d : in std_logic; q : out std_logic); end component; MyDff : DFF Port map (foo, bar); --implicit Port map (d => foo, q => bar);--explicit Verilog: dff u0 (foo, bar); //implicit dff u0 (.foo(d), .bar(q)); //explicit

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**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 output a <= b & c; //same as line above {carry, sum} <= b + c; //carry is //optional

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Flow Logic (I): Case Differentiation in VHDL for select statements and conditional assignment; none in Verilog VHDL: With std_logic_vector’(A) select Q <= “1” when “0” <= “0” when “1” <= “0” when others; VHDL can also use case statements similar to Verilog Verilog: case (A) 0 : Q <= 1; 1 : Q <= 0; default : Q <= 0; endcase

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**Flow Logic (II): If/Then/Else**

Equivalence operators similar to C in Verilog VHDL: Q <= ‘1’ when A = ‘1’ else ‘0’ when B = ‘1’ else ‘0’; Verilog: if (A == 1) begin Q <= 1; end else if (B == 1) begin Q <= 0; end else begin end

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**Combinational Processes**

VHDL: process (a) begin b <= a; end process; Verilog: (a) end

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Sequential Logic Example: D Flip Flop: (note edge triggering is in always statement in Verilog) VHDL: process(clk) begin if rising_edge(clk) q <= d; end if; End process; Verilog: (posedge clk) end

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**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

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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’

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**4-bit Counter Code //Asynchronous Reset 4-bit //Counter**

module counter (clk, reset, enable, count); input clk, reset, enable; wire clk, reset, enable; output count; reg[3:0] count; (reset) //change to //posedge clk for sync reset begin if (reset == 1) begin count <= 0; end (posedge clk) begin if (enable == 1) begin count <= count + 1; end endmodule

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**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 begin clk = 0; reset = 0; enable = 0; end always #5 clk = !clk; //flip clock every 5 ns initial begin $dumpfile ( "counter.vcd" ); $dumpvars; end $display( "\t\ttime,\tclk,\treset,\tenable,\tcount" ); $monitor( "%d,\t%b,\t%b,\t%b,\t%d" ,$time, clk,reset,enable,count); initial #100 $finish; //ends after 100 ns endmodule

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**Introduction to Cryptography**

Tutorial 1 Part 2 Introduction to Cryptography

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**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

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**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

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

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**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

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**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)

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**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

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Example

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**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

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Diagram of A5-1

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**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

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