inst eecs berkeley edu cs61c CS61C Machine Structures A Hardware Description Language HDL for describing testing logic circuits Lecture 24 Verilog I 2004 10 25 text based way to talk about designs easier to simulate before silicon translate into silicon directly Lecturer PSOE Dan Garcia www cs berkeley edu ddgarcia We shut out AZ 38 0 Football team continues to roll The 7 Bears shut out Arizona in a rout McArthur had 6 catches for 94 yards breaking the school record for career yards with 2 768 Rodgers threw three first half touchdowns J J Arrington topped 100 yards for 6th consec game At 23 ASU 3 1 6 1 Sat calbears com CS 61C L24 Verilog I 1 Verilog Overview 1 3 Garcia Fall 2004 U CB No sequential execution normally hardware just runs continuously Verilog A strange version of C with some changes to account for time VHDL is alternative similar and can pick it up easily Verilog simpler to learn CS 61C L24 Verilog I 2 Verilog Overview 2 3 Verilog Overview 3 3 Verilog description composed of modules Verilog has 2 basic modes 1 Structural composition describes that structure of the hardware components including how ports of modules are connected together module Name port list Declarations and Statements endmodule Modules can have instantiations of other modules or use primitives supplied by language Note that Verilog varies from C syntax borrowing from Ada programming language at times endmodule CS 61C L24 Verilog I 3 module contents are builtin gates and or xor not nand nor xnor buf or other modules previously declared 2 Behavoral describes what should be done in a module module contents are C like assignment statements loops Garcia Fall 2004 U CB Example Structural XOR xor built in but module xor X Y Z input X Y which ports input output output Z Default is 1 bit wide data wire notX notY ports connect components XnotY YnotX notX not YnotX X notX X Y notY Y Z and X YnotX notX Y Y XnotY XnotY X notY notY or Z YnotX XnotY endmodule Note order of gates doesn t matter CS 61C L24 Verilog I 5 Garcia Fall 2004 U CB since structure determines relationship Garcia Fall 2004 U CB CS 61C L24 Verilog I 4 Garcia Fall 2004 U CB Example Behavoral XOR in Verilog module xorB X Y Z input X Y output Z reg Z always X or Y Z X Y endmodule Unusual parts of above Verilog always X or Y whenever X or Y changes do the following statement reg is only type of behavoral data that can be changed in assignment so must redeclare Z Default is single bit data types X Y Z CS 61C L24 Verilog I 6 Garcia Fall 2004 U CB Verilog replication hierarchy Example Replicated XOR in Verilog Often in hardware need many copies of an item connected together in a regular way Need way to name each copy Need way to specify how many copies Specify a module with 4 XORs using existing module example module 4xor A B C input 3 0 A B output 3 0 C xorB My4XOR 3 0 X A Y B Z C endmodule Note 1 can associate ports explicitly by name X A Y B Z C or implicitly by order as in C A B C Note 2 must give a name to new instance of xors My4XOR CS 61C L24 Verilog I 7 Garcia Fall 2004 U CB Verilog big idea Time C 3 A 2 B 2 A 2 B 2 C 2 A 1 B 1 A 1 B 1 C 1 A 0 B 0 A 0 B 0 C 0 Garcia Fall 2004 U CB Example Difference from normal prog lang is that time is part of the language part of what trying to describe is when things occur or how long things will take In both structural and behavoral Verilog determine time with n event will take place in n time units structural not 2 notX X says notX does not change until time advances 2 ns behavoral Z 2 A B says Z does not change until time advances 2 ns Default unit is nanoseconds can change CS 61C L24 Verilog I 9 CS 61C L24 Verilog I 8 A 3 B 3 A 3 B 3 Garcia Fall 2004 U CB module test stream output stream reg stream inital begin stream 0 2 stream 1 5 stream 0 3 stream 1 4 stream 0 end endmodule time 1 stream 0 CS 61C L24 Verilog I 10 2 7 10 Initial means do this code once Note Verilog uses begin end vs as in C 2 stream 1 means wait 2 ns before changing stream to 1 Output called a waveform 14 Garcia Fall 2004 U CB Testing in Verilog Testing Verilog Code above just defined a new module Create a test module that instantiates xor Need separate code to test the module just like C Java Since hardware is hard to build major emphasis on testing in HDL Testing modules called test benches in Verilog like a bench in a lab dedicated to testing Can use time to say how things change CS 61C L24 Verilog I 11 Garcia Fall 2004 U CB module testxor reg x y expected wire z xor myxor x x y y z z add testing code endmodule Syntax declare registers instantiate module CS 61C L24 Verilog I 12 Garcia Fall 2004 U CB Testing continued Testing continued Now we write code to try different inputs by assigning to registers Pound sign syntax 10 indicates code should wait simulated time 10 nanoseconds in this case Values of registers can be changed with assignment statements initial begin x 0 y 0 expected 0 10 y 1 expected 1 So far we have the xor module and a testxor module that iterates over all the inputs How do we see if it is correct 10 x 1 y 0 10 y 1 expected 0 end CS 61C L24 Verilog I 13 Garcia Fall 2004 U CB Testing continued CS 61C L24 Verilog I 14 Garcia Fall 2004 U CB Output Use monitor to watch some signals and see every time they change x 0 y 0 z 0 exp 0 time 0 initial monitor x b y b z b exp b time d x y z expected time Our code now iterates over all inputs and for each one prints out the inputs the gate output and the expected output x 0 y 1 z 1 exp 1 time 10 x 1 y 0 z 1 exp 1 time 20 x 1 y 1 z 0 exp 0 time 30 Expected value matches actual value so Verilog works time is system function gives current time CS 61C L24 Verilog I 15 Garcia Fall 2004 U CB Peer Instruction How many mistakes in this module module test X output X initial begin X 0 X 1 end end CS 61C L24 Verilog I 17 CS 61C L24 Verilog I 16 Garcia Fall 2004 U CB Peer Instruction Answer 1 1 2 2 3 …
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