FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Verilog for sequential machines

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Verilog always statement n Use always to wait for clock edge: clock) // start execution at the clock edge begin // insert combinational logic here end

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Verilog state machine clock) // start execution at the clock edge begin if (rst == 1) begin // reset code end else begin // state machine case (state) ‘state0: begin o1 = 0; state = ‘state1; end ‘state1: begin if (i1) o1 = 1; else o1 = 0; state = ‘state0; endcase end // state machine end

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Traffic light controller n Intersection of two roads: –highway (busy); –farm (not busy). n Want to give green light to highway –as much as possible. n Want to give green to farm –when needed. n Must always have –at least one red light.

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Traffic light system highway farm road sensor traffic light

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR System operation n Sensor on farm road –indicates when cars on farm road are waiting for green light. n Must obey –required lengths for green, yellow lights.

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Traffic light machine n Build controller out of two machines: –sequencer which sets colors of lights, etc. –timer which is used to control durations of lights. n Separate counter isolates logical design from clock period. n Separate counter greatly reduces number of states in sequencer.

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Sequencer state transition graph hwy- green farm- green hwy- yellow farm- yellow (cars & long)’ / 0 green red cars & long / 1 green red short’ / 0 yellow red short / 1 yellow red cars & long’ / 0 red green cars’ & long / 1 red green short’ / 0 red yellow short/ 1 red yellow

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Verilog model of controller module sequencer(rst,clk,cars,long,short,hg,hy,hr,fg,fy,fr,count_reset); input rst, clk; /* reset and clock */ input cars; // high when a car is present at the farm road input long, short; /* long and short timers */ output hg, hy, hr; // highway light: green, yellow, red output fg, fy, fr; /* farm light: green, yellow, red */ reg hg, hy, hr, fg, fy, fr; // remember these outputs output count_reset; /* reset the counter */ reg count_reset; // register this value for simplicity // define the state codes ‘define HWY_GREEN 0 ‘define HWY_YX 1 ‘define HWY_YELLOW 2 ‘define HWY_YY 3 ‘define FARM_GREEN 4 ‘define FARM_YX 5 ‘define FARM_YELLOW 6 ‘define FARM_YY 7 reg [2:0] state; // state of the sequencer clk) begin if (rst == 1) begin state = ‘HWY_GREEN; // default state count_reset = 1; end

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Verilog model of controller, cont’d. else begin // state machine count_reset = 0; case (state) ‘HWY_GREEN: begin if (~(cars & long)) state = ‘HWY_GREEN; else begin state = ‘HWY_YX; count_reset = 1; end hg = 1; hy = 0; hr = 0; fg = 0; fy = 0; fr = 1; end ‘HWY_YX: begin state = ‘HWY_YELLOW; hg = 0; hy = 1; hr = 0; fg = 0; fy = 0; fr = 1; end ‘HWY_YELLOW: begin if (~short) state = ‘HWY_YELLOW; else begin state = ‘FARM_YY; end hg = 0; hy = 1; hr = 0; fg = 0; fy = 0; fr = 1; end ‘FARM_YY: begin state = ‘FARM_GREEN; hg = 0; hy = 0; hr = 1; fg = 1; fy = 0; fr = 0; end

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Verilog model of system ‘FARM_GREEN: begin if (cars & ~long) state = ‘FARM_GREEN; else begin state = ‘FARM_YX; count_reset = 1; end hg = 0; hy = 0; hr = 1; fg = 1; fy = 0; fr = 0; end ‘FARM_YX: begin state = ‘FARM_YELLOW; hg = 0; hy = 0; hr = 1; fg = 1; fy = 0; fr = 0; end ‘FARM_YELLOW: begin if (~short) state = ‘FARM_YELLOW; else begin state = ‘HWY_GREEN; end hg = 0; hy = 0; hr = 1; fg = 0; fy = 1; fr = 0; end ‘HWY_YY: begin state = ‘HWY_GREEN; hg = 1; hy = 0; hr = 0; fg = 0; fy = 0; fr = 1; end endcase end // state machine end // always endmodule

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Verilog model of timer module timer(rst,clk,long,short); input rst, clk; // reset and clock output long, short; // long and short timer outputs reg [3:0] tval; // current state of the timer clk) // update the timer and outputs if (rst == 1) begin tval = 4’b0000; short = 0; long = 0; end // reset else begin {long,tval} = tval + 1; // raise long at rollover if (tval == 4’b0100) short = 1’b1; // raise short after 2^2 end // state machine endmodule

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Verilog model of system module tlc(rst,clk,cars,hg,hy,hr,fg,fy,fr); input rst, clk; // reset and clock input cars; // high when a car is present at the farm road output hg, hy, hr; // highway light: green, yellow, red output fg, fy, fr; // farm light: green, yellow, red wire long, short, count_reset; // long and short // timers + counter reset sequencer s1(rst,clk,cars,long,short, hg,hy,hr,fg,fy,fr,count_reset); timer t1(count_reset,clk,long,short); endmodule

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR The synchronous philosophy n All operation is controlled by the clock. –All timing is relative to clock. –Separates functional, performance optimizations. n Put a lot of work into designing the clock network so you don’t have to worry about it throughout the combinational logic.

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Register characteristics n Form of clock signal –used to trigger the register. n How the behavior of data around the clock trigger affects the stored value. n When the stored value is presented at the output. n Whether there is ever a combinational path from input to output.

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Types of registers n Latch: transparent when internal memory is being set. n Flip-flop: not transparent, reading and changing output are separate.

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Types of registers n D-type (data). Q output is determined by the D input at the clocking event. n T-type (toggle). Toggles its state at input event. n SR-type (set/reset). Set or reset by inputs (S=R=1 not allowed). n JK-type. Allows both J and K to be 1, otherwise similar to SR.

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Clock event n Change in clock signal that controls register behavior. –0-1 transition or 1-0 transition. n Data must generally be held constant around the clock event.

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Setup and hold times time clock D changing stable event setup hold

FPGA-Based System Design: Chapter 5 Copyright  2004 Prentice Hall PTR Duty cycle n Percentage of time that clock is active. 50%