Delay timer (LS7212) in Verilog HDL

Today's project is an implementation of a programmable digital delay timer in Verilog HDL. Verilog code for the delay timer is fully presented.

Verilog code for delay timer

The digital delay timer being implemented is CMOS IC LS7212 which is to generate programmable delays. The specification of the delay timer can be easily found here. Basically, the delay timer has 4 operating modes: one-shot (OS), Delayed Operate (DO), Delayed Release(DR), Dual Delay (DD). Those four modes will be selected by inputs mode_a and mode_b.

delay timer in Verilog

The wb[7:0] input is to program the delays according to given equations in the specification of the delay timer. To understand more how it works in each mode and how inputs control delays, kindly refer to its datasheet. The following waveform is how the delays are generated in each mode.


delay timer in Verilog


Below is the Verilog code for the programmable digital delay timer LS7212. 

 //fpga4student.com: FPga projects, Verilog projects, VHDL projects
 // Verilog project: Verilog code for delay timer LS7212 
 module delay_timer_ls7212  
 (   
      input [7:0] wb, // weighting bits  
      input clk, // clock input
      input reset, // timer reset  
      input trigger, // trigger input  
      input mode_a, mode_b, // mode bits A and B  
      output reg delay_out_n // delay output, active low  
 );  
 reg[7:0] PULSE_WIDTH ;  
 reg [7:0] DELAY;  
 reg [7:0] TIMER=0;  
 reg trigger_sync_1=0,trigger_sync_2=0;  
 wire trigger_rising,trigger_falling;  
 reg timer_start=0,out_low=0;  
 wire timer_clear2,timer_clear3,timer_clear;  
 reg [1:0] mode;  
 reg reset_timer1=0,reset_timer2=0,reset_timer=0;  
 wire reset_timer3,reset_det;  
 reg reset_det1=0,reset_det2=0;  
//fpga4student.com: FPga projects, Verilog projects, VHDL projects
 always @(posedge clk)  
 begin  
           trigger_sync_1 <= trigger; // the first Flip-Flop  
           trigger_sync_2 <= trigger_sync_1;// the second Flip-Flop  
           reset_timer1 <= reset_timer;  
           reset_timer2 <= reset_timer1;  
           reset_det1 <= reset;  
           reset_det2 <= reset_det1;  
 end  
 // Identify the zero to one transitions on trigger signal  
 assign trigger_rising = trigger_sync_1 & (~trigger_sync_2);   
 assign trigger_falling = trigger_sync_2 & (~trigger_sync_1);   
 assign reset_timer3 = reset_timer1 & (~reset_timer2);  
 assign reset_det = reset_det2 & (~reset_det1);  
 // sample Mode and wb  
 always @(trigger_rising,trigger_falling,mode_a,mode_b,wb)  
 begin  
      if(trigger_falling == 1 || trigger_rising == 1) begin  
           PULSE_WIDTH = wb;  
           DELAY = (2*wb + 1)/2;  
           mode = {mode_a,mode_b};  
      end  
 end  
 // modes  
 always @(mode,reset,trigger_falling,trigger_rising,TIMER,reset,trigger,PULSE_WIDTH,DELAY,reset_det)  
 begin  
      case(mode)  
                2'b00: // One-Shot Mode  
                          begin  
                               if(reset) begin  
                                    out_low <= 0;  
                                    timer_start <= 0;  
                                    reset_timer <= 1;  
                               end  
                               else if(trigger_rising==1) begin  
                                    out_low <= 1;  
                                    timer_start <= 1;  
                                    reset_timer <= 1;  
                                    end  
                               else if(TIMER>=PULSE_WIDTH) begin  
                                    out_low <= 0;  
                                    timer_start <= 0;  
                                    reset_timer <= 1;  
                               end  
                          end  
                2'b01: // Delayed Operate Mode  
                          begin  
                               if(reset) begin  
                                    out_low <= 0;  
                                    timer_start <= 0;  
                                    reset_timer <= 1;  
                               end  
                               else if(reset_det==1 && trigger==1) begin  
                                    timer_start <= 1;  
                                    reset_timer <= 0;  
                               end  
                               else if(trigger_rising==1) begin  
                                    timer_start <= 1;  
                                    reset_timer <= 0;  
                                    end  
                               else if(trigger_falling==1 || trigger == 0) begin  
                                    out_low <= 0;  
                                    reset_timer <= 1;  
                                    timer_start <= 0;  
                               end  
                               else if(TIMER >= DELAY) begin  
                                    out_low <= 1;  
                                    timer_start <= 0;  
                                    reset_timer <= 1;  
                               end  
                               //else  
                               //     reset_timer <= 0;  
                          end       
                2'b10: // Delayed Release Mode  
                          begin  
                               if(reset) begin  
                                    out_low <= 0;  
                                    timer_start <= 0;  
                                    reset_timer <= 1;  
                               end  
                               else if(trigger_rising==1 || trigger == 1) begin  
                                    out_low <= 1;  
                               end  
                               else if(trigger_falling==1 ) begin  
                                    timer_start <= 1;  
                                    reset_timer <= 0;  
                                    end  
                               else if(TIMER>=DELAY) begin  
                                    out_low <= 0;  
                                    timer_start <= 0;  
                                    reset_timer <= 1;  
                               end  
                          end       
                2'b11: // Delayed Dual Mode  
                          begin  
                               if(reset) begin  
                                    out_low <= 0;  
                                    timer_start <= 0;  
                                    reset_timer <= 1;  
                               end  
                               else if(reset_det==1 && trigger==1) begin  
                                    timer_start <= 1;  
                                    reset_timer <= 0;  
                               end  
                               else if(trigger_falling==1 || trigger_rising==1 ) begin  
                                    timer_start <= 1;  
                                    reset_timer <= 0;  
                                    end  
                               else if(TIMER>=DELAY) begin  
                                    out_low <= trigger;  
                                    timer_start <= 0;  
                                    reset_timer <= 1;  
                               end  
                          end  
           endcase  
 end  
//fpga4student.com: FPga projects, Verilog projects, VHDL projects
 // timer  
 always @(posedge clk or posedge timer_clear)  
 begin  
 if(timer_clear)   
      TIMER <= 0;  
 else if(timer_start)  
      TIMER <= TIMER + 1;  
 end  
 assign timer_clear = reset_timer3 | trigger_rising == 1 | timer_clear3 ;  
 assign timer_clear2 = (trigger_rising == 1)|(trigger_falling == 1);  
 assign timer_clear3 = timer_clear2 & (mode == 2'b11);  
 //delay output  
 always @(posedge clk)  
 begin  
      if(out_low == 1)  
           delay_out_n <= 0;  
      else  
           delay_out_n <= 1;  
 end  
 endmodule  

Testbench Verilog code for the delay timer:

`timescale 1ns / 1ps  
//fpga4student.com: FPga projects, Verilog projects, VHDL projects
// Testbench Verilog code for delay timer
 module tb_ls7212;  
      // Inputs  
      reg [7:0] wb;  
      reg clk;  
      reg reset;  
      reg trigger;  
      reg mode_a;  
      reg mode_b;  
      // Outputs  
      wire delay_out_n;  
      //fpga4student.com: FPga projects, Verilog projects, VHDL projects
      // Instantiate the Unit Under Test (UUT)  
      delay_timer_ls7212 uut (  
           .wb(wb),   
           .clk(clk),   
           .reset(reset),   
           .trigger(trigger),   
           .mode_a(mode_a),   
           .mode_b(mode_b),   
           .delay_out_n(delay_out_n)  
      );  
      initial begin  
           // Initialize Inputs  
           wb = 10;  
           mode_a = 0;  
           mode_b = 0;  
           reset = 0;  
           trigger = 0;  
           #500;  
           trigger = 1;  
           #15000;  
           trigger = 0;  
           #15000;  
    trigger = 1;  
           #2000;  
           trigger = 0;  
           #2000;  
           trigger = 1;       
           #2000;  
           trigger = 0;       
           #20000;  
           trigger = 1;            
           #30000;  
           trigger = 0;  
           #2000;  
           trigger = 1;  
           #2000;  
           trigger = 0;       
           #4000;  
           trigger = 1;       
           #10000;  
           reset = 1;  
           #10000;  
           reset = 0;  
           // Delay Operate  
           // Add stimulus here  
      end  
   initial begin   
   clk = 0;  
   forever #500 clk = ~clk;  
      end  
 endmodule  
//fpga4student.com: FPga projects, Verilog projects, VHDL projects

Simulation waveform for the digital delay timer in Verilog

   One-Shot Mode


delay timer in Verilog

   Delayed Operate Mode

delay timer in Verilog

  Delayed Release Mode

delay timer in Verilog

   Delay Dual Mode

delay timer in Verilog

The Verilog code for the programmable delay timer is synthesizable and can be implemented on FPGAs. 

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