PWM 呼吸灯实验

FPGA实现一个PWM模块（硬件）来控制灯的亮灭。

实验原理

PWM本质上就是一个输出脉冲的硬件，通过改变一个周期高电平（占空比）的时间来对其他的硬件进行控制，比如电机。
呼吸灯的实现利用了人眼的视觉特性，控制灯亮和暗的间隔时间就形成了对灯亮度的调节。
通过一个N比特的计数器（溢出相当于从0开始）和一个值就可以实现一个占空比可调的脉冲输出。

实验步骤

设计PWM模块
用硬件描述语言实现设计
查看硬件设计，添加管脚约束和时钟约束
本地仿真
上板验证

实验记录

设计PWM模块

信号名称	方向	说明
clk	in	时钟输入
rst	in	异步输入复位，高有效
period	in	PWM 脉宽周期（频率）控制。实际是每次计数的步进值，根据系统输入时钟可以计算出pwm输出频率
duty	in	占空比，小于该值时输出低电平，否则输出高电平
pwm_out	out	pwm输出

用硬件描述语言实现设计
ax_pwm.v

`timescale 1ns / 1ps
module ax_pwm
#(
    parameter N = 16
)
(
    input clk,
    input rst,
    input [N-1:0] period,
    input [N-1:0] duty,
    output pwm_out
);


reg[N-1:0] period_r;
reg[N-1:0] duty_r;
reg[N-1:0] period_cnt;
reg pwm_r;

assign pwm_out = pwm_r;

always @(posedge clk or posedge rst)
begin
    if(rst==1)
    begin
        period_r <= {N{1'b0}};
        duty_r <= {N{1'b0}};
    end
    else
    begin
        period_r <= period;
        duty_r <= duty;
    end
end
    
always @(posedge clk or posedge rst)
begin
    if(rst==1)
        period_cnt <= {N{1'b0}};
    else
        period_cnt <= period_cnt + period_r;
end

always @(posedge clk or posedge rst)
begin
    if(rst==1)
    begin
        pwm_r <= 1'b0;
    end
    else
    begin
        if(period_cnt >= duty_r)
            pwm_r <= 1'b1;
       else
            pwm_r <= 1'b0;
    end
end

endmodule

PWM测试模块

`timescale 1ns / 1ps

module pwm_test(
    input  sys_clk_p,    // 时钟是一个200MHz的差分时钟
    input  sys_clk_n,
    input  rst_n,        // 复位由一个按键来控制，按键默认状态为高，所以低复位
    output wire led          // 输出给led
);
localparam CLK_FREQ = 200 ; //200MHz，周期为 1s/200MHz = 1/200 us
localparam US_COUNT = CLK_FREQ ; //1 us counter，200个时钟周期是1us 
localparam MS_COUNT = CLK_FREQ*1000 ; //1 ms counter  1个ms的周期计数

localparam DUTY_STEP = 32'd100000 ; //duty step, 占空比调整大小，单次调整 100000/2^32 = 0.000000000232
localparam DUTY_MIN_VALUE = 32'h6fffffff ; //duty minimum value, 最小占空比 0.437
localparam DUTY_MAX_VALUE = 32'hffffffff ; //duty maximum value, 最大占空比 0.999

localparam IDLE = 0; //IDLE state 
localparam PWM_PLUS = 1; //PWM duty plus state 
localparam PWM_MINUS = 2; //PWM duty minus state 
localparam PWM_GAP = 3; //PWM duty adjustment gap

wire pwm_out; //pwm output 
reg[31:0] period; //pwm step value 
reg[31:0] duty; //duty value 
reg pwm_flag ; //duty value plus and minus flag, 0: plus; 1: minus

reg[3:0] state; 
reg[31:0] timer; //duty adjustment counter 该测试模块计数的

assign led = ~pwm_out ; //led low active

wire clk;
// 差分转单端
IBUFDS IBUFDS_inst(
    .O(clk),
    .I(sys_clk_p),
    .IB(sys_clk_n)
);

always@(posedge clk or negedge rst_n) 
begin 
    if(rst_n == 1'b0) 
    begin  // 复位，设置初始值
        period <= 32'd0; 
        timer <= 32'd0; 
        duty <= 32'd0; 
        pwm_flag <= 1'b0 ; 
        state <= IDLE; 
    end
    else 
    case(state) 
        IDLE: 
        begin 
            // N位计数器，CLK Hz的时钟（周期1/CLK s），要输出n Hz的PWM波，PWM周期为1/n s,要在一个PWM周期计数完一轮,而一个周期会计数((1/n)/(1/CLK))次
            //  2^N/((1/n)/(1/CLK)) = 2^32*n/CLK  
            //  这个地方的时钟写的50M，PWM是800Hz的，周期0.00125s。
            period <= 32'd17179; //The pwm step value, pwm 200Hz(period = 200*2^32/50000000) 
            state <= PWM_PLUS; 
            duty <= DUTY_MIN_VALUE; 
        end 
        PWM_PLUS : 
        begin 
            if (duty > DUTY_MAX_VALUE - DUTY_STEP) //if duty is bigger than DUTY MAX VALUE minus DUTY_STEP , begin to minus duty value 
            begin 
                pwm_flag <= 1'b1 ; 
                duty <= duty - DUTY_STEP ; 
            end 
            else 
            begin 
                pwm_flag <= 1'b0 ; 
                duty <= duty + DUTY_STEP ; 
            end 
            state <= PWM_GAP ; 
        end
        PWM_MINUS : 
        begin 
            if (duty < DUTY_MIN_VALUE + DUTY_STEP) //if duty is little than DUTY MIN VALUE plus duty step, begin to add duty value 
            begin 
                pwm_flag <= 1'b0 ; 
                duty <= duty + DUTY_STEP ; 
            end 
            else 
            begin 
                pwm_flag <= 1'b1 ;
                duty <= duty - DUTY_STEP ; 
            end 
            state <= PWM_GAP ; 
        end 
        PWM_GAP: 
        begin 
            if(timer >= US_COUNT*100) //adjustment gap is 100us 
            begin 
                if (pwm_flag) 
                    state <= PWM_MINUS ; 
                else 
                    state <= PWM_PLUS ; 
                timer <= 32'd0; 
            end 
            else 
                begin 
                    timer <= timer + 32'd1; 
                end 
        end
        default: 
        begin 
            state <= IDLE; 
        end 
    endcase
end

ax_pwm
#(
.N(32)
)
ax_pwm_m0(
    .clk(clk),
    .rst(~rst_n),
    .period(period),
    .duty(duty),
    .pwm_out(pwm_out)
);

endmodule

查看硬件设计，添加管脚约束和时钟约束
约束文件

set_property PACKAGE_PIN AE15 [get_ports led]
set_property PACKAGE_PIN AE14 [get_ports rst_n]
set_property PACKAGE_PIN AE5 [get_ports sys_clk_p]
set_property IOSTANDARD LVCMOS33 [get_ports led]
set_property IOSTANDARD LVCMOS33 [get_ports rst_n]
set_property IOSTANDARD DIFF_SSTL12 [get_ports sys_clk_p]
create_clock -period 5.000 -name sys_clk_p -waveform {0.000 2.500} [get_ports sys_clk_p]

本地仿真
激励文件

`timescale 1ns / 1ps

module pwm_test_tb;
// Inputs
reg sys_clk_p;
wire sys_clk_n;
reg rst_n;
wire led ;

// Instantiate the Unit Under Test (UUT)
pwm_test uut (
	.sys_clk_p(sys_clk_p), 		
	.sys_clk_n(sys_clk_n), 	
	.rst_n		(rst_n),
	.led       (led)
);

initial begin
	// Initialize Inputs
	sys_clk_p = 0;
	rst_n = 0;

	// Wait 100 ns for global reset to finish
	#100;
      rst_n = 1;      

 end

always #2.5 sys_clk_p = ~ sys_clk_p;   //5ns一个周期，产生200MHz时钟源

assign sys_clk_n = ~ sys_clk_p;
   
endmodule