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Opto 0 - LED glow

Do you have an FPGA board with an LED?
Try this simple design.

module LEDglow(clk, LED);
input clk;
output LED;

reg [23:0] cnt;
always @(posedge clk) cnt<=cnt+1;

wire [3:0] PWM_input = cnt[23] ? cnt[22:19] : ~cnt[22:19];
reg [4:0] PWM;
always @(posedge clk) PWM <= PWM[3:0]+PWM_input;

assign LED = PWM[4];

This makes the LED "glow".

Note: Use a "clk" clock signal at around 20MHz. Otherwise the effect might be too slow or too fast to be appreciated.