## Opto 2 - Varying an LED intensity

##### Turning an LED on and off

Here's how to make an LED blink (on and off).

```module LEDblink(clk, LED);
input clk;     // clock typically from 10MHz to 50MHz
output LED;

// create a binary counter
reg [31:0] cnt;
always @(posedge clk) cnt <= cnt+1;

assign LED = cnt[22];    // blink the LED at a few Hz (using the 23th bit of the counter, use a different bit to modify the blinking rate)
endmodule
```
##### Making an LED half-lit
One solution would be doubling the value of the resistor used in series with the LED.

Another solution is to drive the LED FPGA output half of the time. If that's done fast enough, your eye will see the LED half-lit. Anything faster than 100Hz is too fast for the human eye to detect - it averages the light intensity it receives.

```module LEDhalflit(clk, LED);
input clk;     // clk should be at least 200Hz.
// Anything above is fine (most FPGA boards have adequate clocks, running at a few 10's of MHz)
output LED;

reg toggle;
always @(posedge clk) toggle <= ~toggle;     // toggles at half the clk frequency (at least 100Hz)

assign LED = toggle;
endmodule
```
##### Fine-tuning the LED intensity

For more LED intensity control, a PWM is an ideal solution.
Here's an example that uses a 4-bits control to select between 16 intensity levels out of an LED.

```module LED_PWM(clk, PWM_input, LED);
input clk;
input [3:0] PWM_input;     // 16 intensity levels
output LED;

reg [4:0] PWM;
always @(posedge clk) PWM <= PWM[3:0]+PWM_input;

assign LED = PWM[4];
endmodule
```

By continuously changing the LED intensity, the LED appears to "glow".

```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];    // ramp the PWM input up and down

reg [4:0] PWM;
always @(posedge clk) PWM <= PWM[3:0]+PWM_input;

assign LED = PWM[4];
endmodule
```

This is the same example that was presented in part 0.