Lesson 4 of 5

Dim an LED with PWM

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// PWM: the switches set the brightness of an LED on Pmod JB. module top #( parameter CLK_HZ = 100_000_000 ) ( input clk, input [15:0] sw, // SW7-SW0 set the brightness, 0-255 output [7:0] JB, // Pmod JB: JB[0] is pin 1 output [15:0] led ); // A free-running 8-bit counter: 0, 1, ... 255, 0, ... reg [7:0] ramp = 0; always @(posedge clk) ramp <= ramp + 1; // High while the ramp is below the switch value. wire bright = ramp < sw[7:0]; // A one-second heartbeat on LD15, timed from CLK_HZ. localparam HALF = CLK_HZ / 2; reg [31:0] count = 0; reg beat = 1'b0; always @(posedge clk) begin if (count == HALF - 1) begin count <= 0; beat <= ~beat; end else count <= count + 1; end assign JB = {7'b0, bright}; // only JB pin 1 is used; the rest stay low assign led = {beat, 14'b0, bright}; endmodule

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A pin can only be 0 or 3.3 V. Switching fast between them, high for SW7-SW0 out of every 256 clocks, dims an LED on Pmod JB and LD0 together.

A counter and a comparator make PWM
Any FPGA pin can do PWM
The switches set the duty in binary

iynk runs your Verilog as a behavioural model, cycle by cycle on the board clock. It does not synthesise, place or route: no timing, no LUT counts.

Pin	Connects to	What it does
`JB1`	R1 220 Ω → LED anode	The PWM output, JB[0]
`GND`	LED cathode (JB pin 5)	The return path
`SW7`	FPGA pin W13 (sw[7])	Worth 128 of 256: half brightness

Wire it in

Sensors and actuators this board drives, with the real connections.

LEDSupply 3.3 V
Anode to JB pin 1 through 220 Ω, cathode to a Pmod GND pin. Drive it from your design like any LED.
Part pin Board pin
A JB1Wire colour orange
K GNDWire colour black
Careful
The board already adds 200 Ω in series, so the LED stays dim but safe even without your resistor; never feed a Pmod pin from 5 V.

All 5 parts on the Digilent Basys 3 page

Connects to

What it does

JB1

R1 220 Ω → LED anode

The PWM output, JB[0]

GND

LED cathode (JB pin 5)

The return path

SW7

FPGA pin W13 (sw[7])

Worth 128 of 256: half brightness

Wire it in

Sensors and actuators this board drives, with the real connections.

LEDSupply 3.3 V

Anode to JB pin 1 through 220 Ω, cathode to a Pmod GND pin. Drive it from your design like any LED.

Part pin	Board pin
A	JB1Wire colour orange
K	GNDWire colour black

Careful

The board already adds 200 Ω in series, so the LED stays dim but safe even without your resistor; never feed a Pmod pin from 5 V.

All 5 parts on the Digilent Basys 3 page

Dim an LED with PWM

Try it yourself

Serial monitor

Checks

Break it

Inputs

Wire it in

Dim an LED with PWM

Try it yourself

Serial monitor

Checks

Break it

Inputs

Wire it in