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Built-in project · SparkFun Pro Micro

Pro Micro blink

A board with no LED on pin 13 blinks its RX lamp instead, with an LED on pin 9 beside it wired the ordinary way around. The whole build, a SparkFun Pro Micro and 2 more parts, runs here in your browser on the firmware below; open it in the editor to change the wiring or the code and run it again.

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Beginner About 10 minutes to build and run. Runs in your browser. Free, and no account needed.

The parts retail for about $15.15. On Mokxi they're free.

The circuit runs here once the page has loaded.

What you'll learn

  • LEDs that light on LOW versus on HIGH
  • Boards with no pin 13 LED
  • Built-in lamp macros like RXLED1
  • Wiring an LED with a resistor to a pin
  • Difficulty: Beginner
  • Time: About 10 minutes
  • Board: SparkFun Pro Micro
  • Parts: 4

How it works

The SparkFun Pro Micro does not bring pin 13 out, so LED_BUILTIN would light nothing you can see. Instead every ATmega32U4 board has an RX and a TX lamp, each wired from 5 V through a resistor to a pin. The pin has to go LOW to light one.

The sketch blinks the RX lamp, and at the same time blinks an LED you wire from pin 9 through a 220 ohm resistor to ground. That LED lights the ordinary way, on HIGH. Each state lasts half a second, so you can see both kinds of LED side by side.

Parts list

4 parts, plus the jumper wires. They retail for about $15.15; on Mokxi they're free.

Wiring table

Every connection, pin by pin, read from the circuit itself. Each row joins the pin on the left to the pin on the right, by a jumper wire or a breadboard strip.

Part and pinConnects to
LED, red pin CSparkFun Pro Micro pin GND
Resistor, 220 Ω pin 2SparkFun Pro Micro pin 9
LED, red pin AResistor, 220 Ω pin 1

The code

The firmware exactly as the editor opens it. Change a line there and press Run: it compiles in the browser.

sketch.ino · SparkFun Pro Micro
// RX blink: the lamp a board with no pin 13 still has.
//
// Every ATmega32U4 board has an RX and a TX LED, and both are wired from 5 V
// through a resistor to a pin: RX to PB0, TX to PD5. So the pin has to go LOW
// to light one. RXLED1 does that and RXLED0 lets it go dark again.
//
// On a Leonardo or a Micro this blinks the RX lamp beside L. On a SparkFun Pro
// Micro, which does not bring pin 13 out at all, it is the only way to see a
// blink without wiring anything: LED_BUILTIN is still 13 there, and blinking
// it lights nothing.
//
// Pin 9 blinks at the same time the ordinary way round, HIGH for on, so an
// LED and a resistor from pin 9 to GND show the difference side by side.

const int WIRED = 9;

void setup() {
  TX_RX_LED_INIT;
  TXLED0;
  pinMode(WIRED, OUTPUT);
}

void loop() {
  RXLED1;                     // lamp on: PB0 low
  digitalWrite(WIRED, HIGH);  // wired LED on: pin 9 high
  delay(500);
  RXLED0;                     // lamp off: PB0 high
  digitalWrite(WIRED, LOW);
  delay(500);
}

Code walkthrough

The key functions of the sketch, and what each one does.

setup sketch.ino

TX_RX_LED_INIT prepares the RX and TX lamp pins as outputs, and TXLED0 lets the TX lamp go dark. Then pinMode makes the WIRED pin (9) an output.

void setup() {
  TX_RX_LED_INIT;
  TXLED0;
  pinMode(WIRED, OUTPUT);
}

loop sketch.ino

RXLED1 pulls the RX pin low, so the lamp lights, and pin 9 goes HIGH to light your LED. After 500 ms, RXLED0 lets the lamp go dark and pin 9 goes LOW. Then it waits another 500 ms.

void loop() {
  RXLED1;                     // lamp on: PB0 low
  digitalWrite(WIRED, HIGH);  // wired LED on: pin 9 high
  delay(500);
  RXLED0;                     // lamp off: PB0 high
  digitalWrite(WIRED, LOW);
  delay(500);
}

Troubleshooting

The usual mistakes with this circuit, and the fix for each.

  • Nothing blinks on the board This project
    Blinking LED_BUILTIN lights nothing here. The sketch uses RXLED1 and RXLED0 for the lamp that exists.
  • Your LED never lights This project
    Check its direction: the cathode goes to GND, and the resistor connects pin 9 to the anode.
  • The LED is dim, burns out, or the pin reads wrong because the resistor value is off. Resistor
    Read the color bands again. A 220 ohm to 330 ohm resistor suits a typical LED on 5 V. 10k is a common pull-up or pull-down value. Check with a multimeter if unsure.
  • A pull-up or pull-down resistor is missing, so the input floats and reads random values. Resistor
    Tie the input to 5 V or ground through about 10k. Or use INPUT_PULLUP and wire the switch to ground.
  • The LED does not light, or it lights once and then dies. LED
    LEDs have polarity. The longer leg (anode) goes toward the positive side. Always add a series resistor, about 220 ohms to 1 k ohm on a 5 V pin.
  • A blue or white LED stays dark on 3.3 V. LED
    Blue and white LEDs drop about 3 V, so 3.3 V leaves almost nothing for the resistor. Use a 5 V supply, or a smaller resistor and a red or green LED.
  • The upload fails or the wrong board is selected. SparkFun Pro Micro
    Pick SparkFun Pro Micro and match the 5 V 16 MHz or 3.3 V 8 MHz version printed on your board. A wrong choice can make the board hard to program.
  • Serial output seems missing, or the sketch hangs at start. SparkFun Pro Micro
    Serial is on the native USB port, so use while(!Serial) only when you want to wait for a monitor. For Serial1 use pins TX and RX. Pin numbers differ from an Uno, so check the pinout.

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