arduino-oscilloscope/arduino-sketch.ino

// https://www.arduino.cc/reference/en/language/functions/time/micros/
const unsigned long SAMPLE_INTERVAL_MICROSECONDS = 2500L;

void setup() {
    // $ stty -F /dev/ttyUSB0 | grep speed
    Serial.begin(115200);
}

unsigned long last_sample_micros = 0;

void loop() {
    while (micros() - last_sample_micros < SAMPLE_INTERVAL_MICROSECONDS)
        ;
    last_sample_micros = micros();
    // > On ATmega based boards [...], it takes about 100 microseconds
    // > (0.0001 s) > to read an analog input, [...]
    // https://www.arduino.cc/reference/en/language/functions/analog-io/analogread/
    unsigned short reading = analogRead(A0);
    // $ tail --follow /dev/ttyUSB0 | od --format=u2 --endian=big --width=2
    Serial.write(reading >> 8);
    Serial.write(reading & 0xff);
}