arduino-oscilloscope/arduino-sketch.ino

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

void loop() {
    // effective sample rate measured with "Probe Rate" block
    delayMicroseconds(800);
    // > 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
    // 16 bits/sample / 115200 Baud * 1e6 μs/sec = 139 μs/sample
    Serial.write(reading >> 8);
    Serial.write(reading & 0xff);
}