const int LEFT_DIR_PIN = 7;
const int LEFT_SPEED_PIN = 6;
const int RIGHT_DIR_PIN = 4;
const int RIGHT_SPEED_PIN = 5;
const int FORWARD = HIGH;
const int BACKWARD = (FORWARD == HIGH) ? LOW : HIGH;

int turnSpeed = 100;
int turnMs = 400; // подбираем: время поворота на 90°

void drive(int leftDir, int leftSpeed, int rightDir, int rightSpeed) {
  digitalWrite(LEFT_DIR_PIN, leftDir);
  digitalWrite(RIGHT_DIR_PIN, rightDir);
  analogWrite(LEFT_SPEED_PIN, leftSpeed);
  analogWrite(RIGHT_SPEED_PIN, rightSpeed);
}

void stopMotors() {
  analogWrite(LEFT_SPEED_PIN, 0);
  analogWrite(RIGHT_SPEED_PIN, 0);
}

void setupMotors() {
  pinMode(LEFT_DIR_PIN, OUTPUT);
  pinMode(LEFT_SPEED_PIN, OUTPUT);
  pinMode(RIGHT_DIR_PIN, OUTPUT);
  pinMode(RIGHT_SPEED_PIN, OUTPUT);
  stopMotors();
}

void setup() {
  setupMotors();
  delay(2000);

  // Левое колесо назад, правое вперёд: робот крутится на месте налево.
  drive(BACKWARD, turnSpeed, FORWARD, turnSpeed);
  delay(turnMs);
  stopMotors();
}

void loop() {
}