Set the minimum and maximum desired connection intervals in units of 1.25 ms.
BLE.setConnectionInterval(minimum, maximum)
Nothing.
// begin initialization
if (!BLE.begin()) {
Serial.println("starting Bluetooth® Low Energy module failed!");
while (1);
}
// ...
BLE.setConnectionInterval(0x0006, 0x0c80); // 7.5 ms minimum, 4 s maximum