// Try to get the crystal running. If no crystal is present, XI should be
    // grounded, so STABLE counter will never complete. Poor designs might
    // leave XI floating, in which case we may eventually drop through... in
    // this case we rely on PLL not locking, and/or resuscitate counter.
    //
    // Don't touch range setting: user would only have changed if crystal
    // needs it, and running crystal out of range can produce glitchy output.
    // Note writing a "bad" value (non-aax) to RANGE has no effect.
    xosc_hw->ctrl = XOSC_CTRL_ENABLE_VALUE_ENABLE << XOSC_CTRL_ENABLE_LSB;
    while (!(xosc_hw->status & XOSC_STATUS_STABLE_BITS));