XREG_POWER_DOWN
rc = send_seq(priv, {0x00, XREG_POWER_DOWN, 0x00, 0x00});
rc = send_seq(priv, {0x80, XREG_POWER_DOWN, 0x00, 0x00});
if (xc_write_reg(priv, XREG_POWER_DOWN, 0) != 0) {