External spi port

Hello I am trying to plug an external SPI sensor onto my pixhawk 4.
I have written the driver. But px4 is not detecting the sensor.
I tried staring the sensor with
"sensor start -S "
It shows "no instance started (no device on bus?) "
I even tried to check the external SPI pins using oscilloscope, i have seen sclk and CS signal generated, but vanishing in 1-2 seconds.
Anyone any idea what could be the reason.?