External SPI sensor

I am writing a driver for external airspeed sensor.
I have written the base code copying from an existing SPI driver. Compiling is successful but every-time when i try to do sen120 start in NSH console, it is showing “no instance started (no device on bus?) ” .Wiring and all are perfect since i checked with Oscilloscope.
I can see PX trying to send CS and SCLK, but both these signals fading automatically in a couple of seconds time. And my sensor is also working fine.

What could be the problem.?