Can not set watchdog Timeout interval less than 5.46 mSeconds.

To set watchdog Timeout interval, we use the following sequence of steps :

1) 24MHz/512/x = y Hz (OSCCLK/512/x) (where x = 1,2,4,8,16,32,64 => lesser the value of x, less Timeout interval) 

2) Inverse y so that frequency is expressed in  Seconds

 (T = 1 / f )

3) Multiply inversed y with watchdog register size, i.e., if it's a 8 bits register, multiply with 256.

4) Minimally i will get 5.46 mSeconds

I want this timeout interval reduce to 1.5 mSeconds

Following information may help you