thank you guys,i solve the pwm problems.that's minor mistake in my hardware.
In addtion, in my project, i move clockwise will increase intensity of my output.
and move counter clockwise will decrease the intensity.
so i try the shifting instruction
the code i used as shown:

if portb == portb>>1 then
high porta.0
endif
if portb == portb<<1 then
high porta.1
endif
end

if my 1st input is high in portb.0, it's consider shifting right or shifting left??