Solution:

It seems PCL-839 don't have speed ramp up/down like manual description. The speed jumps right to high speed. The fact is the ramp up/down time interval is too short, so user could not feel it. Please increase the difference between high speed and low speed, also set AD value to 1023 the maximum value, you will feel a little ramp up/down. The formula for time interval is: (high speed - low speed) * AD * Tclk, where Tclk = 203ns Tclk is the inverse of PCL-839 system clock 4.9152MHz. User can also change the clock to lower speed, like 1.8432Mhz, and the ramp up/down time interval will be longer.