Signal conditioning circuitry

1. Signal conditioning circuitry for an optical pointing device comprising a photodetector which upon periodic illumination generates a periodic signal, the circuitry comprising:

means for determining the rate of change of said periodic signal independently of the amplitude of said periodic signal, and for generating an output signal representative of said predetermined rate of change; and

means for comparing said output signal to a reference voltage.

2. Circuitry according to claim 1 , wherein the determining means comprises a differentiator.

3. Circuitry according to claim 1 or 2 , wherein the comparing means comprises a comparator.

4. Circuitry according to claim 1 or 2 , wherein the comparing means comprises a Schmitt trigger.

5. Signal conditioning circuitry for an optical pointing device comprising a photodetector which upon periodic illumination generates a periodic signal, the circuitry comprising:

a differentiator receiving as input said periodic signal, the differentiator being operable to determine the rate of change of said periodic signal independently of the amplitude of said periodic signal and to generate an output signal representative of said determined rate of change, and

a Schmitt trigger operable to compare said output signal to a predetermined reference voltage.

6. Signal conditioning circuitry for an optical pointing device, said circuitry comprising a circuit for outputting signals proportional to X-direction movement and a circuit for outputting signals proportional to Y-direction movement, each said circuit comprising a detector arrangement for detecting movement of a respective body of said X- and Y-direction movement and outputting a signal in response thereto, means for determining a rate of change of said signal and outputting a signal representative of said determined rate of change and means for comparing said signal output by said determining means to a reference voltage.

7. Circuitry according to claim 6 , wherein each said body is rotatable about an axis of rotation and the respective detector arrangements each comprise two detectors disposed about the respective axes of rotation so as to provide respective output signals that are out of phase for determining the direction of rotation of the respective body.

8. Circuitry according to claim 7 , wherein each said determining means comprises respective differentiators for receiving signals outputted by the two detectors.

9. Circuitry according to claim 8 , wherein each said comparing means comprises respective Schmitt triggers connected in series with said differentiators.

10. An optical pointing device comprising signal conditioning circuitry as claimed in any one of claims 1 , 5 , or 6 .

11. Signal conditioning circuitry as claimed in to any one of claims 1 , 5 , or 6 wherein said periodic signal is a sinusoidal signal.