Limitations of timing in microprocessors. Microprocessors operate at a rela- tiv

Question

Limitations of timing in microprocessors. Microprocessors operate at a rela- tively low frequency, typically below 50 MHz. Hence the basic cycle time limits the applications that can be handled or limits the accuracy of sensing. Consider an autofocus camera that uses an infrared beam to automatically focus the lens from 0.5 m to 10 m.

What are the requirements of the internal clock if one attempts to measure the time of flight of the beam from the camera to the subject and back? Assume the processor needs a minimum of 10 cycles to measure time. The speed of light in air is 3 108 m/s.

Based on the response in (a), is this method feasible?

Explanation / Answer

Speed of light = 3 * 108 m/s.

The camera has to focus from 0.5 m to 1m. Time taken by light to travel these distances back and forth is

t1 = 2 *d/v = 2 * 0.5 / (3 * 108) = 3.33 * 10-9. The frequency corresponding to this is (1/t1) which is 300 MHz.

t2 = 2 *d/v = 2 * 10 / (3 * 108) = 6.66 * 10-8. The frequency corresponding to this is (1/t1) which is 15 MHz.

The processor needs minimum 10 cycles to measure time . So, the minimum frequency is 0.1 Hz. So, to operate the camera the processor frequency should be minimum 0.1 Hz and maximum 300 MHz.

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