Correlated double sampling to include a controller to increment and invert a count during a first period and a second period

1. A method for correlated double sampling comprising:

setting all bits of a counter;

comparing an offset signal to a first reference signal to define a first period;

incrementing the counter during the first period;

inverting all bits of the counter after the first period;

comparing a sensor signal to a second reference signal to define a second period; and

incrementing the counter during the second period to generate a data value.

2. The method according to claim 1 further including generating the first reference signal and the second reference signal from a single source.

3. The method according to claim 1 wherein the first reference signal has a first ramp during the first period and the second reference signal has a second ramp during the second period.

4. The method according to claim 1 comprising

generating a first comparator output signal based on the comparison of the offset signal to the first reference signal wherein the counter is incremented during the first period responsive to the first comparator output signal; and

generating a second comparator output signal based on the comparison of the sensor signal to the second reference signal wherein the counter is incremented during the second period responsive to the second comparator output signal.

5. The method according to claim 4 wherein a single comparator generates the first comparator output signal and the second comparator output signal and the method comprises:

coupling the offset signal and the first reference signal to first and second inputs of the comparator, respectively, during the first period; and

coupling the sensor signal and the second reference signal to the first and second inputs of the comparator, respectively, during the second period.

6. The method according to claim 5 wherein the offset signal is a pixel reset signal, the sensor signal is a pixel sensor signal, and the data value is a pixel value.

7. The method according to claim 6 wherein the counter is incremented responsive to the first comparator output signal by incrementing the counter until the pixel reset signal equals the first reference signal.

8. The method according to claim 6 wherein the counter is incremented responsive to the second comparator output signal by incrementing the counter until the pixel sensor signal equals the second reference signal.

9. The method according to claim 6 comprising storing the pixel value in a memory.

10. A system for processing image data comprising:

a comparator having a pixel signal input for receiving a pixel signal and a reference signal input for receiving a reference signal, for generating a comparator output;

a counter for generating a count;

a controller for controlling the counter to set the count to all ones, increment the count during a first period responsive to the comparator output, invert all bits of the count, and increment the count during a second period responsive to the comparator output to generate a pixel value; and

a pixel signal generator for generating the pixel signal including a pixel reset signal during the first period and a pixel sensor signal during the second period.

11. The system according to claim 10 comprising a memory for storing the pixel value.

12. The system according to claim 10 comprising a reference signal generator for generating the reference signal having a first ramp portion during the first period and a second ramp portion during the second period.

13. A counter having a plurality of modules arranged in a sequence, each module comprising:

a module output;

a clock input wherein the clock input is connected to the module output of a preceding module in the sequence of modules for the second and subsequent modules in the sequence;

a set signal input for setting all the module outputs to 1 in response to a set signal; and

an inversion signal input for inverting all the module outputs in response to an inversion signal.