Methods and apparatus for histogramming

1. An apparatus, comprising:

a time-to-digital converter (TDC) configured to generate a plurality of signals, each signal having a total number of bits; and

a state machine responsive to the TDC and configured to:

create, in a memory, a first histogram of a first set of signals from the plurality of signals, wherein the first histogram has a fixed number of bins over a first range;

determine a first peak bin from the first histogram;

determine a region of interest of the first histogram based at least partially on a relationship between the first peak bin and half the fixed number of bins, wherein the region of interest is centered at the first peak bin and defined by a start bin and an end bin;

create a second histogram of a second set of signals from the plurality of signals using the total number of bits, wherein the second histogram has the fixed number of bins over the region of interest and wherein the bins of the second histogram have a smaller width than the bins of the first histogram; and

determine a second peak bin from the second histogram.

2. The apparatus according to claim 1 , wherein the state machine uses less than the total number of bits to create the first histogram.

3. The apparatus according to claim 1 , wherein the state machine is further configured to compute a high resolution peak location comprising adding a value of the start bin to a value of the second peak bin.

4. The apparatus according to claim 1 , wherein determining the region of interest comprises computing the start bin and computing the end bin based on the first peak bin.

5. The apparatus according to claim 1 , wherein the start bin is the first peak bin minus half the fixed number of bins plus 1.

6. The apparatus according to claim 1 , wherein the end bin is the first peak bin plus half the fixed number of bins.

7. The apparatus according to claim 1 , wherein creating the second histogram comprises tracking a number of photon events that occur at a timestamp using the total number of bits in the timestamp.

8. A method for histogramming, comprising:

with a state machine, receiving a plurality of signals from a time-to-digital converter;

with the state machine, performing a first histogramming phase comprising:

creating a first histogram of a first set of signals from the plurality of signals, wherein the first histogram has a fixed number of bins over a first range;

determining a first peak bin from the first histogram; and

determining a first region of interest of the first histogram based at least partially on a relationship between the first peak bin and half the fixed number of bins, wherein the first region of interest is centered at the first peak bin and defined by a start bin and an end bin; and

with the state machine, performing a second histogramming phase comprising:

creating a second histogram of a second set of signals from the plurality of signals, wherein the second histogram has the fixed number of bins over the first region of interest; and

determining a second peak bin from the second histogram.

9. The method according to claim 8 , wherein each bin from the first histogram has a first width and each bin from the second histogram has a second width that is less than the first width.

10. The method according to claim 8 , wherein performing the first histogramming phase further comprises determining a third peak bin from the first histogram and determining a second region of interest of the first histogram based on the determined third peak bin.

11. The method according to claim 10 , wherein identifying the first and third peak bins comprises comparing a height of each bin from the first histogram to a predetermined threshold value.

12. The method according to claim 10 , wherein performing the second histogramming phase further comprises creating a third histogram of the second set of signals, wherein the third histogram has the fixed number of bins over the second region of interest and identifying a fourth peak bin from the third histogram.

13. The method according to claim 8 , wherein:

when the first peak bin is greater than or equal to half the fixed number of bins, the start bin is the first peak bin minus half the fixed number of bins plus 1;

when the first peak bin is less than half the fixed number of bins, the start bin is 0;

when the first peak bin plus half the fixed number of bins is less than a total number of virtual bins, the end bin is the first peak bin plus half the fixed number of bins, wherein the total number of virtual bins is equal to a total number of codes output by the time-to-digital converter; and

when the first peak bin plus half the fixed number of bins is greater than or equal to the total number of virtual bins, the end bin is equal to the total number of virtual bins.

14. A system, comprising:

a source configured to transmit a source signal;

a receiver configured to receive a reflected source signal and generate a reception signal according to the reflected source signal;

a time-to-digital converter (TDC) connected to the receiver and configured to generate a plurality of signals, each signal represented by a total number of bits;

a state machine responsive to the TDC and configured to:

create, in a memory, a first histogram of a first set of signals from the plurality of signals, wherein the first histogram has a fixed number of bins over a first range and each bin of the first histogram has a first width;

determine a first peak bin from the first histogram;

determine a first region of interest of the first histogram based at least partially on a relationship between the first peak bin and half the fixed number of bins, wherein the first region of interest is centered at the first peak bin and defined by a start bin and an end bin;

create a second histogram of a second set of signals from the plurality of signals, wherein the second histogram has the fixed number of bins over the first region of interest and each bin of the second histogram has a second width that is less than the first width;

determine a second peak bin from the second histogram; and

compute a high resolution peak location comprising adding a value of the start bin to a value of the second peak bin.

15. The system according to claim 14 , wherein the state machine uses a portion of the total number of bits to create the first histogram and uses the total number of bits to create the second histogram.

16. The system according to claim 14 , wherein determining the first region of interest comprises computing the start bin and computing the end bin based on the first peak bin.

17. The system according to claim 16 , wherein:

the start bin is the first peak bin minus half the fixed number of bins plus 1; and

the end bin is the first peak bin plus half the fixed number of bins.

18. The system according to claim 14 , wherein the state machine is further configured to:

determine a third peak bin from the first histogram; and

determine a second region of interest of the first histogram based on the third peak bin.

19. The system according to claim 18 , wherein the state machine is further configured to:

create a third histogram using the second set of signals, wherein the third histogram has the fixed number of bins over the second region of interest; and

determine a fourth peak bin from the third histogram.