Method and apparatus for measuring time of flight
View Patent ↗SPADs detect photons of a return light pulse and output corresponding pulse signals. First and second counters, when enabled in response to phase measurement value, are configured to count the pulse signals. The phase measurement value is set for a subsequent iteration of the return light pulse in response to processing of first and second count values of the first and second counters, respectively, for a current iteration. If the first count value exceeds the second count value by more than a difference threshold limit, the phase measurement value is decremented for the subsequent iteration. Otherwise, the phase measurement value is incremented for the subsequent iteration. If the difference threshold limit is not satisfied, the phase measurement value may be maintained for the subsequent iteration, but one of the counters is preloaded for the subsequent iteration with a value equal to a magnitude of the difference between the count values.
1. An imaging system, comprising:
an array of pixels arranged in rows and columns, wherein each pixel comprises:
a plurality of single photon avalanche diodes (SPADs);
a first counter having a clock input coupled to outputs of the plurality of SPADs in said pixel and configured to count during a first temporal window;
a second counter having a clock input coupled to outputs of the plurality of SPADs in said pixel and configured to count during a second temporal window; and
a phase selector circuit configured to select a transition time between the first and second temporal windows in response to a phase measurement value; and
a phase processor circuit for each column of said array, wherein the phase processor circuit is configured to process a first count value from the first counter and a second count value from the second counter to determine said phase measurement value.
2. The imaging system of claim 1 , further comprising a clock generator circuit for each row of said array, wherein each clock generator circuit outputs a plurality of clock signals applied to the pixels of the corresponding row, said phase selector circuit receiving said plurality of clock signals as inputs, processing the plurality of clock signals and selecting certain clock signals in response to the phase measurement value to control opening of said first and second temporal windows.
3. The imaging system of claim 1 , further comprising:
a readout circuit configured to readout the phase measurement values from the phase processor circuits; and
a processing circuit configured to process said phase measurement values from the readout circuit and determine an average phase measurement value over time.
4. The imaging system of claim 3 , wherein said average phase measurement value over time is indicative of a time of flight measurement.
5. The imaging system of claim 1 , wherein the phase processor circuit processes the first count value and the second count value to:
determine whether either of the first count value or second count value in a current iteration exceeds an ambient threshold value; and
if so, maintain the phase measurement value for a subsequent iteration and load one of the first and second counters with a value equal to a magnitude of a difference between the first and second count values.
6. The imaging system of claim 5 , wherein said one the first and second counters loaded with the magnitude of the difference for the subsequent iteration is the counter whose count value is higher for the previous iteration.
7. The imaging system of claim 6 , wherein the other of the first and second counters is reset for subsequent iteration.
8. The imaging system of the claim 6 , wherein each of the first and second counters includes a load input configured to support loading of the counter with the magnitude of the difference.
9. The imaging system of claim 1 , wherein the phase processor circuit processes the first count value and the second count value to:
determine whether both the first count value and second count value in a current iteration are less than an ambient threshold value; and
if so, then either:
increment the phase measurement value for a subsequent iteration if the phase measurement value in a current iteration is less than a phase threshold; or
decrement the phase measurement value for the subsequent iteration if the phase measurement value in the current iteration is greater than the phase threshold.
10. The imaging system of claim 1 , wherein the phase processor circuit processes the first count value and the second count value to:
determine whether the first count value is less than the second count value in a current iteration, and if so then increment the phase measurement value for a subsequent iteration; and
determine whether the second count value is less than the first count value in the current iteration, and if so then decrement the phase measurement value for the subsequent iteration.
11. An imaging system, comprising:
a plurality of single photon avalanche diodes (SPADs);
a first counter having a clock input coupled to outputs of the plurality of SPADs and configured to count in response to assertion of a first enable signal;
a second counter having a clock input coupled to outputs of the plurality of SPADs and configured to count in response to assertion of a second enable signal;
a phase selector circuit configured to select a phase of the first and second enable signals in response to a phase measurement value; and
a phase processor circuit configured to process a first count value from the first counter and a second count value from the second counter to determine said phase measurement value.
12. The imaging system of claim 11 , further comprising a clock generator circuit configured to output a plurality of clock signals, said phase selector circuit receiving said plurality of clock signals as inputs, processing the plurality of clock signals and generating the first and second enable signals having phases selected by the phase measurement value.
13. The imaging system of claim 11 , further comprising a processing circuit configured to process said phase measurement values and determine an average phase measurement value over time.
14. The imaging system of claim 13 , wherein said average phase measurement value over time is indicative of a time of flight measurement.
15. The imaging system of claim 11 , wherein the phase processor circuit processes the first count value and the second count value to:
determine whether either of the first count value or second count value in a current iteration exceeds an ambient threshold value; and
if so, maintain the phase measurement value for a subsequent iteration and load one of the first and second counters with a value dependent on a magnitude of a difference between the first and second count values.
16. The imaging system of claim 15 , wherein said one the first and second counters loaded with the magnitude of the difference for the subsequent iteration is the counter whose count value is higher for the previous iteration.
17. The imaging system of claim 16 , wherein the other of the first and second counters is reset for subsequent iteration.
18. The imaging system of the claim 16 , wherein each of the first and second counters includes a load input configured to support loading of the counter with the magnitude of the difference.
19. The imaging system of claim 11 , wherein the phase processor circuit processes the first count value and the second count value to:
determine whether both the first count value and second count value in a current iteration are less than an ambient threshold value; and
if so, then either:
increment the phase measurement value for a subsequent iteration if the phase measurement value in a current iteration is less than a phase threshold; or
decrement the phase measurement value for the subsequent iteration if the phase measurement value in the current iteration is greater than the phase threshold.
20. The imaging system of claim 11 , wherein the phase processor circuit processes the first count value and the second count value to:
determine whether the first count value is less than the second count value in a current iteration, and if so then increment the phase measurement value for a subsequent iteration; and
determine whether the second count value is less than the first count value in the current iteration, and if so then decrement the phase measurement value for the subsequent iteration.