Time-to-digital converter circuit and method for single-photon avalanche diode based depth sensing

1. A self-calibration time-to-digital converter (TDC) integrated circuit for single-photon avalanche diode (SPAD) based depth sensing, the circuit comprising:

a SPAD matrix with a plurality of SPAD pixels arranged in m rows and n columns, wherein the SPAD pixels in each column of SPAD pixels are connected by a column bus;

a global delay-locked loop (DLL) unit with n buffers and n clock signals;

a circular n-way multiplexer for circularly multiplexing n clock signals in the global DLL unit; and

an image signal processing unit for receiving image signals from a column TDC array.

2. The circuit of claim 1 , wherein the circuit further comprises:

a row control unit configured to enable one SPAD pixel in each row for a transmitting signal.

3. The circuit of claim 2 , wherein the circuit further comprises:

the column TDC array with n TDCs, wherein each TDC further comprises a counter and a latch, wherein the latch of each TDC is connected to the circular n-way multiplexer for circular multiplexing.

4. The circuit of claim 3 , wherein the SPAD matrix is implemented in a first layer, and the column TDC array is implemented in a second layer different from the first layer, wherein each column bus is connected to a corresponding column TDC through a corresponding hybrid bond.

5. The circuit of claim 3 , wherein each of the n clock signals in the global DLL is connected to one and only one of the n latches in the column TDC array at a given time, wherein each of the n clock signals in the global DLL is swept through all n latches in the column TDC array in n clock cycles.

6. The circuit of claim 4 , wherein the row control unit is implemented in the second layer.

7. The circuit of claim 4 , wherein the global DLL unit is implemented in the second layer.

8. The circuit of claim 4 , wherein the circular n-way multiplexer is implemented in the second layer.

9. The circuit of claim 4 , wherein the image signal processing unit is implemented in a third layer different from the first layer and the second layer.

10. A three-dimensional (3D) pixel-wise self-calibration time-to-digital converter (TDC) integrated circuit for single-photon avalanche diode (SPAD) based depth sensing, the circuit comprising:

a SPAD matrix with a plurality of SPAD pixels arranged in m rows and n columns implemented in a first layer;

a row control unit configured to enable one and only one SPAD pixel in each row for a transmitting signal;

a global delay-locked loop (DLL) unit with n buffers and n clock signals;

a circular n-way multiplexer for circularly multiplexing n clock signals in the global DLL unit to average out phase variations;

a TDC matrix arranged in m rows and n columns implemented in a second layer below the first layer, wherein each TDC is arranged directly below a corresponding SPAD pixel, wherein the TDC is connected to the corresponding SPAD pixel through a hybrid bond; and

an image signal processing unit for receiving image signals from a column TDC array.

11. The circuit of claim 10 , wherein the image signal processing unit is implemented in the second layer.

12. The circuit of claim 10 , wherein the image signal processing unit is implemented in a third layer below the second layer.

13. The circuit of claim 11 , wherein each TDC is connected to the image signal processing unit through a TSV.

14. The circuit of claim 10 , wherein the global DLL is implemented in the second layer.

15. The circuit of claim 10 , wherein the circular n-way multiplexer is implemented in the second layer.

16. The circuit of claim 10 , wherein each TDC further comprises a counter and a latch, wherein the latch of each TDC is connected to the circular n-way multiplexer for circular multiplexing.

17. A method for time-to-digital converter (TDC) circuit self-calibration, the method comprising the steps of:

in each frame of data collection, TDC transforming a time of flight signal from a single-photon avalanche diode (SPAD) array to depth information;

column TDC quantizing timing signal by delay-locked loop (DLL) phases resulting in an output causing non-uniformity of TDC;

circularly multiplexing the output of the DLL to column TDC latch in each frame data collection;

collecting multiple frames of data; and

averaging non-uniformity of TDC.

18. The method of claim 17 , wherein the averaging of the non-uniformity of TDC is by a histogram method.

19. The method of claim 17 , wherein in the step of circularly multiplexing the output of the DLL, a phase signal order is shifted clockwise.

20. The method of claim 17 , wherein in the step of circularly multiplexing the output of the DLL, a phase signal order is shifted counter-clockwise.

21. The method of claim 17 , wherein in the step of re-routing the global DLL output terminals, a phase signal order is shifted at least one phase in each frame data collection.

22. The method of claim 17 , after circular multiplexing of the plurality of frame data collection, all output data forming a histogram with depth information distribution.

23. The method of claim 17 , wherein the transforming, quantizing, multiplexing, and averaging are performed by a three-dimensional (3D) pixel-wise self-calibration TDC integrated circuit for SPAD based depth sensing comprising:

a SPAD matrix with a plurality of SPAD pixels arranged in m rows and n columns implemented in a first layer;

a global DLL unit with n buffers and n clock signals;

a circular n-way multiplexer for circularly multiplexing n clock signals in the global DLL unit to average out phase variations;

a TDC matrix arranged in m rows and n columns implemented in a second layer below the first layer, wherein each TDC is arranged directly below a corresponding SPAD pixel, wherein the TDC is connected to the corresponding SPAD pixel through a hybrid bond; and

an image signal processing unit for receiving image signals from a column TDC array.