Pixel sensor having hierarchical memory

1. An apparatus comprising:

an array of pixel cells, each pixel cell including:

a photodiode to generate a charge in response to incident light,

a charge sensing unit configured to generate a voltage based on the charge, and

an in-pixel memory configured to store a digital output generated by a quantizer based on quantizing the voltage;

a bus interface;

an off-array frame memory connected to the bus interface and the array of pixel cells, the in-pixel memory of the array of pixel cells being connected to the frame memory via a set of parallel interconnects; and

a frame memory controller configured to:

control the frame memory to receive, via the set of parallel interconnects, the digital outputs from the in-pixel memory of each pixel cell of the array of pixel cells;

store the digital outputs in the frame memory;

fetch the digital outputs from the frame memory; and

transmit the digital outputs to a host device via the bus interface.

2. The apparatus of claim 1 , wherein the frame memory is housed within a same chip package as the array of pixel cells.

3. The apparatus of claim 2 , wherein the in-pixel memory of the array of pixel cells is formed in a first semiconductor substrate;

wherein the frame memory is formed in a second semiconductor substrate, the first semiconductor substrate and the second semiconductor forming a vertical stack; and

wherein the in-pixel memory of the array of pixel cells and the frame memory are connected via a set of parallel vertical interconnects.

4. The apparatus of claim 3 , wherein the set of parallel vertical interconnects comprise at least one of: through silicon vias (TSVs), micro-TSVs, or Copper-Copper bumps.

5. The apparatus of claim 1 , wherein:

the photodiode is a first photodiode;

the charge is a first charge;

the digital output is a first digital output;

the voltage is a first voltage;

each pixel cell of the array of pixel cells further comprises a second photodiode configured to generate a second charge in response to incident light;

the charge sensing unit of the pixel cell is configured to, after outputting the first voltage, output a second voltage based on the second charge;

the pixel cell is configured to, after the first digital output is transferred from the in-pixel memory to the frame memory, use the quantizer to quantize the second voltage to generate a second digital output; and

the in-pixel memory is configured to overwrite the first digital output with the second digital output.

6. The apparatus of claim 5 , wherein the in-pixel memory has sufficient capacity to store the first digital output or the second digital output.

7. The apparatus of claim 5 , wherein the in-pixel memory of the array of pixel cells is connected to the bus interface; and

wherein the in-pixel memory of the array of pixel cells is configured to transmit the second digital outputs to the host device via the bus interface.

8. The apparatus of claim 5 , wherein the first photodiode is configured to generate the first charge within a first exposure period between a first time and a second time;

wherein the second photodiode is configured to generate the second charge within a second exposure period between a third time and a fourth time; and

wherein the fourth time is delayed from the third time by a processing delay, the processing delay including a quantization time by the quantizer to generate the first digital output, an in-pixel memory access time to store the first digital output into the in-pixel memory, and a transfer time of the first digital outputs from the in-pixel memory of the array of pixel cells to the frame memory.

9. The apparatus of claim 8 , wherein the first time is also delayed from the third time by the processing delay.

10. The apparatus of claim 1 , wherein the digital output is a first digital output;

wherein the photodiode is configured to:

store a first portion of the charge as residual charge; and

after being saturated by the residual charge, output a second portion of the charge as overflow charge to the charge sensing unit; and

wherein each pixel cell of the array of pixel cells is configured to:

use the quantizer to quantize the overflow charge stored in the charge sensing unit to generate the first digital output;

transfer the residual charge to the charge sensing unit;

use the quantizer to quantize the residual charge stored in the charge sensing unit to generate a second digital output; and

store the first digital output in the in-pixel memory based on the photodiode being saturated by the residual charge.

11. The apparatus of claim 10 , wherein the charge sensing unit includes a capacitor having a configurable capacity;

wherein the capacitor is configured to have a first capacity to store the overflow charge; and

wherein the capacitor is configured to have a second capacity lower than the first capacity to store the residual charge.

12. The apparatus of claim 1 , wherein the photodiode is configured to store the charge; and

wherein each pixel cell of the array of pixel cells is configured to transfer the charge from the photodiode to the charge sensing unit to generate the voltage.

13. The apparatus of claim 1 ,

wherein the digital outputs are stored as light measurement values in the frame memory;

wherein each pixel cell of the array of pixel cells is configured to:

reset the charge sensing unit to cause the charge sensing unit to output a reset voltage;

use the quantizer to output a dark pixel value based on quantizing the reset voltage; and

store the dark pixel value in the in-pixel memory; and

wherein the apparatus further comprises a digital correlated double sampling (CDS) circuit configured to:

for each pixel cell, fetch the dark pixel value and the light measurement value from the frame memory;

compute a difference between the dark pixel value and light measurement value; and

transmit the difference as a noise-corrected pixel value to the host device via the bus interface.

14. The apparatus of claim 1 , wherein the frame memory controller is configured to:

within a first time, when the in-pixel memory of a first row of pixel cells of the array of pixel cells is connected to a first row of memory cells of the frame memory:

receive a first subset of the digital outputs from the in-pixel memory of the first row of pixel cells of the array of pixel cells; and

store the first subset of the digital outputs at the first row of memory cells of the frame memory; and

within a second time after the first time, when the in-pixel memory of a second row of pixel cells of the array of pixel cells is connected to a second row of memory cells of the frame memory:

receive a second subset of the digital outputs from the in-pixel memory of the second row of pixel cells of the array of pixel cells; and

store the second subset of the digital outputs at the second row of memory cells of the frame memory.

15. The apparatus of claim 1 , wherein the frame memory controller is configured to:

within a first time, when the in-pixel memory of a first row and a second row of pixel cells of the array of pixel cells is connected to, respectively and simultaneously, a first row and a second row of memory cells of the frame memory:

receive a first subset of the digital outputs from the in-pixel memory of the first row of pixel cells;

receive a second subset of the digital outputs from the in-pixel memory of the second row of pixel cells;

store the first subset of the digital outputs at the second row of memory cells of the frame memory; and

store the first subset of the digital outputs at the second row of memory cells of the frame memory; and

within a second time, when the in-pixel memory of a third row and a fourth row of pixel cells of the array of pixel cells is connected to, respectively and simultaneously, a third row and a fourth row of memory cells of the frame memory:

receive a third subset of the digital outputs from the in-pixel memory of the third row of pixel cells;

receive a fourth subset of the digital outputs from the in-pixel memory of the fourth row of pixel cells;

store the third subset of the digital outputs at the third row of memory cells of the frame memory; and

store the fourth subset of the digital outputs at the fourth row of memory cells of the frame memory.

16. The apparatus of claim 1 , wherein the bus interface is a first bus interface;

wherein the apparatus further comprises a second bus interface;

wherein the frame memory controller is configured to:

at a first time, select a first memory cell and a second memory cell of the frame memory;

transmit, via the first bus interface, a first digital output from the first memory cell to the host device;

transmit, via the second bus interface, a second digital output from the second memory cell to the host device;

at a second time, select a third memory cell and a fourth memory cell of the frame memory;

transmit, via the first bus interface, a third digital output from the third memory cell to the host device; and

transmit, via the second bus interface, a fourth digital output from the fourth memory cell to the host device.

17. The apparatus of claim 16 , wherein the photodiode is a first photodiode associated with a first wavelength channel;

wherein each pixel cell of the array of pixel cells further comprises a second photodiode associated with second wavelength channel;

wherein the first digital output and the second digital output are generated for the first photodiode and the second photodiode of a first pixel cell of the array of pixel cells; and

wherein the third digital output and the fourth digital output are generated for the first photodiode and the second photodiode of a second pixel cell of the array of pixel cells.

18. The apparatus of claim 17 , wherein:

the first digital output and the third digital output are generated by the first photodiode of, respectively, the first pixel cell and the second pixel cell; and

the second digital output and the fourth digital output are generated by the second photodiode of, respectively, the first pixel cell and the second pixel cell.

19. The apparatus of claim 17 , wherein:

the first digital output and the third digital output are generated by, respectively, the first photodiode of the first pixel cell and the second photodiode of the second pixel cell; and

the second digital output and the fourth digital output are generated by, respectively, the second photodiode of the first pixel cell and the first photodiode of the second pixel cell.

20. A method comprising:

generating, using a photodiode of each pixel cell of an array of pixel cells, a charge in response to incident light;

generating, using a charge sensing unit of each pixel cell of the array of pixel cells and based on the charge, a voltage;

generating, using a quantizer, a digital output for each pixel cell based on quantizing the voltage output by the charge sensing unit of the pixel cell;

storing, at an in-pixel memory of each pixel cell of the array of pixel cells, the digital output for the respective pixel cell;

receiving, by an off-array frame memory via a set of parallel interconnects, the digital outputs from the in-pixel memory of each pixel cell of the array of pixel cells;

storing the digital outputs in the frame memory; and

transmitting, via a bus interface, the digital outputs from the frame memory to a host device.