Fractional-readout oversampled image sensor
Signals representative of total photocharge integrated within respective image-sensor pixels are read out of the pixels after a first exposure interval that constitutes a first fraction of a frame interval. Signals in excess of a threshold level are read out of the pixels after an ensuing second exposure interval that constitutes a second fraction of the frame interval, leaving residual photocharge within the pixels. After a third exposure interval that constitutes a third fraction of the frame interval, signals representative of a combination of at least the residual photocharge and photocharge integrated within the pixels during the third exposure interval are read out of the pixels.
1. A method of operation within an integrated-circuit image sensor during an image frame interval, the method comprising:
asserting a first control signal pulse having a first magnitude on a transfer-gate signal line coupled to transfer gates of respective pixels within a pixel array to enable photocharge integrated within respective photodiodes of the pixels since the start of the image frame interval to be transferred in its entirety to respective floating diffusion nodes, the transfer-gate signal line constituting the sole signal line that controls charge transfer between the photodiodes and respective floating diffusion nodes;
asserting a second control signal pulse having a magnitude less than the first magnitude on the transfer-gate signal line to enable transfer, from the photodiodes to the respective floating diffusion nodes, of photocharge integrated in excess of a threshold photocharge level within the photodiodes after assertion of the first control pulse; and
asserting a third control signal pulse having the first magnitude on the transfer-gate signal line after assertion of the second control pulse to enable further photocharge to be transferred from the photodiodes to the respective floating diffusion nodes.
2. The method of claim 1 wherein asserting the second control signal pulse having the magnitude less than the first magnitude to enable photocharge in excess of the threshold level to be transferred comprises leaving residual photocharge in the respective pixels after assertion of the second control pulse, and wherein asserting the third control pulse having the first magnitude comprises enabling transfer, to the respective floating diffusion nodes, of both the residual photocharge and photocharge integrated within the respective photodiodes during an interval that transpires between assertion of the second and third control signal pulses.
3. The method of claim 1 further comprising generating respective fractional read-out signals representative of the photocharge transferred to the floating diffusion nodes in response the second control signal pulse and generating respective full read-out signals representative of the photocharge transferred to the floating diffusion nodes in response to the first and third control signal pulses, the fractional read-out signals having a maximum valid signal level lower than a maximum valid signal level of the full read-out signals.
4. The method of claim 3 further comprising digitizing the fractional read-out signals to yield digital pixel values and setting each of the digital pixel values that does not exceed a minimum digital threshold to a predetermined value.
5. The method of claim 1 further comprising generating respective fractional read-out signals representative of the photocharge transferred to the floating diffusion nodes in response the second control signal pulse and generating respective full read-out signals representative of the photocharge transferred to the floating diffusion nodes in response to the first and third control signal pulses, and wherein the threshold level corresponds to an integrated photocharge level above which photon shot noise is a predominant component of total noise in the full read-out signals.
6. An integrated-circuit image sensor comprising:
a pixel array;
a transfer-gate signal line coupled to transfer gates of respective pixels within the pixel array and constituting the sole signal line that controls charge transfer between respective photodiodes of the pixels and respective floating diffusion nodes; and
control circuitry to assert on the transfer-gate signal line during an image frame interval:
a first control signal pulse having a first magnitude to enable photocharge integrated within respective photodiodes of the pixels since the start of the image frame interval to be transferred in its entirety to the respective floating diffusion nodes;
a second control signal pulse having a magnitude less than the first magnitude to enable transfer, from the photodiodes of the pixels to the respective floating diffusion nodes, of photocharge integrated within the photodiodes in excess of a threshold photocharge level after assertion of the first control pulse; and
a third control signal pulse having the first magnitude to enable further photocharge to be transferred from the photodiodes to the respective floating diffusion nodes.
7. The integrated-circuit image sensor of claim 6 wherein the control circuitry to assert the second control signal pulse that enables photocharge in excess of the threshold level to be transferred comprises circuitry to assert the second control signal pulse with a magnitude that leaves residual photocharge in the respective pixels after assertion of the second control pulse, and wherein the control circuitry to assert the third control pulse having the first magnitude comprises control circuitry to enable transfer, to the respective floating diffusion nodes, of both the residual photocharge and photocharge integrated within the respective photodiodes during an interval that transpires between assertion of the second and third control signal pulses.
8. The integrated-circuit image sensor of claim 6 wherein the control circuitry further comprises circuitry to generate (i) respective fractional read-out signals representative of the photocharge transferred to the floating diffusion nodes in response the second control signal pulse and (ii) respective full read-out signals representative of the photocharge transferred to the floating diffusion nodes in response to the first and third control signal pulses, the fractional read-out signals having a maximum valid signal level lower than a maximum valid signal level of the full read-out signals.
9. The integrated-circuit image sensor of claim 8 further comprising logic circuitry to convert the fractional read-out signals to respective digital pixel values and to set each of the digital pixel values that does not exceed a minimum digital threshold to a predetermined value.
10. The integrated-circuit image sensor of claim 6 wherein the control circuitry further comprises circuitry to generate (i) respective fractional read-out signals representative of the photocharge transferred to the floating diffusion nodes in response the second control signal pulse and (ii) respective full read-out signals representative of the photocharge transferred to the floating diffusion nodes in response to the first and third control signal pulses, and wherein the threshold level corresponds to an integrated photocharge level above which photon shot noise is a predominant component of total noise in the full read-out signals.