Method, apparatus, and system providing an imager with pixels having extended dynamic range
The dynamic range of a pixel is increased by using selective photosensor resets during a frame time of image capture at a timing depending on the light intensity that the pixel will be exposed to during the frame time. Pixels that will be exposed to high light intensity are reset later in the frame than pixels that will be exposed to lower light intensity.
1. An imager, comprising:
a pixel for providing an image output signal, said pixel comprising:
a photosensor for accumulating charge;
a storage region for storing charge transferred from the photosensor;
a transfer transistor for transferring charge from the photosensor to the storage region, wherein a drain of the transfer transistor is connected to the photosensor and a source of the transfer transistor is connected to the storage region;
a photosensor reset transistor connected between a selectively enabled control line and a gate of said transfer transistor, the photosensor reset transistor for transmitting a first control signal to a gate of the transfer transistor to reset the charge accumulated on the photosensor in accordance with said first control signal and a second control signal applied to a gate of said photosensor reset transistor.
2. The imager of claim 1 , further comprising a controller for delivering the first control signal to reset said photosensor during the frame time at a timing that corresponds to the value of light intensity sensed by the photosensor.
3. The imager of claim 1 , further comprising:
a reset transistor for resetting the charge on the storage region, wherein a drain of the reset transistor is connected to a supply voltage and a source of the reset transistor is connected to the storage region.
4. The imager of claim 1 , further comprising:
a source follower transistor for converting the charge stored in the storage region to a voltage, wherein a gate of the source follower transistor is connected to the storage region.
5. The imager of claim 4 , further comprising:
a row select transistor for outputting the voltage generated by the source follower transistor, wherein a drain of the row select transistor is connected to a source of the source follower transistor.
6. The imager of claim 1 , further comprising an array of said pixels.
7. a processor system, comprising:
a processor; and
an imager coupled to said processor, said imager comprising:
a pixel for providing an image output signal, said pixel comprising:
a photosensor for accumulating charge;
a storage region for storing charge transferred from the photosensor;
a transfer transistor for transferring charge from the photosensor to the storage region; and
a photosensor reset circuit for selectively resetting charge on said photosensor during an image capture period at a time related to a light level received by said photosensor; and
a photosensor reset transistor connected between a selectively enabled control line and a gate of said transfer transistor, the photosensor reset transistor for transmitting a first control signal to a gate of the transfer transistor to reset the charge accumulated on the photosensor in accordance with said first control signal and a second control signal applied to a gate of said photosensor reset transistor.
8. The processor system of claim 7 , wherein the processor system is a digital camera.
9. The processor system of claim 8 , wherein the digital camera is a still camera.
10. The processor system of claim 8 , wherein the digital camera is a video camera.
11. The processor system of claim 7 , further comprising a controller for controlling the photosensor reset circuit to reset the charge on said photosensor earlier in the image capture period if the light level is lower and later in the image capture period if the light level is higher.
12. The processor system of claim 7 , wherein the imager further comprises an array of said pixels.