PIXEL READ CIRCUITRY

• US 20090153715A1
• Filed: 12/11/2008
• Published: 06/18/2009
• Est. Priority Date: 12/13/2007
• Status: Active Grant

• Current Assignee: Stmicroelectronics France (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SA (STMicroelectronics NV)
• Inventors: Deschamps, Benoit, Barbier, Frederic
• Granted Patent: US 8,159,587 B2
• Time in Patent Office: Days
• US Class Current: 348/308
• CPC Class Codes: H04N 25/583 with different integration times
• CPC Class Codes: H04N 25/589 with different integration times, e.g. short and long exposures

Abstract

A method of reading voltages from an image sensor having an array of pixels, each pixel Having at least one photodiode connectable to a storage node, the method including: controlling each pixel in a row of pixels to store and output a first voltage value at a first instance, a second voltage value at a second instance, and a third voltage value at a third instance, the first, second and third voltage values being representative of charge accumulated by the photodiodes during an integration phase; comparing the first voltage value from each pixel with a reference threshold; sampling for each pixel, based on the comparison, one of the second and third voltage values: and generating an output pixel value based on the sampled one of the second and third voltage values

Claims

1. A method of reading voltages from an image sensor having an array of pixels, each pixel comprising at least one photodiode connectable to a storage node, the method comprising:

controlling each pixel in a row of pixels to store and output a first voltage value at a first instance, a second voltage value at a second instance, and a third voltage value at a third instance, the first, second and third voltage values being representative of charge accumulated by said photodiodes during an integration phase;

comparing the first voltage value from each pixel with a reference threshold;

sampling for each pixel, based on said comparison, one of said second and third voltage values; and

generating an output pixel value based on the sampled one of said second and third voltage values.

2. The method of claim 1, wherein the second instance is after the first instance, and the third instance is after the second instance.

3. The method of claim 1, wherein said first instance is a time period after the start of said integration phase, said second instance is at the end of a first integration period, said second voltage value being representative of charge accumulated by said photodiode during said first integration period, and said third instance is at the end of a second integration period, said third voltage value representative of charge accumulated by said photodiode during said second integration period, said second integration period being longer than said first integration period.

4. The method of claim 3, wherein said step of controlling each pixel to output a first voltage value comprises applying a first voltage level to a gate node of a transistor in each pixel coupled between said photodiode and said storage node, said first voltage level being chosen such that charge accumulated above a first threshold is transferred to said storage node.

5. The method of claim 4, wherein selecting one of the second and third voltage values comprises selecting said second voltage value if said first voltage value indicates that charge accumulated by said photodiode was above said first threshold, and otherwise selecting said third voltage value, and wherein generating an output pixel value based on said third voltage value comprises digitizing the difference between said third voltage value and a reference value, and generating an output pixel value based on said second voltage value comprises digitizing the difference between said second voltage value and said reference value multiplied by T_L/T_S, where T_L is the duration of the long integration period, and T_S is the duration of the short integration period.

6. The method of claim 1, wherein the voltage at said storage node is reset between storage of said first and second voltage values, and is not reset between storage of said second and third voltage values.

7. An image sensor comprising:

an array of pixels, each pixel comprising at least one photodiode and a first transistor associated with said photodiode controllable to connect said photodiode to a storage node in said pixel to store a voltage value representative of a voltage of said photodiode;

row control circuitry arranged to control the first transistors in a row of said pixels to each store first, second and third voltage values representative of voltages of the photodiodes at respective first, second and third times instances during an integration phase of the photodiodes, and to output said first, second and third voltage values; and

output circuitry arranged to sample said first voltage value and to compare said first voltage values for each pixel with a reference threshold, and to sample for each pixel, based on said comparison, one of said second and third voltage values for use in generating an output pixel value.

8. The image sensor of claim 7, further comprising a memory connected to said output circuitry arranged to store a one-bit value associated with each pixel in a row of pixels indicating the result of said comparison.

9. The image sensor of claim 7, wherein, if said third voltage value is selected, said output circuitry is arranged to generate said output pixel value as the digitalized value of the difference between said third voltage value and a reference value, and if said second voltage value is selected, said output circuitry is arranged to generate said output pixel value as the digitalized value of the difference between said second voltage value and said reference value multiplied by a gain factor.

10. A digital camera comprising the image sensor of claim 7.

11. A mobile telephone comprising the image sensor of claim 7.