Short latency fingerprint sensing
An input device includes sensing pixels, each sensing pixel including a photodiode, and a gate driver circuit controlling operation of the sensing pixels. The gate driver circuit includes a set of cascaded gate driver units. Each of the cascaded gate driver units is associated with a subset of the sensing pixels, controlling an operation of the subset of the sensing pixels. The set of cascaded gate driver units output, during a reset phase of the input device, a global reset gate control signal to the sensing pixels in response to a clock signal. Each of the cascaded gate driver units sequentially output, during an operating phase of the input device, an operating gate control signal to the associated subset of the sensing pixels.
1. An input device comprising:
a plurality of sensing pixels, each sensing pixel comprising a photodiode; and
a gate driver circuit configured to:
control operation of the plurality of sensing pixels, the gate driver circuit comprising a set of cascaded gate driver units, wherein each of the cascaded gate driver units is associated with a subset of the plurality of sensing pixels and configured to control an operation of the subset of the plurality of sensing pixels;
output, by the set of cascaded gate driver units, during a reset phase of the input device, a global reset gate control signal to the plurality of sensing pixels in response to a clock signal; and
output, sequentially by each of the cascaded gate driver units, during an operating phase of the input device, an operating gate control signal to the associated subset of the plurality of sensing pixels.
2. The input device of claim 1 , wherein the cascaded gate driver units operate as a shift register.
3. The input device of claim 2 , wherein the cascaded gate driver units comprise, alternatingly:
a first type of gate driver unit that outputs the global reset gate control signal when the clock signal is in a high state; and
a second type of gate driver unit that outputs the global reset gate control signal when the clock signal is in a low state,
thereby causing the shift register to operate on clock half-cycles.
4. The input device of claim 3 ,
wherein each of the first type of gate driver units receives a first global pixel reset request, provided during the reset phase, and
wherein the first global pixel reset request in combination with the clock signal in the high state causes all of the first type of gate driver units to simultaneously output the global reset gate control signal.
5. The input device of claim 4 ,
wherein each of the second type of gate driver units receives a second global pixel reset request, provided during the reset phase, and
wherein the second global pixel reset request in combination with the clock signal in the low state causes all of the second type of gate driver units to simultaneously output the global reset gate control signal.
6. The input device of claim 1 , wherein the cascaded gate driver units sequentially output the operating gate control signal, with only one of the cascaded gate driver units outputting the operating gate control signal at a time.
7. The input device of claim 1 wherein, in each of the sensing pixels, the global reset gate control signal causes an application of a reverse biasing voltage to the photodiode of the sensing pixel.
8. The input device of claim 7 , wherein a duration of the global reset gate control signal is sufficiently long for the reverse biasing voltage to drive the photodiode from a photovoltaic operating mode to a photoconductive operating mode.
9. The input device of claim 7 , wherein the reverse biasing voltage is applied by connecting the sensing pixel to a charge amplifier.
10. The input device of claim 1 , wherein when the global reset gate control signals are emitted during the reset phase, the sensing pixels receiving the global reset gate control signals are in photovoltaic mode.
11. The input device of claim 1 , wherein when the operating gate control signals are emitted during the operating phase, the sensing pixels receiving the operating gate control signals are in photoconductive mode.
12. A method for operating an input device, the method comprising:
during a reset phase of the input device:
providing a first clock signal to a gate driver circuit of the input device;
outputting, by a plurality of cascaded gate driver units of the gate driver circuit, in response to the first clock signal, a global reset gate control signal to a plurality of sensing pixels of the input device, to reset the plurality of sensing pixels,
wherein each of the plurality of sensing pixels comprises a photodiode, and
wherein each of the cascaded gate driver units is associated with a subset of the plurality of sensing pixels, controlling the operation of the subset of the sensing pixels; and
during an operating phase of the input device, following the reset phase:
providing a second clock signal to the gate driver circuit of the input device;
outputting, sequentially by each of the plurality of cascaded gate driver units, in response to the second clock signal, an operating gate control signal to the associated subsets of the plurality of sensing pixels to obtain an image frame comprising a fingerprint image using the plurality of pixels; and
comparing the obtained image frame to archived image frames stored in a fingerprint data repository to determine whether the fingerprint image is recognized.
13. The method of claim 12 , wherein the cascaded gate driver units operate as a shift register.
14. The method of claim 12 , wherein the cascaded gate driver units comprise, alternatingly:
a first type of gate driver unit that outputs the global reset gate control signal when the clock signal is in a high state; and
a second type of gate driver unit that outputs the global reset gate control signal when the clock signal is in a low state,
thereby causing the shift register to operate on clock half-cycles.
15. The method of claim 14 , further comprising:
receiving by each of the first type of gate drive units a first global pixel reset request, during the reset phase,
wherein the first global pixel reset request in combination with the clock signal in the high state causes all of the first type of gate driver units to simultaneously output the global reset gate control signal.
16. The method of claim 14 , further comprising:
receiving by each of the second type of gate drive units a second global pixel reset request, during the reset phase,
wherein the second global pixel reset request in combination with the clock signal in the low state causes all of the second type of gate driver units to simultaneously output the global reset gate control signal.
17. The method of claim 12 , wherein the cascaded gate driver units sequentially output the operating gate control signal, with only one of the cascaded gate driver units outputting the operating gate control signal at a time.
18. The method of claim 12 , wherein, in each of the sensing pixels, the global reset gate control signal causes an application of a reverse biasing voltage to the photodiode of the sensing pixel.
19. The method of claim 18 , wherein a duration of the global reset gate control signal is sufficiently long for the reverse biasing voltage to drive the photodiode from a photovoltaic operating mode to a photoconductive operating mode.
20. The method of claim 18 , wherein the reverse biasing voltage is applied by connecting the sensing pixel to a charge amplifier.