Event-based image sensor and operating method thereof

An event-based image sensor comprising a controller, an address-event representation circuitry, and a plurality of pixel circuits forming a pixel array, the controller connected to each pixel circuit to control said pixel circuits, each pixel circuit comprising:

• a photoreceptor circuit comprising a light-sensitive element and an output, said photoreceptor circuit being configured to deliver at said output a photoreceptor current derived from a light impinging on the light-sensitive element;
• an analog bus selectively connected to a readout unit, the output of the photoreceptor circuit being selectively connected to said analog bus;
• and
• a plurality of current memory cells connected to the analog bus, each current memory cell being adapted to selectively store an electric current flowing in the analog bus and to selectively deliver into said analog bus an electric current stored within said current memory cell;
• wherein each pixel circuit further comprises a current comparator configured to detect a change in the photoreceptor current, said current comparator comprising:
• a current sign detector connected to the analog bus and configured to determine a sign of a value of an electric current flowing in the analog bus resulting from a compared electric current and a threshold current, said compared electric current resulting from a difference between at least the photoreceptor current and a previous photoreceptor current stored in a current memory cell;
• and
• at least one storage cell connected to an output of the current sign detector and having a state conditioned by the output of said current sign detector, the state of said at least one storage cell being representative of a change between the photoreceptor current and the previous photoreceptor current greater than the threshold current;
• wherein said at least one digital storage cell is connected to the address-event representation circuitry so that the address-event representation circuitry monitors the state of the storage cell to determine a change in the light impinging on the light-sensitive element;
• and
• wherein a readout of a pixel circuit through the connection of the analog bus to the readout unit is conditioned by the state of said at least one digital storage cell.

A method of operating an event-based image sensor, the event-based image sensor comprising a controller, an address-event representation circuitry, and a plurality of pixel circuits forming a pixel array, the controller connected to each pixel circuit to control said pixel circuits, the method comprising:

• detecting, with a change detection cycle, a change in light impinging onto a light-sensitive element of a photoreceptor circuit of a pixel circuit, said change detection cycle comprising:
• delivering, with the photoreceptor circuit, a photoreceptor current derived from a light exposure of the light-sensitive element—at the output of said photoreceptor circuit;
• storing the photoreceptor current in a first current memory cell;
• storing a compared electric current, resulting from a difference between the photoreceptor current and a previous photoreceptor current, in a second current memory cell;
• comparing, with a current sign detector, a value of the compared electric current with a value of a threshold current, and if the value of the compared electric current is above the value of the threshold current, setting the state of a digital storage cell to a first state;
• if the storage cell is in the first state, reading out the photoreceptor current, storing the photoreceptor current in the second current memory cell, and setting the state of the storage cell to a second state.