Event-driven image sensor and method of reading the same

A method of reading out an event from a pixel of an event-driven sensor, the method comprising:

• propagating, through a series of column register cells of a column readout circuit, a first token, wherein the column readout circuit is coupled to column output lines of a pixel array of the sensor, the column readout circuit comprising, for each column output line, one of the column register cells coupled to the column output line;
• activating, by one of the column register cells, a column event output signal when it receives the first token while an event is indicated on the column output line;
• and, before or after the propagation of the first token and the activation of the column event output signal:
• propagating, through a series of row register cells of a row readout circuit, a second token, wherein the row readout circuit is coupled to row output lines of the pixel array, the row readout circuit comprising, for each of a plurality of sub-groups of the row output lines, one of the row register cells coupled to a plurality of the row output lines via a logic gate configured to combine row event signals on the plurality of the row output lines to generate a combined row event signal;
• and
• activating, by one of the row register cells, a row event output signal when it receives the second token while an event is indicated by the combined row event signal of the sub-group.

An event-driven sensor comprising:

• a pixel array;
• a column readout circuit coupled to column output lines of the pixel array, the column readout circuit comprising, for each of the column output lines, a column register cell coupled to the column output, the column register cells being coupled in series with each other to propagate a first token, wherein each column register cell is configured to activate a column event output signal when it receives the first token while an event is indicated on the column output line;
• and
• a row readout circuit coupled to row output lines of the pixel array, the row readout circuit comprising, for each of the row output lines, or for each of a plurality of sub-groups of the row output lines, a row register cell coupled to the row output line or lines, the row register cells being coupled in series with each other to propagate a second token, wherein each row register cell is configured to activate a row event output signal when it receives the second token while an event is indicated on the row output line, or on one of the row output lines of the sub-group;
• wherein each column output line is coupled in a daisy-chain to each of the pixels of its column, and each row output line is coupled in a daisy-chain to each of the pixels of its row.