CMOS imager having asynchronous pixel readout in order of pixel illumination

An image sensor comprising:

• a plurality of pixel cells disposed in a two-dimensional array of M rows and N columns, each of the pixel cells containing a photodiode and a comparator circuit, each comparator circuit having a first input terminal connected to the corresponding photodiode and a second input terminal receiving a common reference voltage, each comparator circuit delivering a comparator pulse when an analog signal from the photodiode equals the reference voltage;
• a set of M row lines, each row line connected to said comparator circuit of each pixel cell of a corresponding row of pixel cells;
• a set of N column lines, each column line connected to said comparator circuit of each pixel cell of a corresponding column of pixel cells;
• a row arbiter operable during a predetermined exposure interval having M inputs, each input connected to a corresponding one of said M row lines, said row arbiter
• detecting when a row line receives a comparator pulse from a pixel cell in said corresponding row,
• generating a row signal indicative of said row, said row arbiter selecting a single row if more than one row line receives a comparator pulse, and
• enabling said comparators of pixel cells in said selected row to send said comparator pulses to said corresponding column lines;
• a column arbiter operable during said predetermined exposure interval having N inputs, each input connected to a corresponding one of said N column lines, said column arbiter
• detecting when a column line receives a comparator pulse from a pixel cell in said corresponding column,
• generating a column signal indicative of said column, said column arbiter selecting a single column if more than one column line receives a comparator pulse,
• generating an address valid signal when said column signal is valid;
• and
• conversion hardware operable during said predetermined exposure interval, said conversion hardware converting a time of arrival of an address valid signal during the exposure interval into an illumination level of said corresponding pixel cell.