Gray code counter

1. A gray code counter for generating a gray code having N bits comprising:

a first toggle flip-flop stage including first and second latches;

N−1 cascaded toggle flip-flop stages each including third and fourth latches;

wherein each of the latches has a clock input, a complementary clock input, a data D input, a Q output and a complementary Q output;

wherein, in the first stage, the clock input of the first latch and the complementary clock input of the second latch are for receiving an external clock signal, the complementary clock input of the first latch and the clock input of the second latch are for receiving a complement of the external clock signal, the D input of the first latch is coupled to the complementary Q output of the second latch, and the Q output of the first latch is coupled to the D input of the second latch;

wherein, in each of the N−1 stages, the D input of the third latch is coupled to the complementary Q output of the fourth latch, the Q output of the third latch is coupled to the D input of the fourth latch, the Q output of the fourth latch is coupled to the clock input of the third latch and the complementary clock input of the fourth latch of a successive N−1 th stage, and the complementary Q output of the fourth latch is coupled to the complementary clock input of the third latch and the clock input of the fourth latch of the successive N−1 th stage;

wherein, for the first of the cascaded N−1 stages, the clock input of the third latch and the complementary clock input of the fourth latch are connected to the Q output of the second latch, and the complementary clock input of the third latch and the clock input of the fourth latch are connected to the complementary Q output of the second latch; and

wherein the Q outputs of the third latches of each of the successive N−1 th stages constitute, respectively, a least significant bit through one less than the most significant bit of a gray code generated based on the external clock signal, and wherein the Q output of the fourth latch of the N th stage constitutes the most significant bit of the gray code.

2. A gray code counter for generating a gray code having N bits comprising:

N−1 cascaded toggle flip-flop stages each including first and second latches, wherein each of the latches has a clock input, a complementary clock input, a data D input, a Q output and a complementary Q output;

wherein, in the first stage of the N−1 cascaded stages, the clock input of the first latch and the complementary clock input of the second latch are for receiving an external clock signal, the complementary clock input of the first latch and the clock input of the second latch are for receiving a complement of the external clock signal, the D input of the first latch is coupled to the complementary Q output of the second latch, and the Q output of the first latch is coupled to the D input of the second latch;

wherein, in each of the second through the N−1 th stages, the D input of the first latch is coupled to the complementary Q output of the second latch, the Q output of the first latch is coupled to the D input of the second latch, the Q output of the second latch is coupled to the clock input of the first latch and the complementary clock input of the second latch of a successive N-1 th stage, and the complementary Q output of the second latch is coupled to the complementary clock input of the first latch and the clock input of the second latch of the successive N-1 th stage;

wherein, for the second of the cascaded N-1 stages, the clock input of the first latch and the complementary clock input of the second latch are connected to the Q output of the second latch of the first N−1 th stage, and the complementary clock input of the first latch and the clock input of the second latch are connected to the complementary Q output of the second latch of the first N-1 th stage; and

wherein the Q outputs of the first latches of each of the successive N−1 stages constitute, respectively, a least significant bit through one less than the most significant bit of a gray code generated based on the external clock signal, and wherein the Q output of the second latch of the N th stage constitutes the most significant bit of the gray code.

3. A gray code counter for generating a gray code having N bits comprising:

a chain of N pairs of first and second D-type flip-flops, wherein each of the D-type flip-flops has a clock input, a complementary clock input, a data D input, a Q output and a complementary Q output;

wherein, in the first pair, the Q output of the first flip-flop is coupled to the D input of the second flip-flop, the complementary Q output of the second flip-flop is coupled to the D input of the first flip-flop, and the clock and the complementary clock inputs of each of the first and second flip-flops are for coupling to an external clock signal and a complement of the external clock signal, respectively;

wherein, for a jth pair of the N pairs in the chain except for the N th pair, the Q output of the second flip-flop is coupled to each of the clock inputs of the first and second flip-flops of a pair succeeding the jth pair in the chain, and the complementary Q output of the second flip-flop is coupled to each of the complementary clock inputs of the first and second flip-flops of the pair succeeding the jth pair in the chain;

wherein, for each of the N pairs following the first pair of the N pairs in the chain, the D input, the Q output, the complementary Q output and the complementary clock input of the second flip-flop and the complementary Q output of the first flip-flop are coupled to a three input exclusive-OR gate circuit, and wherein the exclusive OR gate circuit includes an output; and

wherein the Q outputs of the first flip-flops of the successive N pairs in the chain constitute, respectively, the least significant bit and every other more significant bit of a gray code generated based on the external clock signal, and wherein the successive exclusive-OR gate circuit outputs constitute, respectively, one more than the least significant bit and every other more significant bit of the gray code.

4. A gray code counter for generating a gray code having N bits comprising:

a chain of N pairs of first and second D-type flip-flops, wherein each of the D-type flip flops has a clock input, a complementary clock input, a data D input, a Q output and a complementary Q output;

wherein, in the first pair, the Q output of the first flip-flop is coupled to the D input of the second flip-flop, the complementary Q output of the second flip-flop is coupled to the D input of the first flip-flop, and the clock and the complementary clock inputs of each of the first and second flip-flops are for coupling to an external clock signal and a complement of the external clock signal, respectively;

wherein, for a jth pair of the N pairs in the chain except for the Nth pair, the Q output of the second flip-flop is coupled to each of the clock inputs of the first and second flip-flops of a pair succeeding the jth pair in the chain and the complement of the Q output of the second flip-flop is coupled to each of the complementary clock inputs of the first and second flip-flops of the pair succeeding the jth pair in the chain;

wherein, for each of the N pairs following the first of the N pairs in the chain, the D input, and the Q output of the second flip-flop are coupled to first and second inputs, respectively, of an exclusive-OR gate, wherein the exclusive OR gate includes an output coupled to an input of a latch, wherein the latch includes an output and a clock input, a complementary clock input, the clock inputs of the latch being coupled to the clock and complementary clock inputs of the second flip-flop, respectively, and

wherein the Q outputs of the first flip-flops of the successive N pairs in the chain constitute, respectively, the least significant gray code bit value and every other more significant bit generated based on the external clock signal, and wherein the successive latch outputs constitute, respectively, one more than the least significant bit and every other more significant bit of the gray code.

5. The counter of claim 4 , wherein the latch is a D-type flip-flop.

6. A focal plane array apparatus including:

an array of imaging pixels; and

digital binary counters coupled to the respective pixels, where at least one the counters is an N bit gray code counter comprising:

a first toggle flip-flop stage including first and second latches;

N−1 cascaded toggle flip-flop stages each including third and fourth latches;

wherein each of the latches has a clock input, a complementary clock input, a data D input, a Q output and a complementary Q output;

wherein, in the first stage, the clock input of the first latch and the complementary clock input of the second latch are for receiving an external clock signal, the complementary clock input of the first latch and the clock input of the second latch are for receiving a complement of the external clock signal, the D input of the first latch is coupled to the complementary Q output of the second latch, and the Q output of the first latch is coupled to the D input of the second latch;

wherein, in each of the N−1 stages, the D input of the third latch is coupled to the complementary Q output of the fourth latch, the Q output of the third latch is coupled to the D input of the fourth latch, the Q output of the fourth latch is coupled to the clock input of the third latch and the complementary clock input of the fourth latch of a successive N−1 th stage, and the complementary Q output of the fourth latch is coupled to the complementary clock input of the third latch and the clock input of the fourth latch of the successive N−1 th stage;

wherein, for the first of the cascaded N−1 stages, the clock input of the third latch and the complementary clock input of the fourth latch are connected to the Q output of the second latch, and the complementary clock input of the third latch and the clock input of the fourth latch are connected to the complementary Q output of the second latch; and

wherein the Q outputs of the third latches of each of the successive N−1 th stages constitute, respectively, a least significant bit through one less than the most significant bit of a gray code generated based on the external clock signal, and wherein the Q output of the fourth latch of the N th stage constitutes the most significant bit of the gray code.

7. A focal plane array apparatus including:

an array of imaging pixels; and

digital binary counters coupled to the respective pixels, where at least one the counters is an N bit gray code counter comprising:

N−1 cascaded toggle flip-flop stages each including first and second latches, wherein each of the latches has a clock input, a complementary clock input, a data D input, a Q output and a complementary Q output;

wherein, in the first stage of the N−1 cascaded stages, the clock input of the first latch and the complementary clock input of the second latch are for receiving an external clock signal, the complementary clock input of the first latch and the clock input of the second latch are for receiving a complement of the external clock signal, the D input of the first latch is coupled to the complementary Q output of the second latch, and the Q output of the first latch is coupled to the D input of the second latch;

wherein, in each of the second through the N−1 th stages, the D input of the first latch is coupled to the complementary Q output of the second latch, the Q output of the first latch is coupled to the D input of the second latch, the Q output of the second latch is coupled to the clock input of the first latch and the complementary clock input of the second latch of a successive N−1 th stage, and the complementary Q output of the second latch is coupled to the complementary clock input of the first latch and the clock input of the second latch of the successive N−1 th stage;

wherein, for the second of the cascaded N−1 stages, the clock input of the first latch and the complementary clock input of the second latch are connected to the Q output of the second latch of the first N−1 th stage, and the complementary clock input of the first latch and the clock input of the second latch are connected to the complementary Q output of the second latch of the first N−1 th stage; and

wherein the Q outputs of the first latches of each of the successive N−1 stages constitute, respectively, a least significant bit through one less than the most significant bit of a gray code generated based on the external clock signal, and wherein the Q output of the second latch of the N th stage constitutes the most significant bit of the gray code.