Counting to moduli other than 2n is a frequent requirement, the most common being to count through the binary-coded decimal (BCD) 8421 sequence. All that is required is a four-stage counter which, having counted from 0000 to 1001 (i.e., decimal 0 to 9; ten states), resets to 0000 on the next clock pulse. Develop a block diagram of an asynchronous decade counter and show its timing diagram.