One-hot

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In digital circuits, one-hot refers to a group of bits among which the legal combinations of values are only those with a single high (1) bit and all the others low (0). For example, the output of a decoder is usually a one-hot code, and sometimes the state of a state machine is represented by a one-hot code. A similar implementation in which all bits are '1' except one '0' is sometimes called one cold.

For example, a ring counter that has fifteen sequentially-ordered states would have a 'one-hot' implementation of fifteen flip flops chained in series with the Q output of each flip flop connected to the D input of the next and the D input of the first flip flop connected to the Q output of the fifteenth flip flop. The first flip flop in the chain represents the first state, the second represents the second state, and so on to the fifteenth flip flop which represents the last state. Upon reset of the state machine all of the flip flops are reset to '0' except the first in the chain which is set to '1'. The next clock edge arriving at the flip flops advances the one 'hot' bit to the second flip flop. The 'hot' bit advances in this way until the fifteenth state after which the state machine returns to the first state.