one-cold
A set of bits with only a single low (0) bit, with the remaining bits high (1).
one-cold: a bit pattern with exactly one zero
A one-cold value is a binary word in which all bits are set to 1 except for exactly one bit, which is 0. The position of that single 0 bit encodes information. If you have an 8-bit one-cold word, you might see 11111011, where the 0 occupies bit position 2 (counting from 0 on the right). This is the complement of a one-hot encoding, where a single 1 bit identifies a state and the rest are 0.
One-cold encoding is used in digital logic and microcontroller design to represent a limited set of mutually exclusive states. Where a one-hot encoding requires an OR gate to detect activation, one-cold detection uses NAND logic instead. The complementary nature makes one-cold useful in circuits optimized for NAND-based implementations or where pull-up logic is the natural topology.
Practical use and constraints
One-cold appears in bus arbitration, multiplexer selection, and priority encoders where speed and gate count matter. A priority circuit can identify which line is low in constant time by feeding the one-cold word directly into a priority encoder. However, one-cold wastes one entire bit to represent any state; an n-bit one-cold word encodes only n possible values, whereas standard binary uses those n bits to represent 2^n values. This makes one-cold impractical for large state spaces.
The main risk with one-cold is catastrophic failure if noise or a fault causes more than one bit to drop low. The circuit then loses the ability to distinguish which state is actually active. Some designs add error detection or use checksum bits alongside one-cold to catch such faults. One-cold is also harder to read and debug in simulation; humans tend to think in binary or hexadecimal, not in the position of the missing 1.
The term reflects its opposite: one-hot encoding had already become standard in digital design literature, so one-cold was named by contrast. It sees continued use in applications where the complementary logic gates offer a real speed or power advantage, but it remains a specialist tool rather than a default choice.