Techniques are disclosed relating to coding data in an apparatus. In one embodiment, the apparatus includes a coder circuit coupled to a data bus, where the coder circuit is configured to receive an indication that data is being transmitted over the data bus from a first circuit to a second circuit. The coder circuit is configured to perform a coding operation on the data in response to receiving the indication. In some embodiments, the coder circuit is configured to operate in a mode in which the coder circuit captures data of a data transmission via the data bus without being specified as a participant of the data transmission. When the coder circuit is not operating in the mode, the coder circuit is not configured to capture data of a data transmission without being specified as a participant of the data transmission.