Tuning limits are set for operational parameters in a processing stage within a data processing apparatus for processing a signal and outputting it at an output time. If a signal output between the output time and a predetermined time later does not have a stable value, the predetermined time later being before a next output time, an error is signaled. A tuning circuit adjusts an operational parameter of the processing stage in accordance with a tuning limit. A signal passing along a critical path of the processing stage tuned to the tuning limit is expected to reach the output of the processing stage at a preset time later than the output time, the preset time being less than the predetermined time.