Inserting time stamps into a trace stream on the occurrence of a predetermined event

A data processing device generates trace packets. When a predetermined event happens, a flag is set. When a trace packet is generated, the flag is checked. If the flag is set, then a timestamp is generated and the flag is cleared. The timestamp may be appended to the trace packet. The event may be an exception, the end of exception handling, a context switch or a context synchronization. The flag may be set whether or not tracing is enabled. The flag may be in a programmable register of a trace module, which can be written by a software instruction when the event occurs. The trace packet may record data processed, data generated, instructions executed, addresses accessed or values stored in registers by the device.