Universal Measurement and Calibration Protocol (XCP)

Section outline

• 3: Synchronous Data Aquisition
  

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  • In this chapter you will learn about the most important feature of the XCP protocol - measuring data synchronously using the DAQ Mode.

    We will have a look at the different phases as well as the communication structure of the DAQ mode. We will compare the DAQ mode to the already known Polling mode. In short, we will discuss how different DAQ measurement modes with different measuring rates are implemented and how to decide which rates shall be used for measuring our signals.

• What we have learned so far…
  

  The DAQ mode is implemented using XCP events added to the software of an ECU. Each XCP event implements a single DAQ measurement mode. Since the XCP event will be called cyclically at a fixed rate in the ECU’s software, the corresponding DAQ measurement mode will also have a fixed rate.

  Once a XCP event is called, data will be measured – synchronously – and sent to the XCP master. Therefore, during the measurement the XCP master will no longer need to request any data. Only the XCP slave will sent XCP messages. This leads to a significantly lower busload compared to the polling mode.

  Signals should always be measured using a DAQ mode measuring at the same rate as the requested signals are updated in the ECU. Not at a slower rate (we will miss data), nor at a faster rate (we will create additional busload without getting any more information). Therefore, it is important to find out about the perfect DAQ measurement mode of your signals prior to your measurement.

  Besides cyclic XCP events (events called e.g. each 20 ms), also acyclic XCP events can be implemented. These XCP events will be triggered once a certain condition (e.g. shaftcrank reaches a certain position) is fulfilled. They can be used for measuring signals which are updated in the ECU on the same conditions.

  If you are wondering about the best way how to store the information which signal shall be measured using which DAQ measurement mode, the trainer of your upcoming CANape training will know the answer.