Image Processing Reference
In-Depth Information
XCP packet
PID
Fill
DAQ
Time stamp
Data
Identification field
FIGURE .
Structure of an XCP packet.
18.6.1.1 XCP Protocol Layer
heXCPprotocollayeristhehigherlayeroftheXCPprotocolfamily.hislayerimplementsthe
main operations of XCP, which are described in detail in Sections ... through .... ....The XCP
protocol layer itself is independent of a concrete communication protocol (e.g., CAN, FlexRay, etc.).
Data exchange on the XCP protocol layer level is performed by data objects called “XCP packets.”
Figure . illustrates the structure of an XCP packet.
An XCP packet starts with an “Identification Field” containing a “Packet identifier (PID),” which
is used to establish a common understanding about the semantics of the packet's data between the
XCPmasterandtheXCPslave.hePIDisthususedtouniquelyidentifyeachofthefollowingtwo
basic packet types and their respective subtypes:
Command transfer object
(
CTO
)
packets
: his packet type is used for the transfer of generic control
commands from the XCP master to the XCP slave and vice versa. It is used for carrying out “protocol
commands (CMD),” transferring “command responses (RES),” “errors (ERR),” “events (EV),” and for
issuing “service requests (SERV).”
Data transfer object (DTO) packets
: This packet type is used for transferring synchronous data
between the XCP master and the XCP slave device. “Synchronous data acquisition (DAQ) data” are
transferred from the XCP slave to the XCP master whereas “synchronous data stimulation (STIM)
data” are transported from the master to the slave.
The “DAQ Field” is used to uniquely identify the DAQ list (see Section ...) to be used for
data acquisition or stimulation if the XCP packet is of DTO packet type. In case the packet is a CTO
packet, the DAQ field is omitted.
The “Time-Stamp Field” is used in DTO packets to carry a time-stamp provided by the XCP slave
for the respective data acquisition. The length of the time-stamp field may vary between - bytes
depending on the configuration. In case the packet is of CTO type, the time-stamp field is omitted.
Command packets (CMD) are explicitly acknowledged on the XCP protocol layer by sending
either a command response packet (RES) or an error packet (ERR). Event (EV) packets (i.e., packets
informing the XCP master that a specific event has occurred in the XCP slave [e.g., an overload situa-
tion has occurred during data acquisition]), service request (SERV) packets (i.e., packets used by the
slave to request certain services from the master [e.g., a packet containing text that is to be printed
by the master]) and data acquisition packets (DAQ, STIM) are sent asynchronously and unacknowl-
edged at the XCP protocol layer. Therefore, it may not be guaranteed that the master device will
receive these packets when using a nonacknowledged transportation link like UDP/IP.
18.6.1.2 XCP Transport Layers
The protocol layer described in the previous sections is independent from the underlying commu-
nicationprotocol.TobeabletouseXCPontopofdiferentcommunicationprotocols,theXCP
specification deines multiple XCP transport layers that perform the packing of the protocol indepen-
dent XCP packets into frames of the respective communication protocol, by adding an XCP header
containing a “node address field” used to identify the destination ECU, a “counter field” used for
