Image Processing Reference
In-Depth Information
hasactuallybeenupdated,i.e.,theECUgeneratingthesignalwroteafreshvalueofthesignalsincethe
lasttimeithasbeentransmitted.heVolcanosotwareonanECUtransmittingasignalautomatically
clears the update bit when it has been sent. his ensures that a Volcano-based ECU on the receiving
side will know each time the signal has been updated (the application can see this update bit, by using
flags tied to an update bit, see below). Using update bits to their full extent require that the underlying
protocol is “secure.” (Frames cannot be lost without being detected.) he CAN protocol is regarded
as such, but not the local interconnect network (LIN) protocol. herefore the update bit mechanism
is limited to CAN within Volcano.
19.5 Flags
AlagisaVolcanoobjectpurelylocaltoanECU.Itisboundtooneoftwothings:
•
heupdatebitofareceivedVolcanosignal—thelagissetwhentheupdatebitisset
•
The containing frame of a signal—the flag is set when the frame containing the signal is
received (regardless of whether an update bit for the signal is set)
Many flags can be bound to each update bit, or the reception of a containing frame. Volcano sets
all the flags bound to an object when the occurrence is seen. The flags are cleared explicitly by the
application software.
19.6 Timeouts
A timeout is, like the flags, a Volcano object purely local to an ECU. he timeout is declared by the
application programmer and is bound to a subscribed signal. A timeout condition occurs when the
particular signal was not received within the given time limit. In this case, the signal (or/and a number
of other signals) is/are set to a value specified as part of the declaration of the timeout. As with the
flags, the timeout reset mechanism can be bound to either
•
heupdatebitofareceivedVolcanosignal
•
The frame carrying a specific signal
19.7 Frames
A frame is a container capable of carrying a certain amount of data (- bytes for CAN and LIN).
Several signals can be packed into the available data space and transmitted together in one frame
on the network. he total size of a frame is determined by the protocol. A frame can be transmitted
periodically or sporadically. Each frame is assigned a unique “identifier.” The identifier serves two
purposes in the CAN case:
•
Identifying and filtering a frame on reception at an ECU
•
Assigning a priority to a frame
19.7.1 Immediate Frames
Volcano normally hides the existence of network frames from the application designer. However,
under certain cases there is a need to send and receive frames with very short processing latencies.
In these cases direct application support is required. Such frames are designated immediate frames.
