Image Processing Reference
In-Depth Information
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First v_input
at which
signal is
available
First v_output
at which new
value is
available
Frame enters
arbitration
Transmission
completed
Notional
consumption
Notional
generation
Time
T
PL
T
BT
T
T
T
AT
T
SL
Max_age
FIGURE .
Volcano timing model.
The max_age of the signal is the maximum age, measured from notional generation, at which it is
acceptable for notional consumption. he max_age is the overall timing requirement on a signal.
T
PL
(publish latency) is the time from notional generation to the first v_output() call when the
signal value is available to volcano (a write call has been made). It will depend on the properties
of the publishing application. Typical values might be the frame_processing_period (if the signal is
written fresh every period but this is not synchronized with v_output()), the offset between the write
call and v_output() (if the two are synchronized), or the sum of the frame_processing_period and
theperiodofsomelower-rateactivitywhichgeneratesthevalue.hisvaluemustbegivenbythe
application programmer.
T
SL
(subscribe latency), the time from the first v_input that makes the new value available to the
application to the time when the value is “consumed.” he consumption of a signal is a user-defined
event which will depend on the properties of the subscribing function. As an example it can be a lamp
being lit, or an actuator starting to move. his value must be given by the application programmer.
The intervals
T
BT
,
T
T
,and
T
AT
are controlled by the Volcano configuration and are dependent
uponthenatureoftheframeinwhichthesignalistransported.
The value
T
BT
isthetimebeforetransmission(thetimefromthev_outputcalluntiltheframe
enters arbitration on the bus).
T
BT
is a per-frame value which depends on the type of frame carrying
thesignal(seelatersections).histimeissharedbyallsignalsintheframe,andiscommontoall
subscribers to those signals.
The value
T
AT
isthetimeatertransmission(thetimefromwhentheframehasbeensuccess-
fully transmitted on the network until the next v_input call).
T
AT
is a per-frame value which may be
different for each subscribing ECU.
The value
T
T
is the time required to transmit the frame (including the arbitration time) on the
network.
19.13 Jitter
The application programmer at the supplier must also provide information of the “jitter” to the
systems integrator. his information is as follows:
The input_jitter and output_jitter refer to the variability in the time taken to complete the v_input()
and v_output() calls, measured relative to the occurrence of the periodic event causing Volcano
processing to be done (i.e., calls to v_input(), v_gateway(), and v_output() to be made). Figure .
shows how the output_jitter is measured:
