Image Processing Reference
In-Depth Information
T
m
D
m
R
m
w
m
J
m
C
m
Higher priority messages
Message
m
t
req(
)
t
start(
t
win(
t
end(
t
req(
m
m
)
m
)
m
)
m
)
FIGURE .
Worst-caseresponsetimeinCAN.
transmission on the bus is started (i.e., when the node will effectively win the contention), and
t
end
(
m
)
the time the frame is completely received at the intended destination(s).
AsshowninFigure.,theworst-caseresponsetime
R
m
for message
m
can be evaluated as the
sum of the worst-case jitter
J
m
introducedbythenodehardwareandsotware,thedelay
w
m
(possibly)
incurred because of lost contentions, and the time
C
m
takentotransmitseriallyoverthebusallthe
bits of the frame, that is
R
m
=
J
m
+
w
m
+
C
m
15.3.2 Schedulability Analysis
As it can be seen, the problem is now evaluating the transmission delay
w
m
for each message in the
set. Delay
w
m
basically includes two kinds of contributions: at most one message with a priority lower
than
m
, which is already in the process of being transmitted at time
t
start
, and every message with
apriorityhigherthan
m
possibly generated in the time interval when message
m
is queued waiting
to be sent. It is worth remembering that in CAN the (numerically) lower the identifier, the higher the
priority.
The first kind of contribution gives rise to the so called blocking time
(
m
)
.Itcanbecomputedas
the maximum transmission time, evaluated among every message
k
where
k
is (numerically) greater
than
m
)
(
B
m
B
m
=
max
k
m
(
C
k
)
>
The latter contribution, instead, is known as interference. Every message
j
,whosepriorityishigher
than
m
, that is generated (or is queued waiting for transmission) in the time interval from
t
start
(
m
)
to one bit after
t
win
, in fact, will win the contention with message
m
and delay it.
As depicted in Figure ., the number
h
m
,
j
of times the higher-priority message
j
may interfere
with message
m
basically depends on the duration of this time interval and the generation rate of
message
j
(
m
)
w
m
+
J
j
+
t
bit
h
m
,
j
=⌈
⌉
T
j
IntheexampleshowninFigure.,message
j
interferes with message
m
three times (each time
making it lose the contention). On the contrary, the fourth instance of message
j
no longer causes
