DESIGN OF A CUSTOM ASIC INCORPORATING CAN™ AND 1 ...

n w 1
m J n
j
bit
wm Bm Cj
(4.5)
j hpm Tj
A value of zero for w can be used. The iteration process proceeds until convergence (i.e.
n 1 n
m m
0
m
w w ). Equations 4.1 through 4.5 do not assume anything about how identifiers and hence
priorities are chosen [67].
From analysis of Equations 4.1 through 4.5, the minimal latency time occurs when the
CAN bus is running at its maximum speed of 1 Mbps and when a frame consisting of zero
data bytes is transmitted (i.e. sm 0 and bit 1μs ). Solving Equation 4.2 for a value of Cm
yields:
C
C
m
m
34
47 8.5 47 55.5 μsec 11-bit header
4
101
54
67 13.5 67 80.5 μsec 29-bit header
4
The maximum latency time occurs when the bus is running at its slowest speed of 10 kbps and
when a frame consisting of eight data bytes is transmitted (i.e. sm 8and
bit 100μs ).
Resolving Equation 4.2 for Cm yields:
C
C
m
m
(4.6)
34 88 98
47 8 8 10μs 111 10μs 1.355msec=1355μsec (11-bit)
4
4
(4.7)
54 88 118
67 88 10μs 13110μs1.605msec=1605μsec (29-bit)
4
4
From this analysis, the worst-case latency value comes from the bus operating at the
slowest speed of 10 kbps when each message contains the maximum amount of data at eight
bytes and the values for the queuing jitter (Jm) and worst-case queuing delay (wm) are zero. The