ns
s
VBUSclk
32
.
22
390
15
7
222
16
1
=
+
´
+
=
£
m
(
)
]
[
7
1
2
2
min
8
s
VBUScycle
VBUScycle
VBUScycle
t
VBUS
find
m
ss
m
t
islots
clk
m
+
´
+
£
(
)
VBUSclk
VBUScycles
VBUScycles
VBUScycles
t
find
m
ss
m
t
islots
+
´
+
³
2
2
min
8
7
Module Operation
1274
SPNU563A – March 2018
Copyright © 2018, Texas Instruments Incorporated
FlexRay Module
26.2.15 Minimum Peripheral Clock Frequency
In order to calculate the minimum peripheral clock frequency (VBUSclk) the worst case scenario has to be
considered. The worst case scenario depends on the following parameters:
•
maximum payload length
•
minimum minislot length
•
number of configured message buffers (excluding FIFO)
•
used channels (single/dual channel)
Worst case scenario:
•
reception of a message with a maximum payload length in slot n (n is 7,15,23,31,39,...)
•
slot n+1 to n+7 are empty dynamic slots (minislot) and configured as receive buffer
•
the find-sequence (usually started in slot 8,16,24,32,40,...) has to scan the maximum number of
configured buffers
•
the number of concurrent tasks has its maximum value of 3
The find-sequence is executed each 8 slots (slot 8,16,24,32,40,...). It has to be finished until the next find-
sequence is requested.
The duration of a Transient Buffer RAM (TBF) transfer to the Message Buffer RAM (MBF) varies from 4
(header section only) to 68 ( maximum data section) time steps plus a setup time of 6 time steps.
VBUScycles
t2m
= (number of concurrent tasks) x (6 + (number of 4-byte words))
A Slot Status (SS) transfer to the Message Buffer RAM (MBF) has a length of 1 time step plus a setup
time of 4 time steps.
VBUSclk
ss2m
= (number of concurrent tasks) x 5
The find sequence has a maximum length of 128 (maximum number of buffers) time steps plus a setup
time of 2 time steps.
VBUSclk
find
= (number of concurrent tasks) x (2 + (number of configured buffers))
A minislot has a length of 2 to 63 macroticks (MTicks). The minimum nominal macrotick period (MTcycle)
is 1
μ
s. A sequence of 8 minislots has a length of
t
8minislots
= 8 x MTicks x MTcycle
The worst case VCLK cycle period can be calculated as follows:
(32)
(33)
minimum t
8minislots
= 8 * 2 * 1
μ
s = 16
μ
s
maximum VBUScycle
t2m
= 3 * (6 + 68) = 222
maximum VBUScycle
ss2m
= 3 * 5 = 15
maximum VBUScycle
find
= 3 * (2 + 128) = 390
(34)
The minimum peripheral clock frequency for this worst case scenario is 44,8125 MHz.