FlexRay Communication Controller (FlexRAY)
PXN20 Microcontroller Reference Manual, Rev. 1
26-154 Freescale Semiconductor
The controller uses a sequential search algorithm to determine the individual message buffer assigned or
subscribed to the next slot. This search must be finished within one FlexRay slot. The shortest FlexRay
slot is an empty dynamic slot. An empty dynamic slot is a minislot and consists of gdMinislot macroticks
with a nominal duration of gdMacrotick. The minimum duration of a corrected macrotick is
gdMacrotick
min
= 39 µT. This results in a minimum slot length of
Eqn. 26-30
The search engine is located in the CHI and runs on the CHI clock. It evaluates one individual message
buffer per CHI clock cycle. For internal status update and double buffer commit operations, and as a result
of the clock domain crossing jitter, an additional amount of 10 CHI clock cycles is required to ensure
correct operation. For a given number of message buffers and for a given CHI clock frequency f
chi
, this
results in a search duration of
Eqn. 26-31
The message buffer search must be finished within one slot which requires that Equation 26-32 must be
fulfilled
Eqn. 26-32
This results in the formula given in Equation 26-33 which determines the required minimum CHI
frequency for a given number of message buffers that are utilized.
Eqn. 26-33
The minimum CHI frequency for a selected set of relevant protocol parameters is given in Table 26-118.
26.7.4 Protocol Control Command Execution
This section considers the issues of the protocol control command execution.
The application issues any of the protocol control commands listed in the POCCMD field of Table 26-15
by writing the command to the POCCMD field of the Protocol Operation Control Register (POCR). As a
result the controller sets the BSY bit while the command is transferred to the PE. When the PE has
accepted the command, the BSY flag is cleared. All commands are accepted by the PE.
The PE maintains a protocol command vector. For each command that was accepted by the PE, the PE sets
the corresponding command bit in the protocol command vector. If a command is issued while the
corresponding command bit is set, the command is not queued and is lost.
Table 26-118. Minimum f
chi
[MHz] examples (128 message buffers)
pdMicrotick
[ns]
gdMinislot
234567
25.0 70.77 47.18 35.39 28.31 23.59 20.22
50.0 35.39 23.59 17.70 14.16 11.80 10.11
slotmin
39 pdMicrotick gdMinislot=
search
1
f
chi
--------
# MessageBuffers 10+=
f
chi
# MessageBuffers 10+
39 pdMicrotick gdMinislot
----------------------------------------------------------------------
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