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Functional description
• The Bus Domain feeds the Control Host Interface (CHI) submodule and is derived
from the peripheral clock.
• The Oscillator Domain feeds the CAN Protocol Engine (PE) submodule and is
derived directly from a crystal oscillator clock, so that very low jitter performance
can be achieved on the CAN bus.
When CAN_CTRL1[CLKSRC] bit is set, synchronous operation occurs because both
domains are connected to the peripheral clock (creating a 1:1 ratio between the peripheral
and oscillator clocks).
When the two domains are connected to clocks with different frequencies and/or phases,
there are restrictions on the frequency relationship between the two clock domains. In the
case of asynchronous operation, the Bus Domain clock frequency must always be greater
than the Oscillator Domain clock frequency.
Asynchronous operation with a 1:1 ratio between peripheral
and oscillator clocks is not allowed.
When doing matching and arbitration, FlexCAN needs to scan the whole Message Buffer
memory during the time slot of one CAN frame, comprised of a number of CAN bits. In
order to have sufficient time to do that, the following requirements must be observed:
• The peripheral clock frequency can not be smaller than the oscillator clock frequency
• For 16 Mailboxes, the minimum number of peripheral clocks per CAN bit is 16
The minimum number of peripheral clocks per CAN bit determines the minimum
peripheral clock frequency for an expected CAN bit rate. The CAN bit rate depends on
the number of time quanta in a CAN bit, that can be defined by adjusting one or more of
the bit timing values contained in either the Control 1 Register (CAN_CTRL1) or CAN
Bit Time register (CAN_CBT). The tima quantum (Tq) is defined in
minimum number of time quanta per CAN bit must be 8; therefore, the oscillator clock
frequency should be at least 8 times the CAN bit rate.
43.5.10 Modes of operation details
The FlexCAN module has functional modes and low-power modes. See
operation
for an introductory description of all the modes of operation. The following
sub-sections contain functional details on Freeze mode and the low-power modes.
"Permanent Dominant" failure on CAN Bus line is not
supported by FlexCAN. If a Low-Power request or Freeze
1158
NOTE
CAUTION
KV4x Reference Manual, Rev. 2, 02/2015
Preliminary
Protocol
timing. The
Modes of
Freescale Semiconductor, Inc.
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