NXP Semiconductors KE1xF Series, Reference Manual, Page: 1463 / 1483

mode request is done during a "Permanent Dominant", the
corresponding acknowledge can never be asserted.
50.5.10.1 Freeze mode
This mode is requested either by the CPU through the assertion of the HALT bit in the
CAN_MCR Register or when the chip is put into Debug mode. In both cases it is also
necessary that the FRZ bit is asserted in the CAN_MCR Register and the module is not in
a low-power mode.
The acknowledgement is obtained through the assertion by the FlexCAN of FRZ_ACK
bit in the same register. The CPU must only consider the FlexCAN in Freeze mode when
both request and acknowledgement conditions are satisfied.
When Freeze mode is requested, FlexCAN does the following:
• Waits to be in either Intermission, Passive Error, Bus Off or Idle state
• Waits for all internal activities like arbitration, matching, move-in and move-out to
finish. A pending move-in does not prevent going to Freeze mode.
• Ignores the Rx input pin and drives the Tx pin as recessive
• Stops the prescaler, thus halting all CAN protocol activities
• Grants write access to the Error Counters Register, which is read-only in other modes
• Sets the NOT_RDY and FRZ_ACK bits in CAN_MCR
After requesting Freeze mode, the user must wait for the FRZ_ACK bit to be asserted in
CAN_MCR before executing any other action, otherwise FlexCAN may operate in an
unpredictable way. In Freeze mode, all memory mapped registers are accessible, except
for CAN_CTRL1[CLKSRC] bit that can be read but cannot be written.
Exiting Freeze mode is done in one of the following ways:
• CPU negates the FRZ bit in the CAN_MCR Register
• The chip is removed from Debug Mode and/or the HALT bit is negated
The FRZ_ACK bit is negated after the protocol engine recognizes the negation of the
freeze request. When out of Freeze mode, FlexCAN tries to re-synchronize to the CAN
bus by waiting for 11 consecutive recessive bits.
Chapter 50 CAN (FlexCAN)
Kinetis KE1xF Sub-Family Reference Manual, Rev. 4, 06/2019
NXP Semiconductors 1463