Controller area network (beCAN) RM0016
384/449 Doc ID 14587 Rev 8
23.8 Register access protection
Erroneous access to certain configuration registers can cause the hardware to temporarily
disturb the whole CAN network. Therefore the following registers can be modified by
software only while the hardware is in initialization mode:
CAN_BTR1, CAN_BTR2, CAN_FCR1, CAN_FCR2, CAN_FMR1, CAN_FMR2 and
CAN_DGR registers.
Although the transmission of incorrect data will not cause problems at the CAN network
level, it can severely disturb the application. A transmit mailbox can be only modified by
software while it is in empty state, refer to Figure 145: Transmit mailbox states.
The filters must be deactivated before their value can be modified by software. The
modification of the filter configuration (scale or mode) can be done by software only in
initialization mode.
23.9 Clock system
The clock tolerance limit as specified in CAN protocol is 1.58 % at speeds of up to 125
Kbps. For higher baud rates, it is suggested to use a crystal oscillator.
Note: If the clock security system feature is enabled in the CLK controller (Refer to the description
of the CSSEN bit in the Clock security system register (CLK_CSSR) on page 96), there is a
way to put CAN automatically into the recessive state when a main clock failure occurs, so
that the CAN network does not get stuck by the device. However to ensure this, the PG0 I/O
pin must be configured in pull-up mode prior to using the beCAN. In this way, when a failure
occurs and the I/O alternate function is disabled, the line is pulled-up instead of floating.
23.10 beCAN low power modes
Note: If a CAN frame is received in Wait, Halt or Active-halt modes, the microcontroller will be
woken-up but the CAN frame will be lost.
Table 67. beCAN behavior in low power modes
Mode Description
Wait
No effect on beCAN, except that accesses to Tx/Rx mailboxes and filter values are not
possible (CPU clock is stopped).
beCAN interrupts cause the device to exit from WAIT mode.
Slow No effect on beCAN.
Halt/ Active-
halt
beCAN is halted.
A beCAN Rx interrupt causes the device to exit from Halt/Active-halt modes (in fact,
any falling edge driven externally on the Rx pin will wake-up the microcontroller.
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