CONTROLLER AREA NETWORK (bxCAN)
CONTROLLER AREA NETWORK (Cont’d)
Figure 144. bxCAN Operating Modes
RESET
SLEEP
SLAK= 1
INAK = 0
SYNC
SLAK= X
INAK = X
NORMAL
INITIALIZATION
SLAK= 0
INAK = 1
SLAK= 0
INAK = 0
INRQ
10.10.4 Operating Modes
tus of the CAN bus output CANTX is recessive
(high).
bxCAN has three main operating modes: initiali-
zation, normal and sleep. After a hardware reset,
bxCAN is in sleep mode to reduce power con-
sumption and an internal pull-up is active on RX1.
The software requests bxCAN to enter initializa-
tion or sleep mode by setting the INRQ or SLEEP
bits in the CMCR register. Once the mode has
been entered, bxCAN confirms it by setting the
INAK or SLAK bits in the CMSR register and the
internal pull-up is disabled. When neither INAK nor
SLAK are set, bxCAN is in normal mode. Before
entering normal mode bxCAN always has to syn-
chronize on the CAN bus. To synchronize, bx-
CAN waits until the CAN bus is idle, this means 11
consecutive recessive bits have been monitored
on CANRX.
Entering Initialization Mode does not change any
of the configuration registers.
To initialize the CAN Controller, software has to
set up the Bit Timing registers and the filters. If a
filter bank is not used, it is recommended to leave
it non active (leave the corresponding FACT bit
cleared).
10.10.4.2 Normal Mode
Once the initialization has been done, the software
must request the hardware to enter Normal mode,
to synchronize on the CAN bus and start reception
and transmission. Entering Normal mode is done
by clearing the INRQ bit in the CMCR register and
waiting until the hardware has confirmed the re-
quest by clearing the INAK bit in the CMSR regis-
ter. Afterwards, the bxCAN synchronizes with the
data transfer on the CAN bus by waiting for the oc-
currence of a sequence of 11 consecutive reces-
sive bits (≡ Bus Idle) before it can take part in bus
activities and start message transfer.
10.10.4.1 Initialization Mode
The software initialization can be done while the
hardware is in Initialization mode. To enter this
mode the software sets the INRQ bit in the CMCR
register and waits until the hardware has con-
firmed the request by setting the INAK bit in the
CMSR register.
The initialization of the filter values is independent
from Initialization Mode but must be done while the
filter is not active (corresponding FACTx bit
cleared). The filter scale and mode configuration
must be configured before entering Normal Mode.
To leave Initialization mode, the software clears
the INQR bit. bxCAN has left Initialization mode
once the INAK bit has been cleared by hardware.
While in Initialization Mode, all message transfers
to and from the CAN bus are stopped and the sta-
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