ST92F124 datasheet(327/429 Pages) STMICROELECTRONICS

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CONTROLLER AREA NETWORK (bxCAN)

CONTROLLER AREA NETWORK (Cont’d)

Figure 144. bxCAN Operating Modes

10.10.4 Operating Modes

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.

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

firmed the request by setting the INAK bit in the

CMSR register.

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-

tus of the CAN bus output CANTX is recessive

(high).

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.

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.

SLEEP

SYNC

INITIALIZATION

NORMAL

P *

INR

INRQ

RESET

SLAK= 1

INAK = 0

SLAK= X

INAK = X

SLAK= 0

INAK = 1

SLE

SLAK= 0

INAK = 0

INRQ

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