STR720 datasheet(240/401 Pages) STMICROELECTRONICS

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STR720 - CONTROLLER AREA NETWORK (CAN)

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18.7 CAN Communications

18.7.1

Managing Message Objects

The configuration of the Message Objects in the Message RAM (with the exception of the bits

MsgVal, NewDat, IntPnd, and TxRqst) will not be affected by resetting the chip. All the

Message Objects must be initialized by the application software or they must be “not valid”

(MsgVal = ‘0’) and the bit timing must be configured before the application software clears the

Init bit in the CAN Control Register.

The configuration of a Message Object is done by programming Mask, Arbitration, Control

and Data fields of one of the two interface registers to the desired values. By writing to the

corresponding IFn Command Request Register, the IFn Message Buffer Registers are loaded

into the addressed Message Object in the Message RAM.

When the Init bit in the CAN Control Register is cleared, the CAN Protocol Controller state

machine of the CAN_Core and state machine of the Message Handler control the internal

data flow of the CAN Peripheral. Received messages that pass the acceptance filtering are

stored in the Message RAM, messages with pending transmission request are loaded into the

CAN_Core’s Shift Register and are transmitted through the CAN bus.

The application software reads received messages and updates messages to be transmitted

through the IFn Interface Registers. Depending on the configuration, the CPU is interrupted

on certain CAN message and CAN error events.

18.7.2

Message Handler State Machine

The Message Handler controls the data transfer between the Rx/Tx Shift Register of the CAN

Core, the Message RAM and the IFn Registers.

The Message Handler FSM controls the following functions:

•

Data Transfer from IFn Registers to the Message RAM

•

Data Transfer from Message RAM to the IFn Registers

•

Data Transfer from Shift Register to the Message RAM

•

Data Transfer from Message RAM to Shift Register

•

Data Transfer from Shift Register to the Acceptance Filtering unit

•

Scanning of Message RAM for a matching Message Object

•

Handling of TxRqst flags

•

Handling of interrupts.

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