STR720 datasheet(241/401 Pages) STMICROELECTRONICS

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STR720 - CONTROLLER AREA NETWORK (CAN)

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18.7.2.1 Data Transfer from/to Message RAM

When the CPU initiates a data transfer between the IFn Registers and Message RAM, the

Message Handler sets the Busy bit in the respective Command Request Register

(CAN_IFn_CRR). After the transfer has completed, the Busy bit is again cleared (see Figure

33).

The respective Command Mask Register specifies whether a complete Message Object or

only parts of it will be transferred. Due to the structure of the Message RAM, it is not possible

to write single bits/bytes of one Message Object. It is always necessary to write a complete

Message Object into the Message RAM. Therefore, the data transfer from the IFn Registers to

the Message RAM requires a read-modify-write cycle. First, those parts of the Message

Object that are not to be changed are read from the Message RAM and then the complete

contents of the Message Buffer Registers are written into the Message Object.

Figure 33. Data transfer between

IFn Registers and Message RAM

START

WR/RD = 1

Busy = 0

Busy = 1

Read Message Object to IFn

Write IFn to Message RAM

Read Message Object to IFn

Yes

CAN_WAIT_B = 0

CAN_WAIT_B = 1

Write Command Request Register

Yes

STR720 Datasheet(PDF) 241 Page - STMicroelectronics