MAC7200 datasheet(607/976 Pages) FREESCALE

607 / 976 page

The Generic FlexCAN2 Module

MAC7200 Microcontroller Family Reference Manual, Rev. 2

Freescale Semiconductor

529

run) when the CPU writes to the C/S field of that MB (see Section 28.2.7.6.1, “Message Buffer

Deactivation”).

28.2.7.2

Transmit Process

In order to transmit a CAN frame, the CPU should prepare a Message Buffer for transmission by executing

the following procedure:

•

Write ‘1000’ to the Code field of the Control and Status word to keep the MB inactive

•

Write the ID word

•

Write the data bytes

•

Write the Length, Control and Code fields of the Control and Status word to activate the MB

The first and last steps are mandatory. The first write to the Control and Status word is important in case

there was pending reception or transmission. The write operation immediately deactivates the MB,

removing it from any currently ongoing arbitration or ID matching processes, giving time for the CPU to

program the rest of the MB (see Section 28.2.7.6.1, “Message Buffer Deactivation”). Once the MB is

activated in the fourth step, it will participate into the arbitration process and eventually be transmitted

according to its priority. At the end of the successful transmission, the value of the Free Running Timer is

written into the Time Stamp field, the Code field in the Control and Status word is updated, a status flag

is set in the Interrupt Flag Register and an interrupt is generated if allowed by the corresponding Interrupt

Mask Register bit. The new Code field after transmission depends on the code that was used to activate

the MB in step four (see Table 28-7 and Table 28-8 in Section Section 28.2.6.2, “Message Buffer

Structure”).

28.2.7.3

Arbitration Process

The arbitration process is an algorithm executed by the MBM that scans the whole MB memory looking

for the highest priority message to be transmitted. All MBs programmed as transmit buffers will be

scanned to find the lowest ID1 or the lowest MB number, depending on the LBUF bit on the Control

•

During the CRC field of the CAN frame

•

During the error delimiter field of the CAN frame

•

During Intermission, if the winner MB defined in a previous arbitration was deactivated, or if there

was no MB to transmit, but the CPU wrote to the C/S word of any MB after the previous arbitration

finished

•

When MBM is in Idle or Bus Off state and the CPU writes to the C/S word of any MB

•

Upon leaving Freeze Mode

Once the highest priority MB is selected, it is transferred to a temporary storage space called Serial

Message Buffer (SMB), which has the same structure as a normal MB but is not user accessible. This

operation is called “move-out”. At the first opportunity window on the CAN bus, the message on the SMB

Actually, if LBUF is negated, the arbitration considers not only the ID, but also the RTR and IDE bits placed inside the ID

at the same positions they are transmitted in the CAN frame.

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