mcf5282 Freescale Semiconductor, Inc, mcf5282 Datasheet - Page 481

• Manufacturer Part Number: mcf5282
• Manufacturer: Freescale Semiconductor, Inc

Data should never be written into a receive message buffer. If this is done while a message is being
transferred from an SMB, the control/status word will reflect a full or overrun condition, but no interrupt
will be requested.
25.4.3.4 Locking and Releasing Message Buffers
The lock/release/busy mechanism is designed to guarantee data coherency during the receive process. The
following examples demonstrate how the lock/release/busy mechanism will affect FlexCAN operation.
Polling the control/status word of a receive message buffer can lock it, preventing a message from being
transferred into that buffer. If the control/status word of a receive message buffer is read, it should then be
followed by a read of the control/status word of another buffer, or by reading the free-running timer, to
ensure that the locked buffer is unlocked.
25.4.4
The remote frame is a message frame which is transmitted to request a data frame. The FlexCAN can be
configured to transmit a data frame automatically in response to a remote frame, or to transmit a remote
frame and then wait for the responding data frame to be received.
When transmitting a remote frame, the user initializes a message buffer as a transmit message buffer with
the RTR bit set to one. Once this remote frame is transmitted successfully, the transmit message buffer
automatically becomes a receive message buffer, with the same ID as the remote frame which was
transmitted.
When a remote frame is received by the FlexCAN, the remote frame ID is compared to the IDs of all
transmit message buffers programmed with a code of 1010. If there is an exact matching ID, the data frame
in that message buffer is transmitted. If the RTR bit in the matching transmit message buffer is set, the
FlexCAN will transmit a remote frame as a response.
Freescale Semiconductor
1. Reading a control/status word of a message buffer triggers a lock for that message buffer. A new
2. To release a locked message buffer, the CPU either locks another message buffer (by reading its
3. If a receive frame with a matching ID is received during the time the message buffer is locked, the
4. When a locked message buffer is released, if a frame with a matching identifier exists within the
5. If two or more receive frames with matching IDs are received while a message buffer with a
6. If the user reads the control/status word of a receive message buffer while a frame is being
received message frame which matches the message buffer cannot be written into this message
buffer while it is locked.
control/status word) or globally releases any locked message buffer (by reading the free-running
timer).
receive frame will not be immediately transferred into that message buffer, but will remain in the
SMB. There is no indication when this occurs.
SMB, then this frame will be transferred to the matching message buffer.
matching ID is locked, the last received frame with that ID is kept within the serial message
buffer, while all preceding ones are lost. There is no indication when this occurs.
transferred from a serial message buffer, the BUSY code will be indicated. The user should wait
until this code is cleared before continuing to read from the message buffer to ensure data
coherency. In this situation, the read of the control/status word will not lock the message buffer.
Remote Frames
MCF5282 and MCF5216 ColdFire Microcontroller User’s Manual, Rev. 3
FlexCAN
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