Remote Frames - Motorola ColdFire MCF5281 User Manual

Motorola microcontroller user's manual
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Functional Overview
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.
1. Reading a control/status word of a message buffer triggers a lock for that message
buffer. A new received message frame which matches the message buffer cannot be
written into this message buffer while it is locked.
2. To release a locked message buffer, the CPU either locks another message buffer
(by reading its control/status word) or globally releases any locked message buffer
(by reading the free-running timer).
3. If a receive frame with a matching ID is received during the time the message
buffer is locked, the receive frame will not be immediately transferred into that
message buffer, but will remain in the SMB. There is no indication when this
occurs.
4. When a locked message buffer is released, if a frame with a matching identifier
exists within the SMB, then this frame will be transferred to the matching message
buffer.
5. If two or more receive frames with matching IDs are received while a message
buffer with a 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.
6. If the user reads the control/status word of a receive message buffer while a frame
is being 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.
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 Remote Frames

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
25-12
MCF5282 User's Manual
MOTOROLA

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