NXP Semiconductors MPC5605BK, Reference Manual

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Chapter 25 FlexCAN
MPC5606BK Microcontroller Reference Manual, Rev. 2
584 Freescale Semiconductor
25.5.8 Rx FIFO
The receive-only FIFO is enabled by asserting the FEN bit in the MCR. The reset value of this bit is zero
to maintain software backwards compatibility with previous versions of the module that did not have the
FIFO feature. When the FIFO is enabled, the memory region normally occupied by the first 8 MBs
(0x80–0xFF) is now reserved for use of the FIFO engine (see
Section 25.4.3, Rx FIFO structure ).
Management of read and write pointers is done internally by the FIFO engine. The CPU can read the
received frames sequentially, in the order they were received, by repeatedly accessing a Message Buffer
structure at the beginning of the memory.
The FIFO can store as many as 6 frames pending service by the CPU. An interrupt is sent to the CPU when
new frames are available in the FIFO. Upon receiving the interrupt, the CPU must read the frame
(accessing an MB in the 0x80 address) and then clear the interrupt. The act of clearing the interrupt triggers
the FIFO engine to replace the MB in 0x80 with the next frame in the queue, and then issue another
interrupt to the CPU. If the FIFO is full and more frames continue to be received, an OVERFLOW
interrupt is issued to the CPU and subsequent frames are not accepted until the CPU creates space in the
FIFO by reading one or more frames. A warning interrupt is also generated when five frames are
accumulated in the FIFO.
A powerful filtering scheme is provided to accept only frames intended for the target application, thus
reducing the interrupt servicing work load. The filtering criteria is specified by programming a table of
eight 32-bit registers that can be configured to one of the following formats (see also
Section 25.4.3, Rx
FIFO structure ):
• Format A: 8 extended or standard IDs (including IDE and RTR)
• Format B: 16 standard IDs or 16 extended 14-bit ID slices (including IDE and RTR)
• Format C: 32 standard or extended 8-bit ID slices
NOTE
A chosen format is applied to all eight registers of the filter table. It is not
possible to mix formats within the table.
The eight elements of the filter table are individually affected by the first eight Individual Mask registers
(RXIMR0–RXIMR7), allowing very powerful filtering criteria to be defined. The rest of the RXIMR,
starting from RXIM8, continue to affect the regular MBs, starting from MB8. If the BCC bit is negated (or
if the RXIMR are not available for the particular MCU), then the FIFO filter table is affected by the legacy
mask registers as follows: element 6 is affected by RX14MASK, element 7 is affected by RX15MASK
and the other elements (0 to 5) are affected by RXGMASK.
25.5.9 CAN protocol related features
25.5.9.1 Remote frames
Remote frame is a special kind of frame. The user can program a MB to be a Request Remote Frame by
writing the MB as Transmit with the RTR bit set to 1. After the Remote Request frame is transmitted
successfully, the MB becomes a Receive Message Buffer, with the same ID as before.