MPC5604B/C Microcontroller Reference Manual, Rev. 8
Freescale Semiconductor 459
NOTE
If the BUSY bit is asserted or if the MB is empty, then reading the Control
and Status word does not lock the MB.
Deactivation takes precedence over locking. If the CPU deactivates a locked Rx MB, then its lock status
is negated and the MB is marked as invalid for the current matching round. Any pending message on the
SMB will not be transferred anymore to the MB.
22.4.8 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 22.3.3, “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 up to 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 5 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 8
32-bit registers that can be configured to one of the following formats (see also Section 22.3.3, “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 8 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.
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