Freescale Semiconductor MPC5604B - Message Buffer Mode and RX FIFO Mode; Message Buffers

The FlexCAN module is a CAN controller implementation with some enhancements over CAN
2.0B standard, with no transceiver (so the user is free to choose the communication medium).
The block diagram above show different building blocks of this module, we can see that the CAN
Controller itself is joined by a mailbox system that can either work as 64 message buffers or a 6
frame RX FIFO with 56 message buffers. There are 3 FlexCAN modules on MPC5604BxLQ.
These MB( Message Buffers) can be configured to serve different purposes like Rx, Tx or remote
frame buffers, supporting both standard and extended frame formats. An internal 16-bit timer is
provides time stamps for messages. Multiple interrupt vectors(8) for fast and refined servicing
for each FlexCAN module. This module possesses different operating modes like listen only
mode.
Each FlexCAN module possess an RX and a TX signal, in SIUL, TX pad has to be configured at
maximum slew-rate and as open-drain, with output buffer enabled. RX pad has to have its input
buffer.
7.2.
Message Buffer mode and RX FIFO mode
All peripherals possess some registers that are mapped to the memory of the microcontroller, in
FlexCAN’s case, there is an embedded SRAM and it can be configured either as 64 16-byte
Message Buffers (MB) with an intelligent MB manager that can configure and arbiter those. But
this zone can also be configured a FIFO engine that can contain up to 6 frames with a single MB
for accessing to the FIFO. These two engines occupy the same address zone in this embedded
memory and with the current version of the memory mapping header, this selection can only be
done before compiling the project.
7.2.1.
Message Buffers
Figure 118: Message Buffer Structure(R.M. Rev8 – Fig. 22-2)
These message buffers can hold the information contained in both standard and extended
frames, and also provides extra information like the time stamp. Details on fields:
 CODE: a code used for MB configuration (detailed below),
 SRR(Substitute Remote Request): a recessive bit used in extended format, if IDE=1 then
user must set this bit to 1,
 IDE(ID Extended): if set, CAN 2.0B extended format is used, else the standard format is
kept,
 RTR(Remote Transmission Request): When set, a remote frame is transmitted,
 LENGTH: Same as the DLC field on frames, number of used data bytes,
 TIME STAMP: a copy of internal counter’s value when ID field is set either on Rx or Tx.