PXS20RM Datasheet, PDF (842/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

FlexRay Communication Controller
In contrast to the single transmit message buffers, the application can provide new transmission data while
the transmission of the previously provided message data is running. This scheme is called double
buffering and can be considered as a FIFO of depth 2.
Double transmit message buffers are implemented by combining two individual message buffers that form
the two sides of an double transmit message buffer. One side is called the commit side and will be accessed
by the application to provide the message data. The other side is called the transmit side and is used by the
CC to transmit the message data to the FlexRay bus. The two sides are located in adjacent individual
message buffers. The message buffer that implements the commit side has an even message buffer number
2n. The transmit side message buffer follows the commit side message buffer and has the message buffer
number 2n+1. The basic structure and data flow of a double transmit message buffer is given in
Figure 26-140.
Application
message data
MB# 2n
Commit Side
Internal Message
Transfer
message data
MB# 2n+1
Transmit Side
FlexRay Bus
message data
Figure 26-140. Double Transmit Buffer Structure and Data Flow
NOTE
Both the commit and the transmit side must be configured with identical
values except for the Message Buffer Index Registers (FR_MBIDXRn).
26.6.6.4.1 Access Regions
To certain message buffer fields, both the application and the CC have access. To ensure data consistency,
a message buffer locking scheme is implemented, which controls the exclusive access to the data, control,
and status bits of the message buffer.
The access scheme for double transmit message buffers is depicted in Figure 26-141. The given regions
represent fields that can be accessed from both the application and the CC and, thus, require access
restrictions. A description of the regions is given in Table 26-116.
26-130
PXS20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor