FlexRay Communication Controller (FlexRAY)
PXN20 Microcontroller Reference Manual, Rev. 1
26-114 Freescale Semiconductor
26.6.6.4 Double Transmit Message Buffer
The section provides a detailed description of the functionality of the double transmit message buffers.
Double transmit message buffers are used by the application to provide the controller with the message
data to be transmitted over the FlexRay Bus. The controller uses this message buffer to provide
information to the application about the transmission process, and status information about the slot in
which message data was transmitted.
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
controller 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 26-130.
Figure 26-130. 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 (MBIDXRn).
26.6.6.4.1 Access Regions
To certain message buffer fields, both the application and the controller 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 26-131. The given regions
represent fields that can be accessed from both the application and the controller and, thus, require access
restrictions. A description of the regions is given in Table 26-106.
Commit Side Transmit Side
Application FlexRay Bus
MB# 2n
MB# 2n+1
Internal Message
Transfer
message data message data message data
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