PXS20RM Datasheet, PDF (841/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

FlexRay Communication Controller
message buffer data field of the receive shadow buffer. If the number of received bytes is less than
2*FR_MBDSR.MBSEG1DS, the CC writes only the received number of bytes and will not change the
trailing bytes in the message buffer data field of the receive shadow buffer. The same holds for the message
buffer segment 2 with FR_MBDSR.MBSEG2DS.
26.6.6.3.4 Received Message Access
To access the message data received over the FlexRay bus, the application reads the message data stored
in the message buffer data field of the corresponding receive message buffer. The access to the message
buffer data field is described in Section 26.6.3.1, Individual Message Buffers.
The application can read the message buffer data field if the receive message buffer is one of the states
HDis, HDisLck, or HLck. If the message buffer is in one of these states, the CC will not change the content
of the message buffer.
26.6.6.3.5 Receive Shadow Buffers Concept
The receive shadow buffer concept applies only to individual receive message buffers. The intention of
this concept is to ensure that only syntactically and semantically valid received non-null frames are
presented to the application in a receive message buffer. The basic structure of a receive shadow buffer is
described in Section 26.6.3.2, Receive Shadow Buffers.
The receive shadow buffers temporarily store the received frame header and message data. After the slot
boundary the slot status information is generated. If the slot status information indicates the reception of
the valid non-null frame (see Table 26-115), the CC writes the slot status into the slot status field of the
receive shadow buffer and exchanges the content of the Message Buffer Index Registers (FR_MBIDXRn)
with the content of the corresponding internal shadow buffer index register. In all other cases, the CC
writes the slot status into the identified receive message buffer, depending on the slot status and the
FlexRay segment the message buffer is assigned to.
The shadow buffer concept, with its index exchange, results in the fact that the FlexRay memory area
located message buffer associated to an individual receive message buffer changes after successful
reception of a valid frame. This means that the message buffer area in the FlexRay memory area accessed
by the application for reading the received message is different from the initial setting of the message
buffer. Therefore, the application must not rely on the index information written initially into the Message
Buffer Index Registers (FR_MBIDXRn). Instead, the index of the message buffer header field must be
fetched from the Message Buffer Index Registers (FR_MBIDXRn).
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 CC with the message data to
be transmitted over the FlexRay Bus. The CC 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.
Freescale Semiconductor
PXS20 Microcontroller Reference Manual, Rev. 1
26-129