FlexCAN
MPC5606S Microcontroller Reference Manual, Rev. 7
Freescale Semiconductor
701
8-entry ID table from FIFO is scanned first and then, if a match is not found within the FIFO table, the
other MBs are scanned. In the event that the FIFO is full, the matching algorithm will always look for a
matching MB outside the FIFO region.
When the frame is received, it is temporarily stored in a hidden auxiliary MB called Serial Message Buffer
(SMB). The matching process takes place during the CRC field of the received frame. If a matching ID is
found in the FIFO table or in one of the regular MBs, the contents of the SMB will be transferred to the
FIFO or to the matched MB during the 6th bit of the End-of-Frame field of the CAN protocol. This
operation is called move-in. If any protocol error (CRC, ACK, etc.) is detected, than the move-in operation
does not happen.
For the regular mailbox MBs, an MB is said to be free to receive a new frame if the following conditions
are satisfied:
•
The MB is not locked (see
Section 18.4.6.3, Message Buffer lock mechanism
•
The Code field is either EMPTY or else it is FULL or OVERRUN but the CPU has already serviced
the MB (has read the Control and Status word and then unlocked the MB).
If the first MB with a matching ID is not free to receive the new frame, then the matching algorithm keeps
looking for another free MB until it finds one. If it cannot find one that is free, then it will overwrite the
last matching MB (unless it is locked) and set the Code field to OVERRUN (refer to
). If the last matching MB is locked, then the new message remains in the SMB, waiting for the
Section 18.4.6.3, Message Buffer lock mechanism
).
Suppose, for example, that the FIFO is disabled and there are two MBs with the same ID, and FlexCAN
starts receiving messages with that ID. Let us say that these MBs are the second and the fifth in the array.
When the first message arrives, the matching algorithm will find the first match in MB number 2. The code
of this MB is EMPTY, so the message is stored there. When the second message arrives, the matching
algorithm will find MB number 2 again, but it is not free to receive, so it will keep looking and find MB
number 5 and store the message there. If yet another message with the same ID arrives, the matching
algorithm finds out that there are no matching MBs that are free to receive, so it decides to overwrite the
last matched MB, which is number 5. In doing so, it sets the Code field of the MB to indicate OVERRUN.
The ability to match the same ID in more than one MB can be exploited to implement a reception queue
(in addition to the full featured FIFO) to allow more time for the CPU to service the MBs. By programming
more than one MB with the same ID, received messages will be queued into the MBs. The CPU can
examine the Time Stamp field of the MBs to determine the order in which the messages arrived.
The matching algorithm described above can be changed to be the same one used in previous versions of
the FlexCAN module. When the BCC bit in MCR is negated, the matching algorithm stops at the first MB
with a matching ID that it founds, whether this MB is free or not. As a result, the message queueing feature
does not work if the BCC bit is negated.
Matching to a range of IDs is possible by using ID Acceptance Masks. FlexCAN supports individual
masking per MB (see
Section 18.3.4.13, Rx Individual Mask Registers (RXIMR0–RXIMR63)
the matching algorithm, if a mask bit is asserted, then the corresponding ID bit is compared. If the mask
bit is negated, the corresponding ID bit is a don’t-care bit. Please note that the Individual Mask Registers
are implemented in RAM, so they are not initialized out of reset. Also, they can only be programmed if
the BCC bit is asserted and while the module is in Freeze mode.
Содержание MPC5602S
Страница 76: ...Overview MPC5606S Microcontroller Reference Manual Rev 7 74 Freescale Semiconductor...
Страница 82: ...Memory Map MPC5606S Microcontroller Reference Manual Rev 7 80 Freescale Semiconductor...
Страница 112: ...Signal Description MPC5606S Microcontroller Reference Manual Rev 7 110 Freescale Semiconductor...
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Страница 546: ...Error Correction Status Module ECSM MPC5606S Microcontroller Reference Manual Rev 7 544 Freescale Semiconductor...
Страница 669: ...Flash Memory MPC5606S Microcontroller Reference Manual Rev 7 Freescale Semiconductor 667...
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Страница 882: ...LIN Controller LINFlex MPC5606S Microcontroller Reference Manual Rev 7 880 Freescale Semiconductor...
Страница 901: ...Memory Protection Unit MPU MPC5606S Microcontroller Reference Manual Rev 7 Freescale Semiconductor 899...
Страница 902: ...Memory Protection Unit MPU MPC5606S Microcontroller Reference Manual Rev 7 900 Freescale Semiconductor...
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Страница 1213: ...System Integration Unit Lite SIUL MPC5606S Microcontroller Reference Manual Rev 7 Freescale Semiconductor 1211...
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Страница 1252: ...Wakeup Unit WKPU MPC5606S Microcontroller Reference Manual Rev 7 1250 Freescale Semiconductor...
Страница 1258: ...Registers Under Protection MPC5606S Microcontroller Reference Manual Rev 7 1256 Freescale Semiconductor...
Страница 1323: ...Register Map MPC5606S Microcontroller Reference Manual Rev 7 Freescale Semiconductor 1321...
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