34.9.3 Tx Buffer Element

The Tx Buffers section can be configured to hold dedicated Tx Buffers as well as a Tx FIFO / Tx Queue. In case that the Tx Buffers section is shared by dedicated Tx buffers and a Tx FIFO / Tx Queue, the dedicated Tx Buffers start at the beginning of the Tx Buffers section followed by the buffers assigned to the Tx FIFO or Tx Queue. The Tx Handler distinguishes between dedicated Tx Buffers and Tx FIFO / Tx Queue by evaluating the Tx Buffer configuration TXBC.TFQS and TXBC.NDTB. The element size can be configured for storage of CAN FD messages with up to 64 bytes data field via register TXESC.

Table 34-9. Tx Buffer Element
31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0
T0 E

S

I

 X

T

D

 R

T

R

 ID[28:0]
T1 MM[7:0]

E

F

C

F

D

F

 B

R

S

 DLC[3:0]
T2 DB3[7:0] DB2[7:0] DB1[7:0] DB0[7:0]
T3 DB7[7:0] DB6[7:0] DB5[7:0] DB4[7:0]
... ... ... ... ...
Tn DBm[7:0] DBm-1[7:0] DBm-2[7:0] DBm-3[7:0]

  • T0 Bit 31 - ESI: Error State Indicator

    0 : ESI bit in CAN FD format depends only on error passive flag.

    1 : ESI bit in CAN FD format transmitted recessive.

    Note: The ESI bit of the transmit buffer is OR’ed with the error passive flag to decide the value of the ESI bit in the transmitted FD frame. As required by the CAN FD protocol specification, an error active node may optionally transmit the ESI bit recessive, but an error passive node will always transmit the ESI bit recessive.

  • T0 Bit 30 - XTD: Extended Identifier

    0 : 11-bit standard identifier.

    1 : 29-bit extended identifier.

  • T0 Bit 29 - RTR: Remote Transmission Request

    0 : Transmit data frame.

    1 : Transmit remote frame.

    Note: When RTR = ‘1’, the CAN transmits a remote frame according to ISO 11898-1, even if CCCR.CME enables the transmission in CAN FD format.

  • T0 Bits 28:0 - ID[28:0]: Identifier

    Standard or extended identifier depending on bit XTD. A standard identifier is stored into ID[28:18].

  • T1 Bits 31:24 - MM[7:0]: Message Marker

    Written by CPU during Tx Buffer configuration. Copied into Tx Event FIFO element for identification of Tx message status.

  • T1 Bit 23 - EFC: Event FIFO Control

    0 : Don’t store Tx events.

    1 : Store Tx events.

  • T1 Bit 22 - Reserved

  • TR1 Bit 21 - FDF: FD Format

    0 : Frame transmitted in Classic CAN format.

    1 : Frame transmitted in CAN FD format.

  • T1 Bit 20 - BRS: Bit Rate Search

    0 : CAN FD frames transmitted without bit rate switching.

    1 : CAN FD frames transmitted with bit rate switching.

    Note: Bits ESI, FDF, and BRS are only evaluated when CAN FD operation is enabled CCCR.FDOE = ‘1’. Bit BRS is only evaluated when in addition CCCR.BRSE = ‘1’.

  • T1 Bits 19:16 - DLC[3:0]: Data Length Code

    0-8 : CAN + CAN FD: received frame has 0-8 data bytes.

    9-15 : CAN: received frame has 8 data bytes.

    9-15 : CAN FD: received frame has 12/16/20/24/32/48/64 data bytes.

  • T1 Bits 15:0 - Reserved

  • T2 Bits 31:24 - DB3[7:0]: Data Byte 3

  • T2 Bits 23:16 - DB2[7:0]: Data Byte 2

  • T2 Bits 15:8 - DB1[7:0]: Data Byte 1

  • T2 Bits 7:0 - DB0[7:0]: Data Byte 0

  • T3 Bits 31:24 - DB7[7:0]: Data Byte 7

  • T3 Bits 23:16 - DB6[7:0]: Data Byte 6

  • T3 Bits 15:8 - DB5[7:0]: Data Byte 5

  • T3 Bits 7:0 - DB4[7:0]: Data Byte 4

    ...

  • Tn Bits 31:24 - DBm[7:0]: Data Byte m

  • Tn Bits 23:16 - DBm-1[7:0]: Data Byte m-1

  • Tn Bits 15:8 - DBm-2[7:0]: Data Byte m-2

  • Tn Bits 7:0 - DBm-3[7:0]: Data Byte m-3

Note: Depending on the configuration of TXESC, between two and sixteen 32-bit words (Tn = 3 ... 17) are used for storage of a CAN message’s data field.