SLLA521B January   2022  – June 2022 TCAN1145-Q1 , TCAN1146-Q1

 

  1.   Abstract
  2.   Trademarks
  3. 1Introduction – Partial Networking
  4. 2Partial Networking Application
    1. 2.1 Modes of Operation and Partial Networking
    2. 2.2 Sleep Mode and Partial Networking
    3. 2.3 Wake-Up Frame
    4. 2.4 Classical High-speed CAN, CAN FD, and PN
    5. 2.5 Mixed Network Information
  5. 3CAN Frames and Wake-up Frames
    1. 3.1 CAN Frame Structure
    2. 3.2 Wake-up Frames
      1. 3.2.1 ID Field Match
      2. 3.2.2 Data Length Code (DLC) Match
      3. 3.2.3 Data Match
      4. 3.2.4 CRC Match
      5. 3.2.5 Acknowledge Match
    3. 3.3 Error Counter
    4. 3.4 Selective Wake FD Passive
  6. 4Selective Wake Registers
  7. 5Configuring Partial Networking
    1. 5.1 Valid CAN Message ID Example
    2. 5.2 Valid Data Example
  8. 6Summary
  9. 7References
  10. 8Revision History

2.5 Mixed Network Information

Since partial networking Wake Up Frames (WUF) are only recognized as part of a Classical CAN frame, it is not necessary to fully decode CAN FD frames. Also, the faster data rates and switching of data rates within a CAN FD frame, can prove to be too difficult to decode without using a highly accurate external clock source. However, if a CAN frame is not decoded correctly, it will accumulate an error counter that can overflow, and cause an unwanted wake-up. To avoid this problem of CAN FD frames leading to unwanted decode errors, an FD Passive mode is provided.

This key feature of Partial networking capable CAN transceivers is that they can operate seamlessly within a network of CAN FD transceivers when FD Passive mode is enabled by setting the SW_FD_PASSIVE bit, and by providing the data rate ratio of CAN FD:CAN with the FD_DR setting. When CAN FD frames are detected, as indicated by FDF=1, the rest of the frame can be safely ignored so as to not cause an increment to the error counter. CAN bus noise that is smaller than CAN FD bit times will be filtered out, while active FD data can be detected, so that the end of the CAN FD frame can be properly recognized. In this way, a WUF based on Classical CAN may appear at any time and adjacent to CAN-FD frames.

CAN FD passive is only enabled in sleep mode so PN enabled transceivers will ignore CAN FD frames while the transceiver is in sleep mode. These PN enabled transceivers will wake when a valid WUP followed by a valid WUF is received and then will transition to standby mode. The ECU controller changes the PN enabled CAN transceiver to normal mode which allows CAN FD frames to be transmitted and received. The CAN FD PN passive mode should not be disabled. When the go-to-sleep command is issued, the PN enabled transceiver can receive a WUF while transitioning from normal to sleep mode, allowing it to decode the WUF correctly and wake up if valid. If these are disabled, they would have to be re-enabled before placing the PN capable transceiver into sleep mode. What could cause PN to be disabled?

In a typical application, PN would be enabled at power-up and left on to cover the mode transition cases mentioned as well as the transition from normal to sleep mode. If a WUP takes place during this transition, the device already has PN enabled and will be looking for a valid WUF. For the case that sleep mode is entered due to a fault, PN can end up disabled. In this case a WUP will wake up the device even though PN is still configured (SWCFG=1) but it is off due to SW_EN=0. Only the SW_EN bit has to be reset to 1 to completely re-enable PN.

As an example, if a UVIO fault or TSD fault occurs and Failsafe mode is disabled, the device will enter Sleep mode and set SW_EN=0. Since PN is now disabled, a WUP will act as a wake event and transition the device to Standby. As another example, in sleep mode, decoding errors that lead to FRAME_OVF error would set SW_EN=0 and cause the device to transition to standby mode. So, after a fault and before placing the PN capable transceiver to sleep the SW_EN should be set to 1.