FD-CAN - Protocol Error - ACK Error

[TSola.1]

Hello,

We are facing an intermittent CAN-FD issue in the field and would appreciate guidance from the community.

Our system has two devices on the bus (no other device on the bus) using a request–response architecture. The master sends a request every 30 ms and the slave responds. This setup is deployed in hundreds of units running continuously (24 hours). Out of these, around 3 to 5 units per day show acknowledgement errors, which we track through the CAN protocol error counters.

The behaviour is unusual:

• The issue appears randomly on any unit.
• No bus-off condition is ever reported.
• Despite no bus-off, communication between the two nodes temporarily stops.
• Communication recovers automatically after a few seconds without any intervention.

Initially, we suspected a physical wiring problem. We re-checked all connectors and even secured them with glue. The bus has 120-ohm termination at both ends. However, the issue still appears randomly.

Below are the system details:

Microcontroller: STM32G0B1CBT6
Baudrate: 125 kbps
CAN bus length: ~100 cm
Termination: 120 ohms at both ends
FD-CAN Core Clock: 50 MHz
ClockDivider: 1
Bitrate Switching: Disabled
AutoRetransmission: Disabled
TransmitPause: Disabled
ProtocolException: Disabled

Nominal Bit Timing:
• Prescaler = 10
• SyncJumpWidth = 8
• TimeSeg1 = 31
• TimeSeg2 = 8

Data Bit Timing (BRS disabled, same as nominal):
• Prescaler = 10
• SyncJumpWidth = 8
• TimeSeg1 = 31
• TimeSeg2 = 8

Filters:
• StdFiltersNbr = 1
• ExtFiltersNbr = 0

---

Troubleshooting Steps Already Performed

1. Physical wiring check
   • Verified connector seating and cable condition
   • Applied glue to prevent vibration-related disconnection
   • Confirmed correct 120-ohm termination at both ends

2. Error counter monitoring
   • ACK errors observed in protocol error counters
   • No error-warning, error-passive, or bus-off states reported

3. Timing verification
   • Checked nominal bit timing settings
   • Ensured both nodes use identical configurations
   • Bitrate switching is disabled on both sides

4. Bus recovery logic
   • Bus-off recovery is implemented
   • Never triggered during these events

5. Environmental factors
   • Units run 24×7
   • Errors occur randomly across different devices and locations

---

CAN Topology Summary

Master Device
 ↕ (approx. 100 cm cable)
Slave Device

Termination resistors (120 ohms) are present at both ends. No other nodes are connected.

---

Any insights or suggestions on what could cause intermittent ACK errors without bus-off would be greatly appreciated.

[mƎALLEm]

---

@TSola.1 wrote:

1. Clock Source
The nodes are running from the internal RC oscillator. Unfortunately, the hardware design does not include a crystal oscillator, so we cannot switch to an external clock source.

---

Unfortunately, the crystal is something crucial for CAN communication.

+ Read also this article: CAN reception issues: Reasons and general troubleshooting

[Ozone]

I overlooked that.

The internal RC oscillator is not suited for most communication protocols. Especially when non-constant environmental temperatures are involved.

[LCE]

Okay, "HSI", bad idea...

Anyway, I would not give up due to the low bit rate.

Your settings:

Nominal Bit Timing:

• Prescaler = 10

• SyncJumpWidth = 8

• TimeSeg1 = 31

• TimeSeg2 = 8

1) reduce the prescaler so there's more room for fine tuning the segments

2) I think SyncJumpWidth must be smaller than TimeSeg2 (SJW is used to "lengthen" Tseg1 for sync purposes), the maximum should be SJW should be SJWmax = Tseg2 -1 

3) ... but use the maximum SJW! 

4) check your HSI frequency and check for the STM's HSI calibration features (if any - never used that, always using external clock)

[TSola.1]

Post edited by ST moderator to be inline with the community rules for the code sharing. In next time please use </> button to paste your code and a linker script content. Please read this post: How to insert source code.

1. Clock Configuration of STM32G0B1CBT6 with baudrate 12.5 kbps:

hfdcan1.Instance = FDCAN1;

hfdcan1.Init.ClockDivider = FDCAN_CLOCK_DIV1;

hfdcan1.Init.FrameFormat = FDCAN_FRAME_FD_NO_BRS;

hfdcan1.Init.Mode = FDCAN_MODE_NORMAL;

hfdcan1.Init.AutoRetransmission = DISABLE;

hfdcan1.Init.TransmitPause = DISABLE;

hfdcan1.Init.ProtocolException = DISABLE;

hfdcan1.Init.NominalPrescaler = 10;

hfdcan1.Init.NominalSyncJumpWidth = 2;

hfdcan1.Init.NominalTimeSeg1 = 15;

hfdcan1.Init.NominalTimeSeg2 = 5;

hfdcan1.Init.DataPrescaler = 10;

hfdcan1.Init.DataSyncJumpWidth = 2;

hfdcan1.Init.DataTimeSeg1 = 15;

hfdcan1.Init.DataTimeSeg2 = 5;

hfdcan1.Init.StdFiltersNbr = 1;

hfdcan1.Init.ExtFiltersNbr = 0;

hfdcan1.Init.TxFifoQueueMode = FDCAN_TX_FIFO_OPERATION; 

2. Curren Configuration of STM32H743 with baudrate 12.5 kbps:

 

---

hfdcan1.Instance = FDCAN1;

  hfdcan1.Init.FrameFormat = FDCAN_FRAME_FD_NO_BRS;

  hfdcan1.Init.Mode = FDCAN_MODE_NORMAL;

  hfdcan1.Init.AutoRetransmission = DISABLE;

  hfdcan1.Init.TransmitPause = DISABLE;

  hfdcan1.Init.ProtocolException = DISABLE;

  hfdcan1.Init.NominalPrescaler = 20;

  hfdcan1.Init.NominalSyncJumpWidth = 2;

  hfdcan1.Init.NominalTimeSeg1 = 15;

  hfdcan1.Init.NominalTimeSeg2 = 5;

  hfdcan1.Init.DataPrescaler = 20;

  hfdcan1.Init.DataSyncJumpWidth = 2;

  hfdcan1.Init.DataTimeSeg1 = 15;

  hfdcan1.Init.DataTimeSeg2 = 5;

  hfdcan1.Init.MessageRAMOffset = 0;

  hfdcan1.Init.StdFiltersNbr = 1;

  hfdcan1.Init.ExtFiltersNbr = 0;

  hfdcan1.Init.RxFifo0ElmtsNbr = 1;

  hfdcan1.Init.RxFifo0ElmtSize = FDCAN_DATA_BYTES_12;

  hfdcan1.Init.RxFifo1ElmtsNbr = 1;

  hfdcan1.Init.RxFifo1ElmtSize = FDCAN_DATA_BYTES_12;

  hfdcan1.Init.RxBuffersNbr = 0;

  hfdcan1.Init.RxBufferSize = FDCAN_DATA_BYTES_12;

  hfdcan1.Init.TxEventsNbr = 1;

  hfdcan1.Init.TxBuffersNbr = 1;

  hfdcan1.Init.TxFifoQueueElmtsNbr = 1;

  hfdcan1.Init.TxFifoQueueMode = FDCAN_TX_FIFO_OPERATION;

  hfdcan1.Init.TxElmtSize = FDCAN_DATA_BYTES_8;

Will be trying to reduce the prescaler and HSI calibration feature if any.

[mƎALLEm]

Even though I still insist on the usage of a crystal, please post some photos of your CAN network set-up: the different nodes with the CAN bus (the wiring).