EWGF (Error Warning Flag) intermittently set at 1 Mbps CAN on STM32L4 / STM32F4 – works fine at 500 kbps

[Chandana_Arava]

We are facing an intermittent EWGF (Error Warning Flag) issue during CAN communication at 1 Mbps on multiple STM32 MCUs. The same hardware and software setup works reliably at 500 kbps without any CAN warnings.

---

MCUs Used

• STM32L443VCT6

• STM32L443CCT6

• STM32F437ZGT6

All devices communicate together on the same CAN bus.

---

Issue Details

• When CAN bit rate is configured to 1 Mbps, the EWGF bit in CAN_ESR register gets set intermittently

• Communication does not stop immediately, but error counters increase over time

• At 500 kbps, the issue is not observed

• Same CAN transceivers, wiring, and termination are used

---

CAN Configuration

Common Settings

• CAN peripheral clock: 24 MHz

• GPIO speed for CAN TX/RX pins: High speed

• Same clock source and configuration across all MCUs
  
  

  For Bit Rate 500KHz

  CANx->BTR = CAN_TSEG1(12) | CAN_TSEG2(1) | CAN_BRP(2) |CAN_SJW(0);

  For Bit Rate 1MHz (Tried for different TQ , but still getting CAN warning)

  TQ per bit = 8 TQ

  CANx->BTR = CAN_TSEG1(5) | CAN_TSEG2(0) | CAN_BRP(2) |CAN_SJW(0);

  TQ per bit = 12 TQ

  CANx->BTR = CAN_TSEG1(7) | CAN_TSEG2(2) | CAN_BRP(1) |CAN_SJW(0);

  TQ per bit = 24 TQ

  CANx->BTR = CAN_TSEG1(14) | CAN_TSEG2(7) | CAN_BRP(0) |CAN_SJW(0);

[mƎALLEm]

Hello,

You didn't tell:

1- What is the clock source you have used? HSI or an external Crystal? If HSI that's not recommended you need to use a precise clock source over temperature and other constraints such as a crystal.

2- What is the length of your CAN bus? The maximum bitrate you can reach depends on your bus length:

CAN bitrate	Maximum bus length (m)
1 Mbit/s	35
800 kbit/s	50
500 kbit/s	100
250 kbit/s	250
125 kbit/s	500
50 kbit/s	1000
20 kbit/s	2500
10 kbit/s	5000

3- Did you use a twisted pair for a long bus length? 

4- Did you connect two terminating resistors of 120ohm at both sides of the CAN bus?

Please refer to the following knowledge base articles:

CAN (bxCAN) bit time configuration on STM32 MCUs

Using CAN (bxCAN) in Normal mode with STM32 microcontrollers (Part 1)

Using CAN (bxCAN) in Normal mode with STM32 microcontrollers (Part 2)