CAN Bus Timing Calculator

This tool calculates the four critical bit timing register values for CAN controllers: Baud Rate Prescaler (BRP), Synchronization Jump Width (SJW), Time Segment 1 (BS1), and Time Segment 2 (BS2). It ensures compliance with ISO 11898-1 timing constraints and outputs valid, integer-based register settings for STM32 bxCAN peripherals.

The sample point determines when the bus level is sampled during each bit time — ideally placed near the end of the bit to maximize noise immunity and accommodate propagation delay. For robust operation, 75–87.5% is recommended; 80% is widely used for medium-length buses (e.g., 20–100 m) and balances timing tolerance with synchronization stability.

The calculator supports common APB1 clock frequencies (e.g., 36 MHz, 42 MHz, 48 MHz) used in STM32 MCUs (e.g., F0/F1/F3/F4 series). Since CAN timing derives from APB1, an incorrect clock value leads to inaccurate bit rates — always verify your actual peripheral clock using RCC configuration or HAL_RCC_GetPCLK1Freq().

BS1 (TSEG1) covers the propagation segment and phase segment 1; BS2 (TSEG2) covers phase segment 2 and the sample point. Together with BRP and SJW, they define the total number of time quanta per bit (TQ/bit = 1 + BS1 + BS2), where each TQ = BRP × APB1 period. Their ratio affects resynchronization capability and jitter tolerance.

SJW (Synchronization Jump Width) limits how far the controller can adjust the sample point during resynchronization — critical for handling oscillator drift or network jitter. STM32 bxCAN restricts SJW to 1–4 TQ; values ≥ BS1 are invalid, and SJW = 1 is preferred for stable, low-jitter networks unless operating near timing limits.

Longer buses increase signal propagation delay, requiring longer BS1 to ensure the sample point occurs after the dominant edge has settled across the entire network. While this tool doesn’t auto-adjust BS1 for length, the bus length input helps guide conservative timing choices — e.g., >50 m often warrants ≥80% sample point and larger TSEG1.

No — this tool is designed exclusively for classical CAN (ISO 11898-1, up to 1 Mbps). CAN FD uses separate timing configurations for nominal and data phases, different synchronization rules, and extended bit rate capabilities. Use a dedicated CAN FD timing calculator or vendor tools (e.g., ST’s CubeMX or CAN FD Bit Timing Calculator) instead.

First, verify inputs: ensure APB1 clock and desired bit rate are physically achievable (e.g., 1 Mbps requires ≥16 MHz APB1 with minimal TQ/bit). Try relaxing constraints — increase sample point tolerance (±2%), allow SJW=2–4, or confirm bus length isn’t forcing overly conservative timing. Some combinations (e.g., 1 Mbps @ 36 MHz APB1) may be mathematically impossible.

Load the computed BRP, BS1, BS2, and SJW into the CAN_BTR register (or use HAL_CAN_Init() with appropriately configured CAN_Timing struct). Confirm operation using a CAN analyzer or oscilloscope — measure actual bit time and check for ACK errors, bus-off events, or resynchronization warnings in CAN_ESR. Also cross-check with STM32CubeMX’s built-in timing estimator.

While this tool doesn’t configure mode directly, “Normal” mode implies real-world timing constraints (propagation delay, noise, node skew); “Loopback” mode (often selected via CAN_MCR register) bypasses physical transceivers for testing and relaxes timing requirements since there’s no bus delay. Timing parameters calculated for Normal mode remain valid in Loopback — but not vice versa.