Can Bit Timing Calculator

Calculate optimal CAN bus bit timing parameters for your network configuration. Enter your system parameters below to determine the ideal bit rate, sample point, and synchronization jump width.

The Controller Area Network (CAN) protocol is widely used in automotive and industrial applications for robust, real-time communication between electronic control units (ECUs). Proper bit timing configuration is crucial for reliable CAN communication, as it ensures all nodes on the bus can correctly interpret the transmitted data despite variations in oscillator frequencies and signal propagation delays.

Understanding CAN Bit Timing Fundamentals

CAN bit timing is determined by dividing each bit time into discrete segments called Time Quanta (TQ). The bit time is composed of four main segments:

1. Synchronization Segment (SYNC_SEG): Always 1 TQ, used to synchronize all nodes on the bus
2. Propagation Segment (PROP_SEG): Compensates for physical delay times on the bus
3. Phase Segment 1 (PHASE_SEG1): Can be lengthened or shortened for resynchronization
4. Phase Segment 2 (PHASE_SEG2): Can be shortened for resynchronization

Key Bit Timing Parameters

• Baud Rate Prescaler (BRP): Divides the controller clock frequency to determine TQ duration
• Time Segment 1 (TSEG1): Combination of PROP_SEG and PHASE_SEG1 (2-16 TQ)
• Time Segment 2 (TSEG2): PHASE_SEG2 (1-8 TQ)
• Synchronization Jump Width (SJW): Determines maximum resynchronization (1-4 TQ)
• Sample Point: Point where the bit value is read (typically 70-90%)

Bit Timing Calculation Formula

The fundamental relationship between these parameters is:

Bit Time = (BRP) × (1 + TSEG1 + TSEG2) × T_CLK

Where T_CLK is the period of the controller clock.

The sample point is calculated as:

Sample Point = (1 + SYNC_SEG + PROP_SEG + PHASE_SEG1) / Bit Time

Factors Affecting CAN Bit Timing

1. Oscillator Tolerance

All CAN controllers have some frequency tolerance in their oscillators. Typical values range from ±0.1% to ±1.5%. The bit timing must account for these variations to ensure all nodes remain synchronized.

The maximum allowed tolerance between any two nodes is determined by:

Max Tolerance = min(SJW, PHASE_SEG1) / (2 × (1 + SYNC_SEG + PROP_SEG + PHASE_SEG1 + PHASE_SEG2))

2. Bus Length and Propagation Delay

The physical length of the CAN bus affects signal propagation time. The standard propagation delay is approximately 5 ns/m for twisted pair cables.

The maximum theoretical bus length can be calculated as:

Max Length = (PROP_SEG × TQ – Bus Driver Delay) / Propagation Delay

Where typical bus driver delays range from 100-200 ns depending on the transceiver.

Practical Bit Timing Configuration

When configuring CAN bit timing, follow these practical steps:

1. Determine Required Baud Rate:
   • 125 kbps – Common for automotive body control
   • 250 kbps – Typical for powertrain applications
   • 500 kbps – High-speed networks
   • 1 Mbps – Maximum standard CAN speed
2. Select Controller Clock Frequency:
   • Common values: 8 MHz, 16 MHz, 24 MHz, 40 MHz
   • Higher clock frequencies allow more precise TQ resolution
3. Calculate BRP Value:

   BRP = (Clock Frequency) / (2 × Desired Baud Rate × (Minimum TQ Count))

   Minimum TQ count is typically 8-20 for reliable operation

4. Determine Segment Lengths:
   • SYNC_SEG = 1 TQ (fixed)
   • PROP_SEG ≥ (2 × Bus Propagation Delay) / TQ
   • PHASE_SEG1 ≥ SJW
   • PHASE_SEG2 ≥ SJW
   • Total segments should be 8-25 TQ
5. Verify Sample Point:

   Sample point should be between 70-90% of bit time

   Higher sample points provide better noise immunity but less time for resynchronization

Common Bit Timing Configurations

Baud Rate	Clock (MHz)	BRP	TSEG1	TSEG2	SJW	Sample Point	TQ (ns)
125 kbps	16	2	12	5	4	75%	1000
250 kbps	16	1	12	5	4	75%	500
500 kbps	24	1	12	5	4	75%	333
1 Mbps	24	1	6	3	2	75%	167

Advanced Bit Timing Considerations

1. Bit Rate Switching (CAN FD)

CAN FD (Flexible Data-Rate) introduces two bit rates:

• Arbitration Phase: Standard CAN bit rate (up to 1 Mbps)
• Data Phase: Higher bit rate (up to 8 Mbps)

Bit timing must be configured separately for each phase, with special consideration for the transition between rates.

2. Temperature Effects

Oscillator frequencies can vary with temperature. Typical variations:

• Ceramic resonators: ±0.5% over -40°C to +85°C
• Crystal oscillators: ±0.01% over same range

Bit timing should account for worst-case temperature variations in the operating environment.

Troubleshooting Bit Timing Issues

Common symptoms of incorrect bit timing include:

• Error Frames: Excessive error frames indicate synchronization problems
• Bit Stuffing Errors: May occur if sample point is too early or late
• ACK Errors: Receiver nodes not properly acknowledging messages
• Bus Off State: Node disconnects due to excessive errors

Diagnostic steps:

1. Use an oscilloscope to measure actual bit times and sample points
2. Check for consistent TQ durations across the bit time
3. Verify the sample point occurs during the stable portion of the bit
4. Ensure all nodes use compatible bit timing parameters
5. Check for excessive bus loading (should be < 40% for reliable operation)

Regulatory Standards and Best Practices

The CAN protocol is standardized by ISO 11898, which defines the physical and data link layers. Key standards include:

• ISO 11898-1: Data link layer and physical signaling
• ISO 11898-2: High-speed medium access unit (up to 1 Mbps)
• ISO 11898-3: Low-speed fault-tolerant medium access unit
• ISO 11898-5: High-speed medium access unit with low-power mode
• ISO 11898-6: Selective wake-up functionality

Best practices for bit timing configuration:

1. Always use the highest possible clock frequency for better TQ resolution
2. Keep the number of TQs between 8 and 25 for optimal performance
3. Position the sample point between 70% and 90% of the bit time
4. Ensure PROP_SEG is sufficient for the maximum bus length
5. Use the largest possible SJW that meets your tolerance requirements
6. Test bit timing at temperature extremes
7. Verify timing with both minimum and maximum bus loading

Comparison of Bit Timing Strategies

Strategy	Advantages	Disadvantages	Best For
Fixed Sample Point (80%)	Simple to implement, good noise immunity	Less flexible for different bus conditions	Standard applications with consistent bus lengths
Dynamic Sample Point	Adapts to varying bus conditions	More complex implementation	Systems with variable bus lengths or temperatures
Minimum Phase Segments	Maximizes data throughput	Reduced noise immunity	Short bus lengths in controlled environments
Maximum Phase Segments	Excellent noise immunity and tolerance	Reduced data throughput	Long bus lengths or harsh electrical environments

Authoritative Resources

For more detailed information on CAN bit timing, consult these authoritative sources:

• National Institute of Standards and Technology (NIST) – Time and frequency standards relevant to CAN timing
• ISO 11898-1:2015 – The international standard for CAN data link layer and physical signaling
• SAE International – Automotive standards including J1939 which builds on CAN protocol
• Bosch CAN Specification 2.0 – The original CAN protocol specification from Bosch

Frequently Asked Questions

Q: What is the maximum CAN bus length at 1 Mbps?

A: At 1 Mbps with standard transceivers, the practical maximum bus length is about 40 meters. This can be extended to ~100 meters with specialized transceivers and careful bit timing configuration.

Q: Why is 80% a common sample point?

A: An 80% sample point provides a good balance between:

• Sufficient time for resynchronization (20% of bit time remaining)
• Good noise immunity (sampling after most transient effects)
• Compatibility with most CAN controllers

Q: How does bit timing affect error rates?

A: Proper bit timing minimizes:

• Bit errors: From sampling at wrong time
• Stuff errors: From incorrect bit edge detection
• ACK errors: From nodes not recognizing messages
• Form errors: From incorrect bit field lengths

Optimal timing can reduce error rates by 90% or more compared to poor configurations.

Q: Can I use different bit timing on different nodes?

A: No. All nodes on a CAN bus must use identical bit timing parameters to communicate reliably. Even small differences in timing can cause communication failures.