What is CAN Bus Bit Timing?

CAN (Controller Area Network) bus communication relies on precise synchronization between nodes. Unlike Ethernet or USB which use dedicated clock lines, CAN is asynchronous. This means every node on the network must have its bit timing configured identically to ensure data is sampled at the correct moment. Our CAN Bus Bit Timing Calculator helps engineers determine the hardware registers needed for various microcontrollers, including STM32, MCP2515, and others.

How to Use the Calculator

To calculate the correct parameters, you need three primary values: the oscillator frequency (Clock), the desired data transmission speed (Bit Rate), and the preferred sample point. Typically, industrial applications use a sample point around 75% to 80%, while CIA (CAN in Automation) recommendations often lean toward 87.5% for specific protocols. Enter your MHz, kbps, and percentage values, and the tool will iterate through the best prescaler and time quanta (TQ) combinations to match your target with the lowest possible error.

Understanding Time Quanta (TQ)

A single CAN bit is divided into four non-overlapping segments: the Sync Segment, the Propagation Segment, Phase Segment 1, and Phase Segment 2. Collectively, these are made up of Time Quanta (TQ). The total number of TQ in a bit usually ranges from 8 to 25. The sample point is the instant where the state of the bus is read; it occurs between Phase Segment 1 and Phase Segment 2.

Frequently Asked Questions

Why is the Sample Point important?

If the sample point is too early, the signal might not have stabilized across the physical wires. If it is too late, the clock drift between nodes might cause the sampler to miss the bit window entirely. Finding the "sweet spot" ensures robust communication in noisy environments.

What happens if there is a frequency error?

If the calculated bit rate deviates from the target by more than 1.58%, the nodes will likely fail to synchronize, leading to error frames and bus-off conditions. Always aim for a 0% error when choosing your oscillator and prescaler settings.