Configuring CAN Bit Timing

1. The Anatomy of a CAN Bit

To control the bitrate, we don't just set a single frequency. Instead, we define the duration of a single bit by breaking it down into smaller, programmable segments. The fundamental unit of time in CAN timing is the time quantum (plural: time quanta), abbreviated as tq.

A single nominal bit time is composed of several such quanta, organized into three main segments for the STM32's bxCAN peripheral:

1. Synchronization Segment (SYNC_SEG): This is a fixed segment of 1 tq. It's used to align the clocks of all nodes on the bus whenever a message starts (i.e., on a falling edge from recessive to dominant).
2. Bit Segment 1 (BS1): This segment compensates for signal propagation delays across the bus wiring and within the transceivers. Its length is programmable.
3. Bit Segment 2 (BS2): This segment compensates for phase errors and oscillator drift between nodes. Its length is also programmable.

The Sample Point is the specific instant in time where the CAN controller reads the voltage on the bus to determine if the bit is a '0' (dominant) or a '1' (recessive). It is located at the end of Bit Segment 1.

The image below, taken from an STM32 reference manual, beautifully illustrates this structure and the key formulas we'll be using.

To dive deeper into these concepts, the official STMicroelectronics documentation provides an excellent explanation.

2. The Bitrate Calculation

Now that we understand the components, let's assemble the calculation. The final bitrate depends on three factors:

1. CAN Peripheral Clock Frequency ($f_{PCLK}$): This is the input clock to the CAN peripheral, which on the STM32F103 is derived from the APB1 bus clock.
2. Baud Rate Prescaler (BRP): An integer value (from 1 to 1024) that divides the peripheral clock to create the time quantum.
3. Time Segment Registers (TS1 and TS2): These registers hold the number of time quanta for Bit Segment 1 and Bit Segment 2.

The relationships are as follows:

• Time Quantum Duration:
  $t_q=\frac{BRP}{f_{PCLK}}$
• Total Quanta per Bit:
  $T{Q}_{total}=1+(TS1+1)+(TS2+1)$
  (Note: The values written to the TS1 and TS2 fields in the CAN_BTR register are one less than the number of quanta they represent, hence the +1 in the formula).
• Nominal Bit Time:
  $\text{Bit Time}=T{Q}_{total}\times t_q$
• Baud Rate:
  $\text{Baud Rate}=\frac{1}{\text{Bit Time}}=\frac{f_{PCLK}}{BRP\times (1+(TS1+1)+(TS2+1))}$

One more crucial parameter is the Sample Point. It's calculated as a percentage of the total bit time:

$\text{Sample Point (\%) = }\frac{1+(TS1+1)}{T{Q}_{total}}\times 100\%$

For robust communication, especially at higher speeds, the CAN standard recommends a sample point around 87.5%. This allows maximum time for the signal to propagate and stabilize before it's read.

For a clear walkthrough of these formulas and the sample point's role, please review the next sections of the ST article.

3. Practical Configuration for 500 kbit/s

As you can see, for a given clock and desired bitrate, there can be many valid combinations of BRP, TS1, and TS2. Finding the optimal set that also yields a good sample point can be tedious. Professionals typically use specialized calculators for this task.

Let's configure our project for a common automotive bitrate: 500 kbit/s.

Step 1: Find Your CAN Clock Frequency

First, we need to know the clock frequency being fed to the CAN peripheral.

1. Open your project in STM32CubeIDE.
2. Open the .ioc file to view the device configuration tool.
3. Navigate to the "Clock Configuration" tab.
4. Look for the box labeled "APB1 peripheral clocks (MHz)". This is your $f_{PCLK}$. For a typical STM32F103C8 running at its maximum speed of 72MHz, the APB1 clock is usually configured to 36 MHz. Let's proceed assuming this value.

Step 2: Use an Online Calculator

We will use the same online calculator recommended in the ST article and the video below.

Now, let's use the calculator ourselves. The following guide is based on the excellent step-by-step example in the ST article.

After entering the parameters, STM32CubeIDE should show the "Nominal Baud Rate" as 500000 bit/s, confirming your settings are correct.

Conclusion

Excellent work. You have now mastered a fundamental and often confusing aspect of CAN configuration. This meticulous process of timing configuration is what ensures every node on the bus, from a simple sensor to the engine control unit, can communicate reliably.

Key Takeaways:

• CAN bitrate is not set directly; it's a result of the peripheral clock, Baud Rate Prescaler (BRP), and the number of time quanta in Bit Segments 1 & 2 (TS1, TS2).
• The Sample Point is a critical parameter for reliable communication, with a location of ~87.5% being a common professional standard.
• While manual calculation is possible, using an online bit timing calculator is an efficient and reliable method to find optimal timing parameters.
• You have now configured your STM32's CAN peripheral for a 500 kbit/s bus speed, a standard in the automotive world.

With the physical layer connected and the timing correctly configured, our CAN node is finally ready to communicate. In our next lesson, we will write the firmware to implement a function to transmit a standard CAN message onto the bus.