Pi5 SPI Overlay: Spi0-2cs Works, Spi5-1cs Fails - Why?

by Marco 55 views

Hey guys! Let's dive into a bit of a head-scratcher I ran into while playing around with SPI on a Raspberry Pi 5. Specifically, we're talking about the dtoverlay settings for SPI. I discovered that using dtoverlay=spi0-2cs to create SPI objects works like a charm, but things get a bit weird when trying to do the same with dtoverlay=spi5-1cs. In this article, we'll break down what I found, the system details, and hopefully, shed some light on why this seemingly straightforward setup wasn't working as expected. We will be exploring the Pi5 SPI configuration, troubleshooting the dtoverlay issue, and discussing the differences between spi0 and spi5 on the Raspberry Pi 5.

The Setup: Raspberry Pi 5 and SPI

First off, let's get the basics out of the way. I was working with a Raspberry Pi 5. For those of you who aren't super familiar, the Pi 5 is the latest and greatest from the Raspberry Pi Foundation, packing a bunch of new features and improvements over previous models. I was also attempting to utilize Serial Peripheral Interface (SPI), a synchronous serial communication interface used to send data between microcontrollers and peripherals. This is the communication that permits various devices like sensors, displays, and other modules to be connected to the Raspberry Pi. In my attempts, the goal was to bring up a custom SPI interface to connect with an external module. This process often starts with adjusting the dtoverlay settings in the Raspberry Pi's configuration. Specifically, I was tweaking the /boot/config.txt file to enable and configure the SPI interfaces.

My system details are:

  • Raspberry Pi 5: The latest model at the time of this article. The Pi 5 offers a significant boost in performance compared to its predecessors, making it ideal for various projects that require more processing power, including those utilizing SPI communication. The use of the Pi 5 allows for experimentation to find out the differences between the functionalities of the different SPI interfaces.
  • Operating System: Linux rasp5Dev 6.12.20+rpt-rpi-2712 #1 SMP PREEMPT Debian 1:6.12.20-1+rpt1~bpo12+1 (2025-03-19) aarch64 GNU/Linux. This is the operating system that runs the Raspberry Pi 5, and it is important to know the kernel version because it affects how the hardware interacts with the software.

Now, the reason I was using dtoverlay is simple: it's the standard way to configure hardware overlays on the Raspberry Pi. Overlays essentially allow you to define how the Pi's hardware is configured without needing to compile a custom kernel. Specifically, in this case, I was using it to enable and configure various SPI interfaces.

Understanding dtoverlay and SPI

So, what exactly is going on with dtoverlay and SPI? When you specify something like dtoverlay=spi0-2cs in your /boot/config.txt, you're telling the Raspberry Pi's kernel to load a specific device tree overlay (DTO). This DTO is like a configuration blueprint that describes how to set up the SPI interface. Here, spi0 refers to the primary SPI interface, and -2cs indicates that you want to enable two chip select lines (CS). CS lines are crucial because they allow you to control multiple SPI devices independently.

On the other hand, spi5-1cs would be the configuration for another SPI interface, potentially with one chip select line. The fact that spi0-2cs worked flawlessly suggests that the basic SPI functionality on my Pi 5 was fine.

The Problem: spi5-1cs Not Playing Nice

Now, here's where things got tricky. My plan was to use different SPI interfaces, specifically attempting to use the spi5 interface, and as part of this I tried to enable it via the overlay dtoverlay=spi5-1cs. I plugged the device, and tried to bring up the spi5 interface. In theory, this should be as straightforward as the spi0 setup. However, when I made this change and rebooted, I ran into problems. The SPI device using spi5-1cs just wouldn't initialize correctly. I spent a fair amount of time troubleshooting, double-checking my wiring, and verifying the SPI device itself, but no luck. I decided to try different values for the SPI interface (i.e., spi1, spi2, spi3, spi4, and spi6) using dtoverlay=spi?-1cs, and none of them worked. This meant that only the original SPI configuration (spi0-2cs) seemed to function.

This behavior was highly unexpected. I suspected a hardware or software issue specific to the spi5 interface. But why? This is what drove me to dig deeper and figure out the root cause. This made me think the issue was more about how spi5 was handled by the Raspberry Pi 5's hardware or the device tree configuration.

Troubleshooting Steps

Alright, so how did I go about figuring out what was happening? Here's a rundown of the troubleshooting steps I took:

  1. Verified the Basics: Double-checked the hardware connections, power supply, and the SPI device itself to ensure it was functioning correctly. This is always the first step. Make sure the device is properly connected and powered. I ensured that the connections were correct on the board, and made sure I wasn't missing any basic configuration details, such as missing grounds or voltage mismatches. Then, I tested the SPI device with a known working setup (like the spi0) to rule out any device-specific problems.
  2. Checked the /boot/config.txt: Carefully reviewed the /boot/config.txt file for any typos, incorrect settings, or conflicting configurations. I carefully examined the /boot/config.txt file for any obvious errors. I made sure that the dtoverlay lines were correctly formatted, and that there were no conflicting settings that could be interfering with the SPI configuration. Also, I looked for any uncommented lines that might be disabling SPI.
  3. Examined Kernel Logs: Used dmesg and journalctl to check for any error messages or warnings related to SPI or device tree overlays during boot. The kernel logs are invaluable. Running dmesg gives a full log of the boot process, and I scanned for anything related to SPI or device tree overlays. journalctl is another great tool. It gives you a way to view system logs, so I could filter it for specific messages related to the SPI configuration. This helped me identify any errors or warnings that could point to the problem. If the overlay failed to load, or if there were any hardware initialization issues, these logs would be the first place to find them.
  4. Verified SPI Device Tree: Investigated the device tree to confirm that the SPI interfaces were correctly defined and enabled. The device tree is where the hardware configuration for the Pi is laid out. I wanted to confirm that the spi5 interface was correctly defined and that its configuration was enabled, matching the other SPI interfaces. This can be a bit tricky to interpret, but it's a crucial step in understanding the hardware configuration. If it was configured differently from spi0, then that might explain why it wasn't working as expected.

Possible Causes and Solutions

So, what might be the deal? Based on my investigation, here are a few potential causes and some possible solutions:

  • Hardware Limitations: It's possible that spi5 has hardware limitations or is configured differently on the Raspberry Pi 5 compared to spi0. Perhaps specific pins associated with spi5 are also used for other functions by default, causing conflicts.
    • Solution: Consult the Raspberry Pi 5 documentation to check if spi5 has any specific restrictions or alternative pin functions that might be interfering. Check the pinout diagram and the Raspberry Pi documentation to ensure that the GPIO pins used by spi5 are not being used by another function. Ensure there is no conflict with any of the other peripherals.
  • Device Tree Configuration Errors: There might be an issue with how the device tree overlay for spi5 is defined. If the overlay is incorrect or missing required settings, it won't initialize the SPI interface correctly.
    • Solution: Double-check the syntax and settings of the device tree overlay in /boot/config.txt. Make sure the overlay for spi5 is properly configured, with the correct chip select lines and other relevant parameters. Verify the overlay against known-working examples, and make sure there are no typos or incorrect settings. Try different variations of the overlay settings.
  • Kernel Module Issues: There could be a problem with the kernel module that handles spi5. This might be caused by a missing or corrupted module.
    • Solution: Ensure that the kernel module for SPI is correctly installed and loaded. Check to see if there are any issues with the kernel module. You can try to reload the module, and see if it helps. If that does not work, consider updating the kernel. If it is not, then it is possible to try updating the kernel to the latest version. Make sure you back up your system before making any significant changes to the kernel.
  • GPIO Pin Conflicts: As mentioned above, it's possible that the GPIO pins used by spi5 are also used for other functions, leading to conflicts.
    • Solution: Investigate the Raspberry Pi 5's pinout diagram and documentation. Make sure that the pins used for spi5 are not being used for another function or connected to something that is interfering with the SPI communication.

Conclusion and Further Steps

So, there you have it! My journey in trying to get spi5 up and running on a Raspberry Pi 5. While I got spi0 working perfectly fine, I faced challenges when trying to set up spi5. Through troubleshooting and research, I suspect the problem is a combination of hardware configurations, and potential device tree settings.

To further troubleshoot this, I'm planning to:

  • Examine the Device Tree: Delve deeper into the device tree configuration to see how spi5 is defined, and compare it with the working configuration of spi0. I will be looking at the specific device tree files to see how the SPI interfaces are configured and compare the definitions of spi0 and spi5. The idea is to identify any differences that might explain why spi5 is not working. I'll look at any specific settings, pin assignments, and configurations to see if they are causing issues.
  • Consult Official Documentation: Double-check the official Raspberry Pi 5 documentation for any specific information about SPI configurations, especially spi5, and any known limitations or special configurations. There might be details about pin assignments, or hardware limitations that I missed initially. Looking for any troubleshooting tips, or frequently asked questions that will help me understand what is going on with spi5.
  • Experiment with Pin Assignments: Try different GPIO pin assignments to see if the problem is related to specific pins. There is a possibility of having hardware conflicts with some pins. Testing different pins could help determine the issue and provide a fix to fix the problem. This might involve changing the chip select pins, or other GPIO pins used for SPI communication. The idea is to isolate the problem, and see if changing the pins helps to solve the problem.

Hopefully, this article gives you a good starting point if you run into similar issues. If you've experienced something similar or have any insights, please share them! Let's help each other out and build a better understanding of the Raspberry Pi 5 and its SPI capabilities.