This blog post details how to boot an RP2040-based Raspberry Pi Pico W (RP2350) directly from UART, bypassing the usual flash memory boot process. This is achieved by leveraging the ROM bootloader's capability to accept code over UART0. The post provides Python code to send a UF2 file containing a custom linker script and modified boot2 code directly to the Pico W via its UART interface. This custom boot2 then loads subsequent data from the UART, allowing the execution of code without relying on flashed firmware, useful for debugging and development purposes. The process involves setting specific GPIO pins for bootsel mode, utilizing the picotool utility, and establishing a 115200 baud UART connection.
This blog post details the process of booting the Raspberry Pi Pico W (RP2350) microcontroller from its UART interface, rather than the default flash memory. This method allows for rapid prototyping and development by streamlining the code deployment process, eliminating the need for repeated flashing of the microcontroller's internal memory. The author outlines the steps required to achieve this, beginning with the underlying hardware mechanism that enables UART booting.
The RP2350 features a Boot ROM (read-only memory) that executes upon power-up. This Boot ROM checks specific GPIO pins to determine the boot mode. By configuring these BOOTSEL pins appropriately, the RP2350 can be instructed to boot from UART0. This UART boot mode allows the microcontroller to receive program code directly through the serial port.
The post then explains how to implement the UART boot process practically. The first step involves setting the correct BOOTSEL pin configuration to enable UART booting. The author then describes using a USB-to-TTL serial adapter to connect a host computer to the RP2350's UART0 interface. This connection allows the host computer to transmit the program code directly to the RP2350.
The blog post then dives into the software side, providing details about the picotool utility and explaining how it can be used to load programs directly into the RP2350's RAM via the established serial connection. The author highlights the load command within picotool, specifying how it takes a UF2 file (a specialized file format used for flashing microcontrollers) as input and transmits it over the UART to the RP2350. The microcontroller's Boot ROM then executes the received code.
Finally, the author illustrates the entire process with a concrete example, demonstrating how to compile a simple "blink" program and then load and execute it on the RP2350 using the described UART boot method. This example provides a practical walkthrough, making the process easy to replicate for readers. The post concludes by reiterating the benefits of UART booting, particularly for iterative development workflows where rapid code changes and testing are paramount.
Summary of Comments ( 20 )
https://news.ycombinator.com/item?id=43954054
Hacker News users discuss various aspects of booting the RP2350 from UART. Several commenters appreciate the detailed blog post, finding it helpful and well-written. Some discuss alternative approaches like using a Raspberry Pi Pico as a USB-to-serial adapter or leveraging the RP2040's ROM bootloader. A few highlight the challenges of working with UART, including baud rate detection and potential instability. Others delve into the technical details, mentioning the RP2040's USB boot mode and comparing it to other microcontrollers. The overall sentiment is positive, with many praising the author for sharing their knowledge and experience.
The Hacker News post "Booting the RP2350 from UART" (https://news.ycombinator.com/item?id=43954054) has a modest number of comments, sparking a discussion around the intricacies of the RP2040's boot process and the implications of UART booting.
One commenter highlights the usefulness of UART booting for situations where the flash chip is corrupted, allowing for recovery and reprogramming. They express appreciation for the detailed blog post, emphasizing the value of understanding the low-level workings of the microcontroller.
Another commenter focuses on the wider context of microcontroller security, pointing out the potential vulnerabilities introduced by an exposed UART boot mode. They argue that if an attacker gains physical access, they could potentially compromise the device. This comment sparks a brief discussion about the balance between convenient debugging/recovery features and the security risks they entail, with suggestions for mitigating these risks through hardware or software mechanisms.
Another participant dives deeper into the technical aspects of the RP2040's boot ROM, explaining how it checks for specific conditions on the GPIO pins to determine the boot mode. They appreciate the author's clarification on the boot process, noting it corrected their own misunderstanding.
One comment thread delves into the complexities of different boot modes and their respective advantages. Specifically, they discuss the trade-offs between using a dedicated boot pin versus relying on existing GPIOs, considering factors like pin availability and potential conflicts with other functionalities. This discussion also touches on the broader topic of designing robust and flexible bootloaders.
Finally, a commenter reflects on their own experience with the RP2040, expressing frustration with the lack of clear documentation regarding the boot process. They commend the blog post author for providing valuable insights and filling this documentation gap. This sentiment echoes the general appreciation for the article's clarity and depth in explaining a complex technical process.