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.
The Raspberry Pi RP2040 microcontroller is now available as a standalone chip, the RP2350, allowing manufacturers to integrate it into their own designs. This dual-core Cortex-M0+ processor with 264KB of SRAM is offered in two package variants: a QFN-56 and a future LGA package with stacked external flash memory. The QFN-56 is available for immediate purchase, offering a cost-effective solution for diverse embedded applications. The stacked memory variant promises simplified design and reduced board space, though pricing and availability for that package are yet to be announced.
Hacker News users generally expressed excitement about the RP2350's availability and potential, particularly its low cost and the possibilities opened by the stacked memory variant. Several commenters discussed the chip's suitability for specific applications, including robotics, industrial control, and amateur radio. Some expressed skepticism about the real-world performance impact of the stacked memory, wanting to see benchmarks. Others debated the merits of the RP2350 compared to other microcontrollers like the ESP32, highlighting trade-offs in processing power, memory, and peripheral options. A few users also mentioned challenges they've faced with Raspberry Pi availability in the past, hoping this release would be smoother.
The blog post details the author's experience porting Rust to the RockPro64 (RP2350) single-board computer. They successfully brought up a minimal Rust environment, including core libraries, allowing basic "Hello, world!" functionality and interaction with GPIO pins. The process involved building a custom cross-compilation toolchain based on a pre-built Debian image, navigating architectural differences like the lack of an MMU, and implementing necessary drivers. While challenging, this achievement lays the groundwork for more complex Rust development on the RP2350, potentially opening doors for embedded systems applications.
HN commenters generally express enthusiasm for Rust's increasing viability on embedded platforms, particularly the RP2040. Several users discuss the benefits of Rust's memory safety and performance in this context, comparing it favorably to C/C++. Some point out the challenges of working with Rust on resource-constrained devices, like managing memory allocation and dealing with abstractions that can add overhead. A few commenters also mention specific crates like rp-pico and embassy, highlighting their usefulness for embedded Rust development on the RP2040. There's also discussion around build times, tooling, and the learning curve associated with Rust, with some suggesting that the ecosystem is still maturing but rapidly improving. Finally, some users share their own experiences and projects using Rust on embedded systems.
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.