Romram: Using QSPI RAM with RP2040's SSI in read-write mode

The article discusses the challenges and solutions for using external QSPI RAM with the RP2040 microcontroller, particularly focusing on achieving read-write capabilities. The RP2040's XIP mode allows for read and execute operations but not writes, which limits the utility of external RAM. The author proposes a method to boot from flash memory, load it into RAM, and then run from RAM, utilizing simple logic gates to manage memory access. However, the inability to write directly to the RAM presents a significant hurdle.

To address this, the author suggests using a HardFault handler to emulate write operations. By leveraging the microcontroller's MPU, the handler can intercept write attempts, emulate them, and flush the cache as needed. This approach allows for the execution of unmodified software that requires RAM. The article also highlights potential pitfalls, such as the need to manage stack pointers and the timing of write operations to avoid conflicts with the XIP cache.

The performance of this setup is discussed, noting that read and execute operations are at native speed, while write speeds depend on the emulation method used. The author reports achieving a write speed of 36Mbit/s for memory copy operations, indicating that while there are complexities, the solution can be effective for applications requiring additional RAM on the RP2040. Overall, the article provides insights into the innovative use of hardware features to overcome limitations in microcontroller memory management.