5 ms·

You could use the DMA engine to reconfigure the function routing using a pair of DMA channels.

• dmitrygr 4y ago
  Sadly in many chips this is not possible. Eg STM32H7 has 3 types of DMA units and each has a complex table of where they can read and write from. Two kinds of DMA cannot write to anywhere that contains a DMA controller. Rp2040 is rather unique in how powerful its DMA units are due to their reach. But it also has faults. Its DMA units seemingly cannot access GPIO, since that is bolted directly to the core (via the single cycle IO port). The only way to affect pins from DMA is via PIO as far as I can tell.
  • sgtnoodle 4y ago
    Reminds me of a memorable Ethernet peripheral. The peripheral would use the MCU's RAM for its own buffering. There were frame headers that both the peripheral and CPU would need to manipulate. Being an M7 the CPU had a data cache, though, and so the CPU couldn't safely manipulate the headers without first completely disabling the cache. I ended up using a DMA channel to indirectly manipulate the headers, bypassing the data cache. It was pretty silly but worked, and was way more efficient than disabling the data cache.
    • dmitrygr 4y ago
      On an m7, you can use the MPU to assign non-cacheable attribute to a memory area. As small as 32bytes and as large as 4GB. Any size representable as 2^n - k * 2^(n - 3) for k = 0..7 and n >= 8
      • sgtnoodle 4y ago
        The particular driver was being retrofitted into a large enough variety of existing firmwares that we really wanted to avoid introducing any sort of MPU or link time configuration. The DMA channel approach was the least impactful path.