Interrupt free V-USB

Starting with V2.0, Micronucleus is going to use an interrupt free modification of the software USB implementation V-USB. This provides significant benefits for the bootloader, as it is not necessary anymore to patch the interrupt vector of the user program. A surprising side effect was a speed up of the V-USB data transmission,  which may also be helpful in other applications. Here, I try to give a rough overview about the meandering work that led to this achievement.

Previous versions of Micronucleus (and also the Trinket bootloader) use an ingenious mechanism devised by Louis of embedded creations to patch the interrupt vector transparently to the user program. Although this approach works very well, it still adds a lot of complexity to the bootloader, will add a couple of cycles of interrupt delay, and carries the risk of breaking the user program in a few rare cases. Removing this burden allows for a drastic reduction in code size and improved robustness. Continue reading