NORbert: chasing a ground bounce through the quad I/O address phase

Found a weird bug in NORbert last week. Quad I/O reads (0xEB, the 1-4-4 mode) driven by an FT4222 would silently return 0xFF at specific SPI clock frequencies. 1-1-1, 1-1-2, 1-2-2 and 1-1-4 all worked fine. 10, 15 and 20 MHz failed; 12 and 30 MHz worked. Same data, same emulator, only the SPI clock changed. Peak 'this is supposed to be the fun hobby project' energy. The symptom Flashprog verify…