I believe what Braben was referring to is the IRQ the APU sends when it's done playing an audio sample. This can be used as a hardware timer by playing a silent

I believe what Braben was referring to is the IRQ the APU sends when it's done playing an audio sample. This can be used as a hardware timer by playing a silent sample (all zeroes) and varying the sample length and pitch to adjust the amount of time before the IRQ happens. There's a Twitter thread (https://togetter.com/li/753345 https://togetter.com/li/753345) from the programmer of Guardian Legend (a game that uses the APU IRQ for measuring when to switch from displaying the main gameplay to displaying the HUD) that mentions something similar to Braben's story. After the game came out, he started getting reports that it wasn't working correctly on a small number of consoles. It turned out out that Guardian Legend was the first game to use the APU IRQ and Nintendo had never tested that feature at manufacture. After this, Nintendo started instructing developers to not use the APU IRQ.