GBA clock frequency slightly faster than NDS?

[ivanivanivan]

So the GBA clock frequency is 2^24 = 16.77216MHz, and the NDS bus clock frequency is 33.513982MHz. This is almost double, but if you calculate the error, it's off by 1207ppm:
16.777216 / (33.513982/2)
Assuming the NDS divides the clock by two...This would mean the GBA is 0.12% faster than the NDS running GBA games.Has anybody confirmed?

See the GBATEK document, section "DS Memory Timings" subsection "System Clock", and section "LCD Dimensions and Timings" subsection "System Clock"


I also read this note at the bottom of "LCD Dimensions and Timings. "Note: The NDS is applying some sort of frameskip to GBA games, about every 3 seconds there will by a missing (or maybe: inserted) frame, ie. a GBA game that is updating the display in sync with GBA interlace will get offsync on NDS consoles."

Do you think this might have to do with it? Anyways, I think it may have implications in speedrunning, a 0.12% difference is not negligible. That's 1 second on a 16 minute run, if my math is correct.

 

[ghjfdtg]

Yes, this is well known. All DS consoles with native GBA hardware run ever so slightly slower. The reason is the clock crystal of the DS. The output frame rate is however very slightly faster than GBA so it will show the same frame twice every few seconds.

 

[ivanivanivan]

Thanks. That's weird.

I also confirmed it on a frequency counter. For future record, these are captures of the Nintendo DS clock output (on slot 2). It's the same clock no matter if it's in NDS or GBA mode. The output is very close to 16.756991MHz. It's just dividing the bus clock by 2. So indeed, the bus clock is 33.513982MHz.