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Amiga Hardware Issues and discussion / I made a modified accelerator, it works, but has a problem
« Last post by r00tb33r on April 18, 2024, 09:03:08 AM »I made modifications to this simple 14MHz accelerator by Dennis:
https://github.com/Mathesar/14mhz-accelerator
It runs at either 21 or 28MHz with a configurable PLL. Photo attached, along with SysInfo and problem shown in ATK.
It's quite speedy (approximately 16MHz 030 A3000 equivalent according to SysInfo result), largely thanks to the zero waitstate memory at that processor frequency, games like Hard Drivin' and 4D Sports Driving (Stunts) have a nice frame rate, and DREAD demo is super smooth. Every game that runs on the unmodified 14MHz accelerator also runs with the 28MHz modification. The problem is that music routines run too fast in most games. A Goggle search pointed me to a gentleman with similar sound symptoms caused by a weak /DTACK pullup, which ultimately led me to discover that my Amiga is having much higher interrupt frequency as measured in ATK. This problem is not present with unmodified 14MHz accelerator.
My mind isn't as sharp as it was after I finished my university computer engineering degree more than a decade ago, so I'd like to ask for some help figuring out what causes the higher interrupt count and figure out what I should do to improve it.
The Amiga board I am using here is an NTSC 500 rev.5, and the only modifications I made to the accelerator are substituting a configurable PLL, a 10ns delay line to line up the clock edges, and using the buffered 7MHz reference clock from the PLL instead of the one from the system to feed the /DTACK and /AS synchronizing flip-flops because it has a nicer waveform with less skew.
Help is appreciated.
https://github.com/Mathesar/14mhz-accelerator
It runs at either 21 or 28MHz with a configurable PLL. Photo attached, along with SysInfo and problem shown in ATK.
It's quite speedy (approximately 16MHz 030 A3000 equivalent according to SysInfo result), largely thanks to the zero waitstate memory at that processor frequency, games like Hard Drivin' and 4D Sports Driving (Stunts) have a nice frame rate, and DREAD demo is super smooth. Every game that runs on the unmodified 14MHz accelerator also runs with the 28MHz modification. The problem is that music routines run too fast in most games. A Goggle search pointed me to a gentleman with similar sound symptoms caused by a weak /DTACK pullup, which ultimately led me to discover that my Amiga is having much higher interrupt frequency as measured in ATK. This problem is not present with unmodified 14MHz accelerator.
My mind isn't as sharp as it was after I finished my university computer engineering degree more than a decade ago, so I'd like to ask for some help figuring out what causes the higher interrupt count and figure out what I should do to improve it.
The Amiga board I am using here is an NTSC 500 rev.5, and the only modifications I made to the accelerator are substituting a configurable PLL, a 10ns delay line to line up the clock edges, and using the buffered 7MHz reference clock from the PLL instead of the one from the system to feed the /DTACK and /AS synchronizing flip-flops because it has a nicer waveform with less skew.
Help is appreciated.