Amiga bus frequency max?

[psxphill]

What is the bus frequency limit for Amiga without CPU ?
 
Ie what is the limit without any CPU to throttle.

I were mainly thinking on the local bus that is physically wired to Agnus (OCS/ECS/AGA), Paula and Denise etc.

For access to chip ram it's 7mhz for OCS/ECS & 14mhz for AGA. The CPU only gets spare cycles, so the limit is irrelevant to whether there is a CPU or not.
 
The CPU only has access to Agnus, which has it's own register bus that connects to paula and denise. I believe the register bus runs at the same speed as chip ram.
 
1. dma'd data from chip ram gets sent across this bus, so it has to be as fast as chip ram.
 
2. There would be no benefit to have run it at a faster clock, because nothing could have made use of it.
 
However it also suffers contention from dma & copper in Agnus. I don't know whether the contention of the register and chip ram bus is handled separately or whether it's combined.
 
Is that what you wanted to know?

[psxphill]

I think I read somewhere that it can access Agnus at 28Mhz.

I would like to see where you read that.

[psxphill]

So clocking the CPU socket (bus) faster than 28 MHz is pointless except for any onboard memory (60 ns dram?).

That depends, A 68000 doesn't access ram on every cycle.
 
With the cheap 14mhz 68000 hack, it has to synchronise with the 7mhz clock when accessing the bus. So quick instructions aren't really affected, but long instructions like MULT/DIV can run quicker. But you don't get faster chip ram or access to registers.
 
For an 020 the code could be running from cache and not accessing any ram at all. But generally if you have an accelerator then you need local fast ram too.
 
For big box Amiga's, Zorro 2 cards have to be accessed at 7mhz but Zorro 3 cards have no fixed clock in the specification so they should run quicker.

[psxphill]

That's true, but big box Amiga CPU's have a 25MHz (16Mhz on some 3k's) connection to the motherboard, so that's one limit.

A2000 don't, but yeah A3000 and A4000 have a 50mhz clock on the motherboard. I'd assumed it was used for something else, but it could limit access to 25mhz. There is a clock source jumper, but I don't know what it's for.

[psxphill]

The clock jumpers determine whether the it uses the clocks from the motherboard or the ones on the CPU card.

Yeah, so with a 33mhz CPU card is there still a 25mhz limit?
 
The crystal on the motherboard is either a 32mhz for a 16mhz 68030 or 50mhz for a 25mhz 68030.