Help with cartridge signals an ROM with RAM

[krank]

Hello,
I am new here so hello to all!

I would like to make my own cartridge. I just need to clarify a few signals. So far as I understand if I only want a 2MB rom plus ram then I simply hook everything to the address and data lines (and control) and decode the addresses above 0x200000 to enable the ram and below that address to the rom. Is this correct so far?

Now if I would like to have a 4MB rom and no ram I can simply hook it up to the address and data lines and thats it.
But if I would like to have 4MB rom and also some ram then the memory addresses above (and equal) 0x200000 are overlapping. To switch between the rom or ram the mega drive (or genesis) sets the data D0 to either 1 or 0 and then pulses the /Time signal. Is this correct?

Thanks a lot
Kind regards

 

[DamageX]

That is how existing games do it. For instance, Phantasy Star 4 does a write to $A130F1 (which causes /Time to go low) to switch between ROM and SRAM.

 

[krank]

Thanks for the reply. So I suppose the actual address does not matter as long as it is in the $A13000 to $A130FF space. Is that correct?

One more question to clarify something. When I look at the address space then there is A1 to A23. Taking A0 in account that is a space of 8MB from which only 4MB are used. But there is nothing to decode if the upper bit is 0 or 1. So how does the rom know that the A23 is low or high. This would mean that the rom could collide with anything that had the same address but also A23 is 1. Or am I missing something here?

 

[DamageX]

Yes, anything in $A13000 to $A130FF

A1~A23 allows the cartridge bus to address 16MB of space. A0 is absent because the 68000 normally accesses 16-bit words. (If it needs to write a byte individually it uses /LWR or /UWR.) Cartridges can ignore A22 and A23 though, because the /CE_0 signal is only active for the first 4MB.

So a 4MB cart with SRAM should be using D0~D15, A1~A21, /CAS_0, /CE_0, /LWR, and /TIME

 

[krank]

@DamageX: Thanks a lot for the details.