16BIT ROM CART THOUGHTS

[keli]

So as peter says, we can use LDS OR UDS to switch the ROM's chip enable lines low.. but the question is, does that break anything...

My question was that I don't think it would matter, as the CPU will only read high or low byte from the bus anyway, so if we put 16bit data on the bus, it shouldn't matter... This is what I really want to confirm before I start building a 16bit ROM cart...

From my playing around with FPGAs and the ST bus, I have found out that for reads, always putting 16 bits of data on the bus when either /LDS or /UDS are active low works perfectly fine. It's only when writing you have to be careful only to grab the correct bits, as the CPU will only be driving the corresponding half of the data bus. This is of course irrelevant for a ROM as you will only ever see read cycles. (The GLUE will produce a bus error if the CPU tries to write to the address space assigned to the cartridge port, and the cartridge port does not have a /WE pin anyway.)

[exxos]

From my playing around with FPGAs and the ST bus, I have found out that for reads, always putting 16 bits of data on the bus when either /LDS or /UDS are active low works perfectly fine. It's only when writing you have to be careful only to grab the correct bits, as the CPU will only be driving the corresponding half of the data bus. This is of course irrelevant for a ROM as you will only ever see read cycles. (The GLUE will produce a bus error if the CPU tries to write to the address space assigned to the cartridge port, and the cartridge port does not have a /WE pin anyway.)

Yeah I think it should be ok.

I was going to just use LDS, but that would limit to 16bit only reads to the cartridge.. I don't know what Arne did, his diagram shows 2x 8bit roms. So if he ignored UDS, then parts of the diagnostic cart may not work as it wouldn't be able to do 8bit reads

[Petari]

If you check some ST schematic, will see that Hi and Lo ROM chip /CE pins are connected to same line, so no separated UDS and LDS activation.

[exxos]

If you check some ST schematic, will see that Hi and Lo ROM chip /CE pins are connected to same line, so no separated UDS and LDS activation.

I was going by the diagram at start of thread, it uses LDS & UDS.

Also same here.. http://dev-docs.exxoshost.co.uk/Test_Cartridge.pdf

[Petari]

If you check some ST schematic, will see that Hi and Lo ROM chip /CE pins are connected to same line, so no separated UDS and LDS activation.

I was going by the diagram at start of thread, it uses LDS & UDS.

Also same here.. http://dev-docs.exxoshost.co.uk/Test_Cartridge.pdf

My point is that if it is OK with ST ROMs, will be OK with cartridge ROM too .

[exxos]

I've redone the schematic for the 16bit PLCC ROM...

I tried to autorouter on 2 layers.. Eagle gave up at 40% and basically went nuts

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It would likely have to be done on 4 layers which just makes it very time consuming to do

It's got to the point where I just don't have the patience anymore to sit puzzling for hours over routing pcbs.. I've already spent ages trying to route previous designs, so i'm not really motivated anymore to spend more time on this.

So If anyone wants to have a crack at routing it...

cartridge2.zip

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[rpineau]

Challenge accepted :

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[exxos]

Challenge accepted :

Routing pcbs is making me

[exxos]

Been fiddling around with Rodolphe's layout a bit..

Mostly added poly fill for 0V top and bottom layers. Manually routed some 5V and 0V traces. I think its pretty much ready to go off to fab for a prototype run..

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[exxos]

Test PCBs arrived... Next to find some time to build and test...

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