DREAMING: Breadboard, DDR socket and an mBed

[nyteschayde]

So I have been drooling over all the ideas of things to build while pouring over sites like sparkfun.com, mbed.org, and others. One thing I was thinking might be fun is to somehow get a SIMM or DIMM socket, plug it into a breadboard with some RAM and then write some sort of (magic) interface on the mbed microcontroller to make a RAM card for an A1200.

The things I think I'd need are

• Some sort of software memory controller (not sure how complicated this is at all; any ideas would be great. Had thought about looking at WinUAE sources for how memory was controlled on the Amiga)
• Some code on the mbed to initialize the RAM in AutoConfig
• More understanding on the pin outs of the trap door access bus
• And actually doing it of course.

I've also been wondering how hardware accelerators actually work. Somehow they need to tell the Amiga to use whatever is on their card vs. the onboard CPU.

• Is it as simple as the ROM remap? (i.e. if rom is located at address b, then in memory set the new address to address c?)
• Early A500 CPU upgrades literally took out the 68000 and put in a 68010 or 68020 into the same slot. I know it's totally naive to assume this but how are the pins on the motherboard that connect to motherboard CPU rerouted to pins on the CPU on an accelerator board over the trapdor expansion bus? It seems like it would have to be that those pins map to some registers of the motherboard CPU or something and somehow those registers are rerouted to the registers of the CPU on the accelerator board.

Obviously I understand only bits and pieces here. It would be nice if anybody with excess amounts of time and boredom who actually knows this stuff could explain some (or all of it) to me.


Thanks

[nyteschayde]

Wow, both of you seem to know a lot more than me. I'd love to sit down for an hour, work on something simple with either of you and pick your brains and make notes. Given I'm in California, though that will likely not happen.

[nyteschayde]

Regarding the number of I/Os on the microcontroller (of any type), do they have to match 1:1 to each pinout on the CPU or do they only need to communicate with the pinouts on the trapdoor bus edge connector (and are those the same number of pins as the CPU?).

Also, why do they need to match? I was of the (likely misguided) impression that by the time the data reached the bus edge connector we'd be going through some software layer already and wouldn't need quite soo many GPIOs. Look at me, using acronyms like GPIO.

Can you recommend a getting started guide for things like resistors, capacitors, diodes, etc...? My only understanding of them comes from the world of car audio installation and does not directly relate to all the things needed in the world of microcontroller hackery.

Final question in this reply, do you think SDCard speeds are fast enough for a stock A1200 SLOW RAM? I mean the CPU is running at what? 14MHz? Maybe I have a totally misguided understanding of BUS speeds on the stock A1200.

[nyteschayde]

All I've ever heard was 3.3v or 5v. What is the new "common" voltage for such things if even 3.3v is getting old?