Using SDRAM with a MC68000

Question

I'm currently working on developing myself a computer based on the Motorola 68000 CPU and am currently working on the RAM interface. Since I want a basic multitasking OS, I want to use 1 MB of RAM. SRAM of that size is hideously expensive (around $25) but SDRAM of the same size and speeds exceeding what I need are much cheaper.

The question is this: what would I need to do to make SDRAM work with a MC68000? I know that it needs a refresh clock, so would that clock need to be my system clock (20 MHz) or my bus clock (System Clock/4 = 5 MHz)? What else do I need to look out for if I want to use SDRAM?

Answer 1

Sounds like a very educational project.

Typically, DRAM manufacturers specify that each row must have its storage cell capacitors refreshed every 64 ms or less.

My understanding is that you can use any one of the following 4 ways to keep the DRAM refreshed:

(a) My understanding is that all SDRAM has an internal on-chip timer that automatically refreshes the SDRAM when the SDRAM is placed in self-refresh mode and the clock to the SRDRAM is stopped. My understanding is that most simple systems don't bother with self-refresh mode, and instead use one of the other methods of refreshing DRAM:

(b) Some systems have special refresh hardware that periodically pauses the CPU, performs a refresh, then resumes the CPU.

(c) A few systems have a special "refresh interrupt" -- a timer signal periodically triggers a hardware interrupt, and the software in the interrupt handler does a refresh, and returns. (Some systems interrupt once every 64 ms, and the interrupt handler reads N bytes -- one byte from every DRAM row -- refreshing all the DRAM in one whack, then returns. Other systems interrupt once every 64/N ms, increment a row counter and read one byte from that DRAM row, then return). The "refresh interrupt" approach requires the least amount of hardware. Alas, a "refresh interrupt" has the drawback that minor bugs in the refresh interrupt software, or bugs in any other software that delays the refresh interrupt too long, cause weird difficult-to-reproduce problems elsewhere when memory becomes corrupted.

(d) Many early computer systems had special DMA video hardware that pauses the CPU, reads video data from the DRAM, and sends it to the video hardware. Many of them are set up such that the process of reading out all the video data, as a side effect, also reads at least 1 byte from every row of DRAM, indirectly refreshing all DRAM.

p.s.: Are you actually using a (effectively) 32-bit 68000, like the original Macintosh and Palm Pilot? If so, you may find it useful to check out the Minimig project, which uses a 68000 (and lots of SRAM), and the FPGA "soft cores" that execute the 68000 instruction set.

Or are you actually using an 8-bit Motorola 6800? If so, I highly recommend you check out the N8VEM Home Brew Computer Project.

Answer 2

The book 68000 Hardware Manual will show you how to build a basic DRAM interface for "oldschool" DRAM, which you should be able to find dirt cheap and fast enough that you have 0 wait states.

On the other hand, US$25 might be well worth the design convenience of just using SRAM—and it looks to me like, given you don't need very fast SRAM to keep up with a 68000, that you could probably find more than 4MB for quite a bit cheaper. (Even if you need to use a latch or something to interface a slow CPU with very fast modern SRAM.)

Oh, and if you can find one, try to use a 68010 instead of a 68000. It's electrically compatible but the supervisor mode has a couple bug fixes for bus error recovery, so it can implement virtualization.

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EDIT: Some of the links on the page linked above are now broken. This is an alternate link to the same document: http://bitsavers.informatik.uni-stuttgart.de/pdf/peripheralTechnology/68000%20Hardware%20Manual.pdf

Answer 3

One answer missing from this ancient question is that you will have to look for a long time if you want to find SDRAM that can run on the 5 volts that you want to run your 68000 CPU at. You may find that adding the bi-directional level converter to your system adds too much cost and complexity to the project.