"DRAM DRAM - Oh Tatoo, you're so funny"

The SOCoCo-80 Project

DRAM is a Tough Nut to Crack

Dram operation

Well, it’s been a few weeks since my last post. I haven’t forgotten about the project. I had a few life bumps to deal with. That being said (or typed in this case), I have made some strides with the project. As I mentioned before, this project is going to happen in phases. Phase one was to develop a working SOC using a 6809 model as it’s CPU template. Phase two will be a hardware component for the Color Computer. I’m still on phase one.   The video uses the SDRAM of the DE-1 board. Dealing with the timing between the CPU and the video is not for the faint at heart. I decided to use the Dynamic RAM (DRAM) for the CPU. DRAM unlike SRAM has to be processed by a procedure sequence. This process includes initialization, read access, write access and refresh. There’s also burst functionality and bus width problems to deal with.   In my search for an easy solution nothing I tried fit until I came across a small test module by someone named lsilvest. He/She wrote a three part module that tests the DRAM on the de-1 board. I used one of his/her modules as a driver base. Then I wrote my own interface core. This core seems to work except I’m having some issues with the write cycle. I hope to have those issues cleared up soon.   One thing about using DRAM is that because it’s a “procedural” device, I have to slow the CPU way down to allow the RAM to go through its motions. The DRAM root frequency is 133.3MHz. The controller has to allow for clock cycles to complete tasks. The interface controller is then running at 50MHz to allow the DRAM controller to do it’s thing. Now, to interface with the CPU, I have to run the CPU even slower to allow the interface module to do its thing. I haven’t determined what speed I can run the CPU at yet but it looks like it’s going to be around 12MHz or so. We’ll see. Once I get the CPU running fully with DRAM and after I get the video linked to the CPU, I will return to the DRAM driver and make it work more efficiently. This, hopefully, will allow the CPU to run faster.

© 2013 – Franklin Laboratories