Retromaster’s Electronics Projects

…related to old computers and other assorted stuff…


Posted by retromaster on May 6, 2010

After some intense debugging, I managed to get the SDRAM on the UFE board to work.

First, I replaced the SDRAM chip with another one from the same module. At the same time, I also reworked the PIC32 solder connections. Unfortunately, that wasn’t enough. Then, I decided to make use of the logic analyzer on my oscilloscope. Lacking the proper means to attach the probes to the signal lines, I had to employ a little trick. I soldered very thin enameled magnet wire to the SDRAM pins. I soldered the other ends of the wires to a header, to which I attached the logic analyzer probes. I also had to slow the PIC32 clock quite a bit so that my 40Mhz oscilloscope would be able to show what was going on.

The oscilloscope shot shows the moment when I realised where my mistake was. The upper four signals are the SDRAM control lines, the red one is the clock, and the analog signal is the DQ0 line. It seems that the “read” command was registering with the SDRAM, but I was reading the data lines too early, I needed to wait for another clock edge to read it. Fixing that in the code gave me the right result. What a relief!

Anyway, several bug fixes later, now I am able to read and write the whole SDRAM chip without any apparent problems. I can fill the whole memory with a pseudo-random pattern and I am able to read it back. I haven’t really measured the throughput yet, but I am quite sure that it will be more than enough for my application. The SDRAM is clocked at close to 20Mhz (sysclk/4) and I use a burst size of 8. It could be clocked faster, even at sysclk perhaps with a slightly different design. The throughput would not increase as much though, since there would be more NOP cycles while the PIC32 is preparing data, etc. Anyway, there is even a little room for optimization in my current design. For example, my arrangement of the address lines turned out to be suboptimal, so I plan to fix that in the next PCB revision, to provide a slight increase in throughput.

I plan to test the SDRAM access functionality some more and implement some missing features such as the self refresh mode. Afterwards, my intention is to release source code so that it can provide an example for those who would like to interface SDRAM to a PIC32 (or any other microcontroller without a built-in SDRAM interface for that matter).


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