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Testing Amiga DRAM

A friend of mine has an Amiga A500. As usual, the machine was kept in a storage until its retro value was significant, and during those years of storage the battery of the “memory & RTC” expansion had leaked and grown a thick fur on the PCB.

The module didn’t work even after cleaning up the mess. I told I could take a look at it, and sure enough I soon had the module in my hands. As I don’t have an A500 of my own, I decided that the best way to troubleshoot would be to talk to it through the connector, and so I built a testbench out of an Olimex LPC1114 board. The microcontroller has 5V tolerant GPIO pins, which is a bonus.

Amiga DRAM test circuit

It turned out that the microcontroller was quite slow with its bit arithmetics and the debugger on so that the timing didn’t become an issue. After a few hours of debugging with my logic analyzer and fixing a few bad pin choices (eg. mapping an address line to a SWD line) I was finally able to write to each location of one of the chips and read it all back. After repeating it for the other chips I found out that one of the chips was faulty, and the board shall return to its rightful owner after I get a new IC from an eBay surplus store.

Of course, it would have been more elegant to program this thing with Assembly and carefully take note of the timings, but I wrote it in C and made sure that the cycles are not too long. After all, the performance of this device doesn’t matter as testing the chip even with my unoptimized code┬átakes only a few seconds.

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