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PART 1: WHATıS IT ALL ABOUT?
by George Martin

Start ı Narrowing Specs ı Selecting the CPU ı Packaging ı The Software ı Memory ı Your Job ı Sources and PDF

MEMORY

Letıs move ahead a bit and look at devices for the DRAM memory. An off-the-shelf DRAM module only requires a mating connector on our circuit board. This would benefit us in two ways. First, the memory design and packaging is done. Secondly, we could leverage the quantities of memory through the PC industry to keep the cost per megabyte down.

Thatıs both good and bad news. The PC industry is moving so fast that we wonıt be able to buy these parts long term. Try to buy a five-year-old DRAM module. Nobody wants to know you. I believe we need to purchase DRAM devices and do the assembly ourselves. We could then assemble our devices onto modules.

The DRAM industry has just gone through a change. The 1 MB ı 16 devices that were the mainstay of the PC industry have been replaced by the next larger sizes. Mainline manufacturers, such as Micron, have dropped the 1 MB x 16 from their inventory. But other manufacturers, such as ISSI have picked up the product line.

From ISSI, you can get 1 MB ı 16 DRAM with extended data out (EDO) page-mode access, in either 50- or 60-ns access time. Packaging is 42-pin SOJ and operating voltage is 3.3 V. It comes in both commercial (0ı70ıC) and industrial (ı40ı85ıC) operating temperatures. Power consumption is 0.5-mA standby current and 160-mA operating current.

Iım mentioning this device as just one example of a DRAM that suits our needs. It meets our design goals and four of these devices provide 8 MB of DRAM. To understate the situation, there are a number of manufacturers offering embeddable DRAM devices. You need to match their features with your requirements.

A lot of design tradeoffs go into the DRAM selectionıtoo many to mention here. If your particular design requirements donıt match up with this selection, I suggest that you start by finding the DRAM you need because so much of your system design depends on it.

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