Demystifcation of PC100 SDRAM
memory modules (Part 1).

Preface

Since PC100 SDRAM memory modules are manufactured there has been more and more peoples complaining about how difficult it is to differentiate a real PC100 memory module. The most frequent problems encountered are: the inability to distinguish a PC100 memory module from a non PC100 memory module, the inability to clearly identify the working speed and CAS latency of a PC100 memory module and unreliable operation with some PC100 memory modules. All these questions will be addressed and answered in the course of this document to the best of our knowledge.

Introduction

Before PC100 memory modules were manufactured and even with early PC100 memory modules it was very frequent to get mismatch between two different memory modules resulting in computer crashes and this was happening even with memory modules of the same brand same model. The problem was the lack of standardized specifications, non uniformized manufacturing process and uncontrolled manufacturing tolerances between the different manufacturers.

Some of the reasons why there have been some functional problems with 100MHz memory are summarized by the two following possibilities:

1- Memory manufacturers not following Intel certification process. This certification process has been put in place especially to allow memory manufacturers to provide parts for compatibility testing. However, this certification test is not obligatory so it is left at the own manufacturer discretion to use it or not. Besides, providing parts for this test doesn't automatically signify that all the parts manufacturer by a specific supplier will meet the compatibility test as there are variations in the parts manufactured by a supplier and it is not all the parts of a manufacturer that will meet the same specs even if they were manufactured in the same day. You must bear in mind that the PC100 certification specifications are very difficult to achieve, and many manufacturers are still having difficulty in meeting all associated specifications.

2 - System manufacturers not providing robust environments for the PC100 memory. The system environment, which is including the motherboard, the memory connectors and the Chipset, has a significant impact on the overall robustness of the memory subsystem. While some manufacturers have done extensive design simulation and timing analysis to present a much more stable environment, with good timing margins, some others have neglected this in their design to cut R&D costs and therefore they cannot offer a robust environment to the memory in use.

In other words, you always get what you paid for, so this is sometimes what it means when a motherboard is less expensive than a similar one. Purchasing a well-designed motherboard, and memory from a reputable supplier is always a good insurance to reduce risk even if this is not a guarantee either. The problem is not in the memory alone, but as memory manufacturers will learn from their own design and production difficulties we can hope to see within the next year more memory subsystem independent memory modules to emerge where the internal performance of the memory modules will be much less affected by their environment.

Another part of the problem is that the higher the frequency you goes the worst the problems you are facing with. For instance, at frequencies below 30Mhz (defined as the High Frequency "HF" of the radio spectrum ) , the design rules are very well controlled and not very critical. From a certain point of view a length of wire below 30Mhz can be considered as a straight length of wire. However, at frequencies of 100Mhz and above (defined as the Very High Frequency "VHF" of the radio spectrum) , a length of wire can no longer be considered as a straight wire but as a resonating circuit including capacitance, inductance with input and output impedances. According to this, every length of trace on a printed circuit has a resonating frequency and all its residual radio frequency components must be calculated and taken into account to get a well balanced circuit.

To develop this type of memory, a set of specifications has been developed by Intel and was endorsed by most of the memory manufacturers. Intel established a very precise set of specifications and guide lines to ensure compatibility between memory modules of any brands. The Intel PC100 compliance specifications are ensuring robust memory operation from suppliers that meet these specifications and this is a great benefit to both the industry and the end users. In addition to Intel providing specs for PC100 devices and DIMMs, Intel has released module gerber (raw card) design files. Vendors using these raw card design files will have much more consistency than those using their own raw card design files.

Continued on next page (Specifications Information)