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Why do some SDRAMs work and some don't?

Two 1GB SODIMMS A while ago I was trying to track down the answer to "Why does this 512MB SODIMM just appear as 256MB?" without any success. There are plenty of theories out there, some total rubbish, some semi-rubbish, some cogently argued, but none I believed. Unfortunately, for reasons given below, this article also falls into the "My best guess" category. What has prompted this article was an ebay purchase where the supplier messed up totally and instead of sending me the two 1GB SODIMMS I'd ordered, he sent me two 1GB Micron SODIMMS which had different part numbers. When I plugged them in in turn into a Neoware CA19 one appeared as 1GB and one as 512MB.

A little history

Way back when you could expand the amount of memory in your PC by buying specific integrated circuits and pushing them into the empty IC sockets on the motherboard. Alternatively you bought a memory board and stuffed it into a socket on the backplane of your system. Over time this changed towards the idea of the SIMM (Single-In-Line-Module) where the 8 or 16 ICs you needed were mounted on a small circuit board that plugged straight into the motherboard which already had the necessary memory controller and interface circuits.

The SIMMs and technology evolved and in order to help the motherboard keep track of exactly what was being plugged in PPD (Parallel Presence Detect) was introduced. There were also timing parameters that had to set by hand in the BIOS.

When the SDRAM PC100 standard was introduced it also included SPD - Serial Presence Detect. This was an onboard 128 byte EEPROM that included a wealth of information about the SDRAM which enabled the BIOS to automatically configure the memory controller for it without user intervention. The SPD specification has continued to evolve with each new generation of memory type. If you are interested in what is in the SPD EEPROM then a good summary is in this Wikipedia article.

SDRAM Memory Organisation

This is really a complex topic which I'll over simplify.

The basic building block at the heart of the memory chip is a matrix of memory cells. They are accessed by a sequential operation of selecting a particular row and then a particular column. The row address and the column address are sent over the same address bus. Multiplexing the address in this way reduced the pin count on the IC. Also it reflects the way that the matrix is organised in that an entire row is read at once and then the column element selects just one or more bits of the row to route to the output pin(s). In the olden days the memory chips were organised x1 - they delivered a single bit of information on each read. i.e. If your memory bus was 16 bits wide you would need to use 16 ICs. Over time the IC's data width has increased with x4, x8 and x16 devices and the external data bus has migrated to 64-bit wide.

You can obviously increase the capacity of a (SO)DIMM by putting more ICs on it. This introduces the concept of having "banks" of memory - you now also have to select which "bank" of ICs you are accessing. As chip densities increased, as well increasing the data width, the chips can also have internal banks.

For example the PC133 SDRAM unbuffered SO-DIMM standard specifies two reference designs which can be populated by various types of x16 SDRAMs:

Raw
Card
Version
SO-DIMM
Capacity
SO-DIMM
Organization
SDRAM
Density
SDRAM
Organization
# of
SDRAMs
SDRAM
Package Type
# of
Physical
Banks
# of Banks
in SDRAM
# Address bits
row/col/banks
A32 MB4MX6464 Mb4M X 16454 lead TSOP1412/8/2
B64 MB8Mx6464 Mb4M x 16854 lead TSOP2412/8/2
A64 MB8Mx64128 Mb8M x 16454 lead TSOP1412/9/2
B128 MB16Mx64128 Mb8M x 16854 lead TSOP2412/9/2
A128 MB16Mx64256 Mb16M x 16454 lead TSOP1413/9/2
B256 MB32Mx64256 Mb16M x 16854 lead TSOP2413/9/2
A256 MB32Mx64512 Mb32M x 16454 lead TSOP1413/10/2
B512 MB64Mx64512 Mb32M x 16854 lead TSOP2413/10/2

One would hope that any PC133 compatible memory controller would support these reference designs.

The various SO-DIMM specifications are downloadable from the Jedec site but you have to register first.

Accessing Memory

Memory access is done via the Memory Controller. This essentially takes the linear address from the CPU and translates it into the complex series of signals necessary to access a specific set of memory cells on the chips on the SODIMM. Crudely, whatever the device, the linear address breaks down into a row, column and bank. In order for it to do this properly it has to be configured by the BIOS based on the information it reads out from the SPD(s) on the various memory sticks. All the SDRAM/DDR/DDR2 etc bit does is alter the complexity of the clocking as each new standard is designed to get the selected data out of the (SO)DIMM more quickly. (cf DDR standing for Double Data Rate).

Limitation

The fundamental limitation is the memory controller. What is it capable of doing? Another question is has the BIOS initialised it properly? I would sort-of assume that the two are reasonably matched - if the memory controller can support X then the BIOS will set it up correctly. If it can't support X then what does the BIOS do? In some cases it sits there complaining loudly - one long endless beep!

Density

In checking things out on the web you'll often find mention of 'high density' and 'low density' parts. I don't know if there is any formal definition of these terms but my take is that it is referring to the memory capacity of the chip used, not how many they've squeezed onto the SODIMM. As a result it's a little counter intuitive - a SODIMM packed with ICs may well be a 'low density' part.

The high density parts are likely to be those that appear towards the end of a particular standard's lifecycle as memory technology has improved and higher capacity chips become available.

Checking things out

So, if we assume the manufacturers haven't limited things further, what we end up with is really quite simple:

This is where I grind to a halt. A lot of thin clients use VIA processors and chip sets. VIA do NOT publish their datasheets. They are available under NDA but I get the impression that even then they are not easy to get hold of. You really have to be in the OEM volume market to get them. I also haven't found one for the SiS Northbridge used in the Nec D380. Ho-hum.

VIA CLE266

Having just said that the datasheets are not available, the one for the CLE266 has escaped into the wild!

Some key points from it:

It does state that "x4 DRAMs supported by SDR only"

The following table shows the Memory Address Map type encoding:

codeChip sizeColumn addresses
000 16Mb8-bit, 9-bit, 10-bit
001 64/128Mb8-bit
010 64/128Mb9-bit
011 64/128Mb10/11-bit
100 -reserved-
101 256Mb8-bit
110 256Mb9-bit
111 256Mb10/11-bit

Depending on the chip size and organisation (x4,x8,x16) they have page sizes of 2k/4k/8k.

So far I haven't had a reasonable set of working/not quite working SO-DIMMs to check out a CLE266 equipped thin client.

VIA CN700

According VIA's website the CN700 memory controller supports DDR2 533/400 or DDR400/333/266.

The above having been said we now enter the realm of speculation as we have no datasheet.....

Two different 1GB SODIMMs Having received two Micron 1GB SODIMMS - one organised 2Rx8 (8 chips per side) and one 2Rx16 (4 chips per side) - I tried them out in a Neoware CA19.

If we look at the respective datasheets we find the following basic information:

ParameterMT16HT...MT8HT...
Refresh Count8K8K
Row address16K A[13:0]8K A[12:0]
Device bank address4 BA[1:0]8 BA[2:0]
Device configuration512Mb (64 Meg x 8)1Gb (64 Meg x 16)
Column address1K A[9:0]1K A[9:0]
Module rank address2 S#[1:0]2 S#[1:0]

The three factors that stand out here are:

  1. The MT8 part uses 1Gbit devices organised as 64Mb x 16 whilst the MT16 part uses 512Mbit devices organised as 64Mb x 8.
  2. The MT8 part has a 13-bit wide row address whilst MT16 part has a 14-bit row address
  3. The MT8 part is split into 4 banks whilst the MT16 part is split into 8 banks.

On the face it (1) shouldn't matter. The SODIMM taken as a whole still delivers 64 bits of data.

I assume (2) and (3) are a consequence of (1). I would guess the significant parameter is (3), but in the absence of the CN700 datasheet it's difficult to know if that is the reason. However Google does produce a few hits of "The CN700 does not support (SO)DIMMs with 1Gbit devices."

Checking the DDR2 specification from the Jedec site you can see that once the individual chip size is 1Gb or greater the number of banks increases from 4 to 8.

So my best guess is that the CN700 will only support 4 memory banks. If you are buying a 1GB memory stick for a CN700 based device, avoid any that have only 8 chips on board. That should improve the odds of what you buy actually delivering its stated capacity.

 


Any comments? email me.    Last update July 2011