DDR4 Sdram – Initialization, Training and Calibration (systemverilog.io)

From my understanding, memory training is/was a closely held secret of memory makers and EDA IP houses who sold memory controller IP to all the chip vendors. This in turn makes fully open motherboard firmware almost impossible as no one can write code for memory training to bring up the chip. That piece of code has to be loaded as a blob - if you can get the blob.

Wow, what goes on in a RAM module is so much more complex than I thought.

Implementing DDR3 training for our packet queuing chip (custom memory controller) was my first project at work. We had originally hoped to use the same training params for all parts. That wasn't reliable even over a small number of testing systems in the chamber. DDR3 RAM parts were super cheap compared to what we had used in previous generations, and you get what you pay for with a huge amount of device variation. So we implemented a relatively long training process to be run on each device during our board testing, and saved those per-lane skews. But we found the effects of temperature, and particularly system noise, were too great once the system was sending full-rate traffic. (The training had to be done one interface at a time, with pedestrian data-rates). We then ended up with a quick re-training pass to re-center the eyes. It still wasn't perfect - slower ram chips (with smaller eyes) would report ECC correctables when all interfaces were doing worst-case patterns at temperature extremes. We spent a lot of time making those interfaces robust, and ended up relying more on ECC than we had intended. But those chips have been shipping ever since and will have seen traffic from most of us.

I did some RE'ing of BIOS code back in the days of the first SDR SDRAM and the calibration part was reasonably straightforward; basically sweeping some controller registers through their ranges while doing repeated read/write operations with lots of transitions in the data (e.g. 0x55555555 <> 0xAAAAAAAA) to find the boundaries where errors occurred, and then choosing the middle of the range.

While the article does mention periodic calibration, I wonder if there are controllers which will automatically and continuously adapt the signal to keep the eye centered, like a PLL.

Does anybody else's fingers also tingle like this is written by an AI?

The formatting is strangely inconsistent, highlighting only some numbers and some variables in fixed-width font. Also there's odd statements like that the reference resistor keeps its value "at all temperatures", which is just not true. Other phrases like "poly-silicon resistor" are highlighted, and then not explained. All in all, I find this article to be quite a mess and not a clear explanation.