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 controll

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.