DDR4 SDRAM – Initialization, Training and Calibration
systemverilog.io
systemverilog.io
> if you program the CAS Write Latency to 9, once the ASIC/uP launches the Column Address, it will need to launch the different data bits at different times so that they all arrive at the DRAMs at a CWL of 9.
and:
> there could be changes in Voltage and Temperature during its course of operation. To keep the signal integrity and data access reliable, some of the parameters that were trained during initialization and read/write training have to be re-run.
So now the chips have to dynamically tune the circuity behind each pin to suite the length of each trace it is connected to, and it's temperature.
Keep those fingers well away from the board during operation. In fact I imagine a cockroach strolling across the motherboard looking for a comfortable spot at the right temperature could well wreck havoc.
Value every bit of RAM you have, as it was won in an uneven battle against entropy, and laws of nature.
I think I wrote before that in my teenage years, I thought of studying for a process engineer. After living in Singapore, and meeting 2 retired TSMC senior engineers there, I decided against that.
I had kind of a mentee-mentor relationship with them. Their told me of horrors of "studying for a PhD for a coffee porter job," and me having to be ready to endure that for many years to have a remotest chance to get into semi RnD.
The industry was just too competitive, and market dynamics are keeping to get more and more adverse against newcomers with each passing year.
Each new generation of FABs is getting more and more automated, which means more and more experienced process specialists are being thrown on the job market with each new fab closure, and each of them wanting to go from FAB to RnD.
They themselves said their decision to quit was thanks to that, and it happened many years prior to them migrating to Singapore, and that it would've been even worse at the time they were mentoring me.
Since there is some curiosity around temperature and voltage variation - here are some more details for you folks to geek out on.
When you build a system with a DRAM interface, you typically specify 2 parameters - A temperature range you guarantee its operation within. For example, this range could be 0C-80C. - Maximum rate of change of temperature your system can handle. Example, +/-2C/min.
Now, to test if the system can withstand the above 2 parameters, while the firmware is being developed it is put in a Thermal Chamber and experiments such this are conducted: - Do a cold soak for a few hours (i.e., power down the system and leave it in a 0C chamber for a few hours). - Then power on the system and let the DRAM interface calibrate at this low temp - Then start a stress test which reads and writes to the memory, and simultaneously ramp up the temperature of the chamber at the specified rate upto your maximum (2C/min upto 80C in our example here) - If the test fails, it typically means the signal integrity is not good enough. Then you go back to the lab, probe the DRAM interface and observe the signals on an oscilloscope (if you have to). Then re-calculate/fiddle around with 6 parameters until you have it all working. These parameters are 1. The drive strength of transistors at the Processor when its writing data to memory 2. The termination resistance of the transistors at the Processor when it is Reading data back from memory 3. Voltage reference (Vref) - This is the value the PHY[++] uses to decide if a voltage level is a binary-0 or 1 4. The set of 3 parameters above exist on the DRAMs as well. Making it a total of 6.
It is easy to imagine what drive strength and termination of transistors mean. But Vref is a bit more interesting.
In DDR4, binary-1 is represented by a 1.2V signal, but binary-0 is a floating voltage value. It could be 0.2V or 0.4V, or whatever. It depends on the termination at either end of the PCB trace. This type of a circuit is called POD (Pseudo Open Drain). Since the level of binary-0 is variable, the DDR controller calibration logic has to figure out where to place Vref so it can reliably decode 1s and 0s.
Lastly, just like the cold soak experiment, we also do hot-soaks with a ramp down and several other modalities to ensure the system is solid.
The PHY has delay registers within it which you can read to figure out the result of calibration. When you power on a system after a cold-soak vs a hot-soak, you'll see different values in these delay registers.
PHYs these days are very robust. They typically don't need periodic calibration (re-tuning of delay registers) while operating in a typical data center environment. Of course, it's a different story if the system if sitting somewhere off on an oil rig.
— [++] The PHY is separate from the DDR controller. This is the actual analog circuits at the edge of the processor sending out and receiving signals on the PCB.
Wow, the cold/warm soaks sound like they make for a very slow iterative process when it doesn't work on the first try. Do you have several systems soaked at the same time? So if the first you test fails, you can adjust something and test on a second setup while the first is re-soaked?
I also thought much of the difference in length on the PCB is compensated by with those wiggly traces (so all have equal-ish length), but you still need to compensate for it? Or is it just to gain a larger error margin?
Thermal chambers are quite expensive, around $100,000 per unit. So bigger shops such as Intel, AMD, Qualcomm probably have many. But I would be surprised if smaller companies have more than a couple.
It is a painful process when a company develops their first system. As you would guess, once they have a proven PCB design with DDR controller firmware, the DDR sub-system design is reused in subsequent systems.
Now say you've been shipping the system for a couple of years. There is one situation under which the above experiments will need to be performed again.
Say your system uses a 16GB DIMM. Micron and Samsung, the DIMM makers, are always trying to improve their manufacturing process, moving to the next node (14nm to 7nm) and so on. So every couple of years you'll find them EOL-ing (End of life) a certain 16GB DIMM for a newer one. There is a chance you'll start seeing failures with the new 16GB DIMM.
> I also thought much of the difference in length on the PCB is compensated by with those wiggly traces (so all have equal-ish length), but you still need to compensate for it? Or is it just to gain a larger error margin?
You are partially correct.
Check out this image: https://www.systemverilog.io/ddr4-initialization-and-calibra...
PCB Board designers match the length of the data lines, which are hooked up in a star-topology from the processor to different DRAMs on the DIMM.
But the address lines are hooked up using fly-by topology. So data signals launched from the processor arrive at all the DRAMs at the same time. But the clocks and address signals that are launched from the processor will reach each DRAM on the DIMM at different times. So, initial calibration compensates for this.
In some applications, it is not enough. Number one example is if you have your PC in an open air environment, and you have significant humidity changes. In such cases, PCB trace capacitance can change enough to lead to a crash.
How one re-runs training when the chip is online is a very big thing in mobile SoC world, and this is why link training programs are often shipped encrypted by some SoC vendors.
Yes, before signing-off on a system, we carefully look at the calibration result and see if there is enough margin.
Now what do I mean by margin? - The delay registers hold some sort of an integral value and there is a certain range to it, for example 0 to 7. You typically don't want the result to be at the ends. Because you want to give periodic calibration (which could run due to big temperature swings) a little margin.
If the calibration result does end up at the ends of the range, we have to play around with other DRAM timing parameters, which are managed by the DDR controller.
Then what about a PC? Might my workstation become unstable if I don't restart it if the room temperature changes?
It's amazing that anything works at all.