Theory: procODT, RTT and CAD_BUS

I want to bring special attention to the important terms "procODT" and "RTT", describe what they influence, how to tune them and what they can tell us.

As I mentioned earlier, users were faced with a huge number of problems when the first generation of Zen processors were released. There was a panic in the reviews and there were very few real experts on the forums. Memory could not be overclocked in the literal sense. After some time, the first presets from the respected "Stilt" appeared, they were a miracle for the AMD community, but, nevertheless, secrets and dependencies were not revealed. And today I will close the biggest hole in the documentation and guides for the last two years.

One of the most frequent Ryzen memory questions that's encountered on the forums is "what does memory overclocking depend on?"
In our case, the success of overclocking depends on three components: the motherboard, the IMC (memory controller) and the memory itself.

Motherboard Choices

All boards based on the 3xx chipset have a T-topology and the maximum memory clock is in most cases limited to 3466 MHz while not interfering with CAD. If we manage to configure the CAD, then we can get 3600 MHz. In order to understand why overclocking is limited to such a rather low frequency, we should look into the PCB of the motherboard.


Each signal trace on the PCB is a conductor, a signal line. And each trace can adversely affect other traces. On top of that there is the likelihood of parasitic connections (parasitic inductance and high-frequency interference). To combat negative connections, every motherboard designer must correctly design all the signal traces.


Pictured above is such a design change, which adds a "curve", replacing the straight line, which can dramatically change the capabilities of the signal line.

Also, the form factor, the number of PCB layers and the composition of conductors affect the quality of the motherboard. For boards, a more expensive product is often given more design-time and usually uses higher quality base components.

How to recognize a high-quality motherboard? procODT. And the lower the working procODT, the better the results you can get on this motherboard. Especially for a better understanding, I created several tables for you that can show you the differences.



As a result, we see the colossal difference between motherboards. I consider this one of the main problems of users. And in their choice, I think the reviewers of motherboards are guilty. Over the past two years I have not seen reviews in YouTube, where the topology of the motherboard and its capabilities were reviewed. They look at the box, its content, RGB modes, what the VRM cooling looks like or what a beautiful hairstyle / T-shirt the reviewer is wearing

There are no reviews in which you'll find real calculations on the VRM capabilities. Instead, numbers are used that exist only in the datasheet under ideal conditions at 25°C, with perfect surface mounting. One value multiplied by others.

As a bonus, I have two recommendations for you, what you should pay attention to when buying a motherboard.
1) This is the voltage step for DRAM and the VTT DDR step. There are boards that have a VDRAM step of 0.01 V, and some have 0.005 V. That is, we get 1.35 V, 1.36 V and so on in the first case, in the second case we get 1.35 V, 1.355 V, 1.36 V. In the second case, we will significantly increase the chance to stabilize DRAM, because any excess voltage can cause errors.

2) The DRAM voltage that you set in the BIOS will not always be exact. It may be lower, it may be higher. Sometimes there are situations when VTT DDR doesn't end up being half the DRAM voltage. You will need to adjust other values accordingly. Recall the formula, VTT DDR = 1/2 * vDRAM. Boards that have the step size automatically gain an advantage.

Memory Choices

I often hear on forums "there are samsung b-die there, but they work at a low frequency and with tremendous voltage, this can not be, AMD is to blame." I will explain. The RAM module consists not only of chips from a specific manufacturer, but also of a PCB on which we'll find some signal lines. Capacitors (strapping) and of course chip binning have a huge impact.

For example, we can find RAM from Corsair using Samsung b-die chips, but you won't find records being broken with this product (if you look at the Ryzen platform). In contrast, there is G.Skill Sniper-X 3400c16, which doesn't look special compared to the Corsair modules. However, if we compare the achieved frequencies, then it will turn out to be something like 3200 - 3466 MHz versus 3666 - 3733 MHz in favor of the G-Skill kit. Therefore, when choosing a RAM, I advise you to visit the forums.

Multiple memory chips combined for a single module, which makes it possible that one of the chips can have distinctive frequency-voltage characteristics. Such chips may require a few steps more voltage to stabilize at a certain frequency than its siblings. With this, the other chips could become unstable due to the increased voltage. The ideal option for the user is the purchase of a kit that has a factory overclocking of over 3,600 MHz. This will give you additional assurance that all chips can reach the targeted frequency.

Memory Controller

Both generations of Ryzen are in most cases limited to a memory clock of 1733 - 1766 MHz (DDR-3466 to DDR-3525). Of course, there are instances that can operate at higher frequencies. To make life easier for our memory controller, it is possible to memory modules that are capable of operating at a very low procODT, which significantly alters the signal matching. The working range of the memory controller's procODT, according to AMD, is in the range of 40 - 60 ohms. 68 ohm is already outside the green zone. Stability in this area will strongly depend on the CAD settings and the quality of the motherboard.


The table above shows how procODT / RTT should be changed with frequency

To help understand, imagine the dials of a mechanical watch. procODT will count the hours, RTT_PARK will count the minutes, and CAD will act as the hand for seconds. For each frequency, the dials on the watch will show different results.

To stabilize the system, first we have to check the neighboring RTT / PARK values and only then try to change the procODT. You should not be in a hurry to change the CAD, as there are too many variables and you may lose a lot of time trying to stabilize the system. There are several algorithms for selecting CAD, but at the moment I cannot say with certainty how effective they are. I believe that CAD can not have dramatic differences from the base value 24-24-24-24 and in most cases, one of the values can be jiggled up or down. That is, option 24-30-24-24 may have an additional margin of safety for the frequency of 3466+ MHz.

Keep in mind that each BIOS update can change the working procODT or RTT.

Dependence of Voltage on procODT and RTT

I did a little test in which I used different settings for procODT and DRAM voltage. The idea is to avoid a BSOD during the test.



Based on the results obtained in this simple and short test, we can conclude: after changing procODT, the stable working voltage of DRAM may change. There is also a slight effect of RTT on the working voltage of the memory.

Cold Boot or Double Start

There are no users of Ryzen who have not encountered a cold or double start (sometimes even triple). I can assure you right away that there is nothing wrong with that. This is closely related to memory training. When the system could not start the first time, an algorithm is triggered, which changes some of the settings inaccessible to the user and tries to start the system again. This phenomenon can be influenced by procODT, RTT and CAD.

Sometimes an external clock generates a double start. In any case, try to follow the recommendations of the calculator.