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Antec ISK 110 with 3400G using cTDP

23 Jun 2021 - by 'Maurits van der Schee'

I've built another PC. This time I wanted to create a silent but powerful small form factor PC. It aims to replace my AMD 3400G midi tower with something equally powerful and quiet that fits my desk. I've chosen an ASRock Fatal1ty B450 Gaming-ITX/ac motherboard with a on-board WiFi 5 (802.11ac) in an Antec ISK110 VESA-U3 small form factor case. I transferred the AMD 3400G quad core, the 2x16 GB of G.Skill Ripjaws 3600 a the 2 TB ADATA XPG SX8200 Pro NVMe drive. It is a small, pretty, super fast and not too noisy little box. The case costs 91 euros and has a built-in 90W power supply and comes with an IEC-C5 (also known as "clover") power cable included. Unfortunately the cable I received was UK type instead of EU type.

Antec ISK110 VESA-U3

Note that I run Xubuntu 20.04 LTS and it works great, I'm very pleased with the results!

List of materials

The materials used in this build:

 91 Antec ISK110 VESA-U3 90W PSU SFF case
169 AMD 3400G CPU with integrated graphics
101 ASRock Fatal1ty B450 Gaming-ITX/ac motherboard
191 G.Skill Ripjaws DDR4 DIMM 2x16GB 3600
240 ADATA XPG SX8200 Pro 2TB M.2 Nvme SSD
 41 Noctua NH-L9a-AM4 CPU fan
--- +
833 Total

Due to some price increases this build may have been 10-15% cheaper a few months ago.

Is 90 watt enough for the 3400G?

No, it is not. The CPU has a TDP of 65 Watt and although the machine boots, it will reboot under heavy load (e.g. instant reboot when running stress -c 8). I measured peaks near 90 watt (on the socket) under normal usage. I want my machines to be rock solid, so I was looking for a solution.

The CPU's TDP of the 65 Watt is not the maximum draw from the CPU. TDP stands for Thermal Design Power, and it is the heat a component is expected to output when under normal load. You should count with roughly 1.5 times the TDP rating to calculate the expected power draw under full load. For our system the CPU with TDP of 65 Watt has an expeced power draw under full load of 65 x 1.5 = 97.5 Watt which explains the reboots.

cTDP to the rescue

Configurable TDP (cTDP) adjust the behavior of the CPU, so that it consumes less power. When you have a configured TDP the CPU slows down (throttles) automatically to match this configured TDP value. This is typically used in very thin devices with low cooling capabilities or when a quite mode is preferred over the maximum performance mode.

The ASRock B450-ITX motherboard has a BIOS setting where you can set the cTDP. Unfortunately it is not called "cTDP" which makes it hard to find. Setting cTDP to 35 Watt gives an expected 52.5 Watt under full load, which in practice was around 64 Watt on the socket. This is well within the limits of the power supply and the machine didn't get very hot, nor loud, nor did it suddenly reboot. Great results so far!

Setting the cTPD BIOS setting to 35 watt on a ASRock Fatal1ty B450 Gaming-ITX/ac motherboard

You need to go to "Advanced > AMD Overclocking" and set "Precision Boost Overdrive" to "Advanced", "PBO Limits" to "Manual" and "PPT Limit [W]" to "35" to set the cTDP of the AMD CPU to 35 watt.

cTDP and performance impact

One would expect half the performance at half the TDP, but that's is not how this works. Normal peak usage requires only 66% (deduced from the 1.5 rule) of the machine's TDP. Therefor normal load tasks lose only 16% of performance at half the TDP (or 12% at 35 instead of 65 Watt). Light tasks are not affected at all. This is also visible in real world scenario like gaming, as you can see here:

The cTDP does not affect the game performance much on an AMD 3400G (source)

As you can see game performance is hardly affected by a lower configured TDP. Still "PPT Limit" may not be exactly the same as cTDP, so I my advice is to experiment a bit with this variable and measure the power usage (in watt) on the wall socket to get optimal results. When I set the "PPT Limit" to 50 the power draw on the wall socket stays safely under 90 watt under full load (around 88 watt).

RAM runs slow at 2133 MHz

I also noted that the machine didn't want to load the rated 3600 speed of the GSkill DDR4 RAM using XMP 2.0 in AUTO mode and feel back to 2133 MHz. When choosing the 3200 profile it did run at 3200 MHz.

cTDP conclusions

Configurable TDP (cTDP or PPT Limit) is a great tool to lower the power draw of your system with integrated graphics. It influences real world performance less than one would expect, allowing for a quiet PC with a smaller form factor using the same powerful components.

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