ROG Maximus IX/X Apex Overclocking Guide (Advanced)

The ASUS ROG team prepared a Z270/Z370 Kaby Lake & Coffee Lake Maximus IX/X  Apex overclocking guide for those who wish to get the most out of their K-series processors. The guide was originally published by Elmor on the HWBOT forums and is shared at with explicit permission.

Additional BIOS and tools will be available in the HWBOT forum thread.

Guide Version v1.1

Basic settings

Maximus X Apex overclocking toolsFirst of all, enable LN2 mode

PLL Termination Voltage is needed for high cache frequency and on some chips also high core frequency. First try booting with ~1.5V. If you need higher to enter the OS, you can use Eventual PLL Termination Voltage. Kaby Lake will typically lock up if applying more than 1.85V.

Set Internal PLL voltage to 1.095V if you have problems with lockups when applying ratios. If you get random freezes at high clocks you can try to lower it to 0.9V.

Enable RSVD/CBB Fix switch at -120°C for no CBB (cold boot bug) on most chips.

For most other settings trust Auto values.


LN2 methodology

First try the new RSVD/CBB Fix switch. It will use more aggressive boot voltages which not all CPUs may function with but helps you to POST with Safe Boot even at very low temperatures. LN2 Mode has to be enabled for it to work and will only assert when powering up the system. If used at ambient temperatures it will most likely cause 00 POST code. After it has been enabled DMI/CPU VCCIO/Core PLL Voltages will not apply changes from BIOS.

Start with PLL Bandwidth Level 2 which should give no CB (cold bug) on most chips. If not then increase step by step until you can go down to the lowest temperature.

If your CPU is not working well with the RSVD/CBB Fix switch, you can manually tune the below settings until it works.

  • DMI Boot Voltage and DMI Voltage to 1.85 (range 1.75 to 1.85V)
  • CPU VCCIO Boot Voltage and CPU VCCIO Voltage to 1.4V (range 1.35V to 1.45V, if increasing DMI Boot Voltage you should increase this as well)
  • Core PLL Boot Voltage and Core PLL Voltage to 2.2V (range 1.8V to 2.4V)
  • CPU Standby Boot Voltage to 1.35V (range 1.2V to 1.5V, if too high you’ll get 00 at POST)
  • PLL Bandwidth might need 1 step higher than what gives you no CB in OS.


CMD Rate 1T may cause additional training at 4F post code, just wait.

Tertiary timings needs to be re-tuned compared to Skylake.

You can use DLLBwEn value 2-4 to tune for high memory frequency (4300+)

When using Samsung B-die at high voltage you might get stuck at 22 for 30-60 seconds. After you might additionally get stuck at 04 code for a few seconds. Just wait.

Usually it will be easier to train at a lower DRAM Voltage and use Eventual DRAM Voltage + manual DRAM VTT Voltage for best results.



If you set DLLBwEn, don’t use Adaptive mode for CPU Core Voltage.

If you disable Thermal Monitor, Slow Mode will not function.

DIMM.2 slot is for an external PCB supporting 2x M.2 SSDs.

During certain scenarios it’s possible to get 2x Safe Boot, just save settings again.

Just like on Skylake you need to use ISO2RAM method to install Windows XP and slipstreaming AHCI + USB drivers on Windows 7. Note that Z270 uses updated drivers.

Using IGPU when installing Windows XP will result in black screen during text-based setup.

For initial settings, try one of the suitable Overclocker’s profiles. For memory look under Memory presets and 2x8GB Samsung New B-die.


Safe voltage ranges

Note that the listed values are not guaranteed and only for reference.

Ambient LN2
CPU Core Voltage Up to 1.45V Up to 1.95V
Dram Voltage (1) Up to 2.10V Up to 2.20V
System Agent Voltage Up to 1.40V Up to 1.50V
CPU VCCIO Voltage Up to 1.40V Up to 1.50V
PLL Termination Voltage Up to 1.20V Up to 1.90V
CPU Graphics Voltage Up to 1.40V Up to 1.70V
Core PLL Voltage (2) Up to 1.60V Up to 2.30V

(1) Depends on your DRAM sticks, the limit is considered from CPU IMC side.

(2) If used together with IGPU, the limit is 1.6V.


Common POST codes

Q-Code display Description
06 This post code is normal after first BIOS save + exit, wait for reset
19 If you get 19 boot loop, disable Adaptive mode CPU voltage
1A Indicates Safe Boot is engaged
22 B-die high voltage issue, might have to wait one minute
25 Common CBB code, check Core PLL Voltage
53 DRAM Detection
2b/3d/49/55 DRAM Training
4F DRAM Training, might have to wait a few seconds to pass
71 Internal PLL Voltage might be too high


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