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CMOS Reset Fix

About

  • Some machines will report a "Boot Self Test Error" when shutting down or rebooting, due to a CMOS Reset triggered by macOS.
  • When using Clover, checking ACPI/DSDT/Fixes/FixRTC will resolve this issue.
  • When using OpenCore, there are 2 Methods available – use either or.

Method 1: Using a Kext and a Kernel Patch

This is the official method suggested by the OpenCore developers.

  • Add RTCMemoryFixup.kext to EFI/OC/Kexts and config.plist
  • Enable Kernel\Patch for "Disable RTC wake scheduling" (copy it over from the Sample.plist included in the OpenCore package if it's missing.)
  • Save and reboot.

The problem should be gone, the next time you reboot or shutdown the system.

Method 2: Fixing the Real Time Clock via SSDT

This approach uses an SSDT hotfix to solve the CMOS reset issue. It adds a fake RTC device for macOS to play with (see SSDT-RTC0 for details).

Instructions

  • Remove the interrupt number from RTC PNP0B00 part _CRS.

    Device (RTC)
{
    Name (_HID, EisaId ("PNP0B00"))
    Name (_CRS, ResourceTemplate ()
    Name (_CRS, ResourceTemplate ()) {
        IO (Decode16,
            0x0070,
            0x0070,
            0x01,
            0x08, /* or 0x02, test to be sure */
            )
        IRQNoFlags () /* Delete this line */
            {8} /* Delete this line */
    })
}

Patch: SSDT-RTC0-NoFlags

  • Disables the original RTC device

  • If RTC does not include the _STA method, disable RTC using the following:

    External(_SB.PCI0.LPCB.RTC, DeviceObj)
Scope (_SB.PCI0.LPCB.RTC)
RTC) {
    Method (_STA, 0, NotSerialized)
    {
        If (_OSI ("Darwin"))
        {
            Return (Zero)
        }
        Else
        {
            Return (0x0F)
        }
    }
}

    • If _STA is present in RTC, use the preset variable method to disable RTC. The variable in the example is STAS and should be used with care about the effect of STAS on other devices and components.

      External (STAS, FieldUnitObj)
Scope (\)
{
    If (_OSI ("Darwin"))
    {
        STAS = 2
    }
}

NOTES

  • The device name and path in the patch must match the one used in your system's DSDT.
  • If the RTC of your systems is disabled by for some reason, a fake RTC is required for it to work properly. In the case that a "Boot self-test error" occurs, remove the interrupt from the SSDT: 
      IRQNoFlags () /* delete this line */
      {8} /* Delete this line */

Thanks @Chic Cheung, @Noctis for all your hard work!