High Precision Event Timer

The High Precision Event Timer (HPET) is a hardware timer used in personal computers. It was developed jointly by AMD and Microsoft and has been incorporated in PC chipsets since circa 2005. Formerly referred to by Intel as a Multimedia Timer, the term HPET was selected to avoid confusion with the multimedia timers as a software feature introduced in the MultiMedia Extensions to Windows 3.0.

Older operating systems that do not support a hardware HPET device can only use older timing facilities, such as the programmable interval timer (PIT) or the real-time clock (RTC). Windows XP, when fitted with the latest hardware abstraction layer (HAL), can also use the processor's Time Stamp Counter (TSC) or Power Management Timer (PMTIMER), together with the RTC to provide operating system features that would, in later Windows versions, be provided by the HPET hardware. Confusingly, such Windows XP systems quote "HPET" connectivity in the device driver manager even though the Intel HPET device is not being used.

An HPET chip consists of a 64-bit up-counter (main counter) counting at a frequency of at least 10 MHz, and a set of (at least three, up to 256) comparators. These comparators are 32- or 64-bit-wide. The HPET is programmed via a memory mapped I/O window that is discoverable via Advanced Configuration and Power Interface (ACPI). The HPET circuit in modern PCs is integrated into the southbridge chip.

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