Software Developers Hpet Spec 1 0a – Download as PDF File .pdf), Text File .txt ) or read online. Updated HPET web link, added WSPT and WDAT, updated WDRT description and web link. Clarified that the endian-ness of data value. High Precision Event Timer Driver for Linux The High Precision Event Timer ( HPET) hardware follows a specification by Intel and Microsoft, revision 1.
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The implicit transformation sprcification a special extra spec into placement-isms is arcane. From Wikipedia, the free encyclopedia. If we do get down to the virt driver and the trait is set, and the driver for whatever reason e. Setting this bit to 1 enables triggering of interrupts.
Implementations have been observed in which the period register is off by parts per million or more. When a corresponding timer interrupt is active, this bit is set. When the interrupt is generated, however, the hardware will increase the value in comparator register by the last value written to it! Reads will return current value of the main counter. 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.
This means that next write to timer N comparator register will have the usual meaning, while second next write will write directly to the accumulator. Views Read Edit View history. Webarchive template archiveis links Articles needing additional references from February All articles needing additional references Articles to be expanded from February All articles to be expanded Articles using small message boxes All articles with unsourced statements Articles with unsourced statements from December I am enabling the timers only when I actually use them, so there’s no “real” initialization of comparators here.
The driver uses the hw: Namely, you probably want to use some of ISA interrupts – or, at very least, be able to use them at one point unambiguously. Common Devices Interrupts Time. Keep in mind you have to initialize both the main counter and all of the comparators.
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High Precision Event Timer – Wikipedia
Update User Documentation for image properties . The following is the procedure you need to perform to initialize main counter and comparators in order to receive interrupts. Compared to these older timer circuits, the HPET has higher frequency and wider bit counters although they can be driven in bit mode.
This has the express advantage of being independent of the CPU frequency and still provides a very reasonable sub-microsecond resolution and accuracy. Timer specificayion Interrupt Routing Capability. While and RTC can be put into an HPET-like one-shot mode, the set-up process is so slow that their one-shot mode is not used in practice for tasks requiring precise scheduling.
It consists of usually bit main counter which counts upas well as from 3 to 32 32 or 64 bit wide comparators. Since the original specification for HPET in calls for a high resolution counter, which is then exposed by the QueryPerformanceFrequency and QueryPerformanceCounter API calls already available since Windowsit is the QueryPerformanceFrequency that can shed light on spcification this “high precision” counter is actually being provided.
If she forgets hw: The following options to use Trait were considered, but ultimatedly we chose a simpler approach without using Trait.
High Precision Event Timer
If an illegal value is written, then value read back from this field will not match the written value. Keep in mind that allowed interrupt routing may be insane.
The operator has to remember to set both extra specs, which is kind of gross UX. In one-shot mode the comparator fires an interrupt once when the main counter reaches the value stored in the comparator’s register, while in the periodic mode the interrupts are generated at specified intervals.
This page is not meant as a full description of HPET, only as a lightweight introduction.
Last zpecification on Fri Dec 21 Save minimal tick either from ACPI table or configuration register. The following operating systems are known to be able to use HPET: If you need any information not covered by this article, consult the HPET specification.
HPET offers two operating modes: HPET supports three interrupt mapping options: For periodic mode, similarly to one-shot mode, you write a value at which an interrupt shall be generated to the comparator register.