High Precision Event Timer Driver

Input Device
Windows 2000/XP/2003/NT

High Precision Event Timer is used in personal computer systems as a hardware timer. Most computer chipsets incorporate this timer especially the ones developed by Intel and Microsoft. This timer has had previous names before such as multimedia timer but now it is commonly referred to as HPET. The HPET circuit used in new generation computers is integrated into the Northbridge chip. These makes these new generation computers run a much faster HPET at 100 MHz or above. However the older generation computer systems were not able to support this timer and had to use the older timing facilities also known as real time clocks. This timer has a variety of features including 29 additional 32-bit timers that are used for random interrupt generation for the user software applications.

However some operating systems are known to fail to configure these interrupts that occur on user software interrupt routines especially on the systems with multicore CPUs. The HPET 32/64-bit extended parts are only reached through memory mapped I/O which is set up via ACPI by the BIOS. Any motherboard on a computer is only able to handle a maximum of 8 HPET chips. However the computers that have FSB and DDR-RAM only need 2 chips. This basically means that modern computers include 6 PC 8253- Timer compatible counters as well as 58 of the 32-bit HPETs for 64 interrupt as well as IRQ capable timers in all. Each timer is able to generate an interrupt when there are equal least significant bits with corresponding bits of the 64-bit counter value. The timers can be in either periodic mode or in one shoot mode.

In the case of periodic mode the interrupts will be generated at a specified interval while in the case of one shot mode the timer will fire one interrupt whenever the main counter has reached the specific value that is stored in the register of the timer. High Precision Event Timer is known to produce high resolution periodic interrupts as compared to RTC and in most cases it is used to synchronize stream using multimedia features. This thereby reduces the need to use other timestamp calculations as well as provide smooth playback. Most of the operating systems that were developed before HPET was introduced in the market are not able to work with HPET. It can only be used with the new computer systems that have timer facilities.

Posted in Input Device by Luciano Ruffalo
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