Hardware Reference
In-Depth Information
Figure 3.49. An ICS 9250 frequency timing generator chip with a 14.318MHz crystal.
The RTC in the original PC was notoriously inaccurate, so starting with the IBM AT in 1984, IBM
added a separate 32.768KHz crystal to count time independent from the speed of the system. This
crystal is used on all modern motherboards as well.
Figure 3.50
shows a 32.768KHz crystal next to a
chipset South Bridge or I/O controller hub, which contains the RTC circuitry and CMOS RAM.
Figure 3.50. Chipset South Bridge (I/O controller hub) incorporating an RTC, along with the
32.768KHz clock crystal.
Most frequency synthesizer chips used on PC motherboards are made by a handful of companies,
including Integrated Device Technology (
www.idt.com
;
formerly Integrated Circuit Systems) and
Cypress Semiconductor (
www.cypress.com
; formerly International Microcircuits Inc. [IMI]). These
chips use phased locked loop (PLL) circuitry to generate synchronized processor, PCI, AGP, and
other bus timing signals that are derived from a single 14.318MHz crystal. The crystal and frequency
synthesizer chip are usually situated near the processor and main chipset component of the
motherboard.
The amazing thing about these chips is that most of them are programmable and adjustable, so they
