Understanding GTL Reference Voltage
Because of the popularity of Intel quadcore processors the method of overclocking has somewhat changed. Most everyone knows that when the CPU and board are pushed to the edge success will hinge on the adjustment of CPU and Northbridge GTL Reference voltages. As important as these settings are to a stable overclock few users know what is being accomplished when these values are changed. For most users, both as to why they need adjusting and how to go about finding the correct values, these settings are shrouded in mystery. GTL (Gunning Transceiver Logic) was invented in 1991 by William Gunning while at Xerox. It was created because of the need for a faster chip-to-chip interface. It is an open-drain, low power switching circuit. Today there are several variations of this specification in use. They differ primarily in the voltages used, signal edge enhancements and slew rates. In the past Intel used a bus technology known as GTL+. The major difference between GTL and GTL+ is that the noise margin around VREF was increased. Currently Intel is using AGTL+ (Assisted Gunning Transceiver Logic +). The most notable difference between AGTL+ and GTL+ is the addition of an active pMOS pull-up transistor on the output of the driver. This is to assist the termination resistors during the first clock of a low-to-high voltage transition. Throughout the following examples all references will be based on the AGTL+ specification. By substituting the correct voltages these examples are valid for most versions of GTL.
Definition of terms used in this article.
- VTT is the termination voltage. It is also the voltage VREF is derived from.
- VTT(min) is the minimum termination voltage.
- VREF is the reference voltage. Normally 2/3(VTT)
- VIH (min) is the minimum specified valid input high level voltage. For this example it is 100mV above the reference voltage.
- VIL (max) is the maximum specified valid input low level voltage. For this example it is 100mV below the reference voltage.
- VOH (min) is the minimum output high level voltage. VOH(min) is equal to VTT(min).
- VOL (max) is the maximum output low level voltage. This value is equal to 1/3(VTT).
VTT sets the base voltage levels between ground and itself for the entire GTL circuit with the exception of the threshold voltage. VOL (max) is VTT(1/3) and VREF is VTT(2/3). A change to VTT also changes all the other voltage limits of the circuit. Increasing VTT not only strengthens the signal but also acts as a rather course control for what GTL Reference voltage is normally used for.
- Overshoot (sometimes referred to as undershoot for falling transitions), is the amount by which the signal's voltage level extends above or below VTT or ground. Limiting overshoot is necessary to insure the flight time meets the design requirements and to avoid damaging the components connected on the GTL bus
- Ringback is the amount by which a signal rebounds below the logic high lower limit (or above the logic low upper limit) after an overshoot event has occurred. Ringback must be limited to prevent inadvertent false switching of these digital signals.
- Settling time is a measurement of oscillations on the GTL waveform (usually caused by reflections on the transmission line traces). This term measures the amount of time required for oscillations to dampen to a level that will not increase the flight time of the next transition.