Intel 820E 3.5.1. VREF Guard Band , 3.5.2. Ringback Levels , 3.5.3. Overdrive Region

Intel® 820E Chipset
R
Design Guide 161
3.5.1. VREF Guard Band
To account for noise sources that may affect the way an AGTL+ signal becomes valid at a receiver, VREF
is shifted by ∆VREF for measuring the minimum and maximum flight times. The VREF guard band region
is bounded by VREF – ∆VREF and VREF + ∆VREF. ∆VREF has a value of 100 mV, which accounts for the
following noise sources:
• Motherboard coupling
• VTT noise
• VREF noise
3.5.2. Ringback Levels
The example topology covered in this guideline assumes a ringback tolerance allowed to within 200 mV
of 2/3 VTT. Since VTT is specified with an approximate total tolerance of ±11%, this implies a 2/3 VTT
(VREF) range, from approximately 0.89 V to 1.11 V. This sets the absolute ringback limits as follows:
• 1.3 V (1.1 V + 200 mV) for rising-edge ringback
• 0.69 V (0.89 V – 200 mV) for falling-edge ringback
A violation of these ringback limits requires flight time correction as documented in the Intel® Pentium®
III Processor Developer’s Manual.
3.5.3. Overdrive Region
The overdrive region is the voltage range at a receiver, from VREF to VREF + 200 mV, for a low-to-
high-going signal, and from VREF to VREF – 200 mV for a high-to-low-going signal. The overdrive
regions encompass the VREF guard band, so when VREF is shifted by ∆VREF for timing measurements, the
overdrive region does not shift by ∆VREF. Figure 84 depicts this relationship. Corrections for edge rate
and ringback are documented in the Intel® Pentium® II Processor Developer’s Manual. However, there is
an exception to the documented correction method: The Intel® Pentium® III Processor Developer’s
Manual states that extrapolations should be made from the last crossing of the overdrive region back to
VREF. Simulations performed on this topology should extrapolate back to the appropriate VREF guard
band boundary, and not to VREF. So, for maximum rising-edge correction, extrapolate back to VREF +
∆VREF. For maximum falling-edge corrections, extrapolate back to VREF – ∆VREF.
Figure 84. Overdrive Region and VREF Guard Band
overdrive_vref_guard
V
REF
+ 200 mV
∆
∆∆
∆
∆
∆∆
∆
V
REF
+ 100 mV
V
REF
V
REF
- 100 mV
V
REF
- 200 mV
V
REF
V
REF
V
REF
Guardband
Overdrive Region (200 mV)
Overdrive Region (200 mV)