Specifications

Intel
®
810E2 Chipset Platform
R
Design Guide
131
4.4.1. V
REF
Guardband
To account for noise sources that may affect the way an AGTL+ signal becomes valid at a receiver, V
REF
is shifted by ∆V
REF
for measuring minimum and maximum flight times. The V
REF
Guardband region is
bounded by V
REF
- ∆V
REF
and V
REF
+ ∆V
REF.
∆V
REF
has a value of 100 mV, which accounts for the
following noise sources:
• Motherboard coupling
• V
TT noise
• V
REF noise
4.4.2. Ringback Levels
The example topology covered in this guideline assumes ringback tolerance allowed to within 200 mV of
2/3 V
TT. Since VTT is specified with approximate total ±11% tolerance, this implies a
2/3 V
TT (VREF) range from approximately 0.89 V to 1.11 V. This places the absolute ringback limits at:
• 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
®
II Processor Developer’s Manual.
4.4.3. Overdrive Region
The overdrive region is the voltage range, at a receiver, from V
REF
to V
REF
+ 200 mV for a low-to- high
going signal and V
REF
to V
REF
- 200 mV for a high-to-low going signal. The overdrive regions encompass
the V
REF
Guardband. So, when V
REF
is shifted by ∆V
REF
for timing measurements, the overdrive region
does not shift by ∆V
REF
. The figure below 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
®
II Processor Developer’s Manual
states that extrapolations should be made from the last crossing of the overdrive region back to V
REF
.
Simulations performed on this topology should extrapolate back to the appropriate V
REF
Guardband
boundary, and not V
REF
. So, for maximum rising edge correction, extrapolate back to V
REF
+ ∆V
REF
. For
maximum falling edge corrections, extrapolate back to V
REF
- ∆V
REF
.
Figure 78. Overdrive Region and V
REF
Guardband
overdrive_vref_guard.vsd
V
REF
+ 200 mV
∆
∆∆
∆
∆
∆∆
∆
Overdrive Region (200 mV)
Overdrive Region (200 mV)
V
REF
+ 100 mV
V
REF
V
REF
- 100 mV
V
REF
- 200 mV
V
REF
V
REF V
REF
Guardband