Intel CHIPSET 820E - Manual (Page 161)

Intel ® 820E Chipset
R
Design Guide 161
3.5.1. V
REF
Guard Band
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 the minimum and maximum flight times. The V
REF
guard band 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
3.5.2. Ringback Levels
The example topology covered in this guideline assumes a ringback tolerance allowed to within 200 mV
of 2/3 V
TT
. Since V
TT
is specified with an approximate total tolerance of ±11%, this implies a 2/3 V
TT
(V
REF
) 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 V
REF
to V
REF
+ 200 mV, for a low-to-
high-going signal, and from V
REF
to V
REF
– 200 mV for a high-to-low-going signal. The overdrive
regions encompass the V
REF
guard band, so when V
REF
is shifted by ∆ V
REF
for timing measurements, the
overdrive region does not shift by ∆ V
REF
. 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
V
REF
. Simulations performed on this topology should extrapolate back to the appropriate V
REF
guard
band boundary, and not to 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 84. Overdrive Region and V
REF
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)