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Advanced System Bus Design
5.4
Definitions of Flight Time Measurements/
Corrections and Signal Quality
Acceptable signal quality must be maintained over all operating conditions to ensure reliable
operation. Signal Quality is defined by four parameters: Overshoot, Undershoot, Settling Limit,
and Ringback. Timings are measured at the pins of the driver and receiver, while signal integrity is
observed at the receiver chip pad. When signal integrity at the pad violates the following guidelines
and adjustments need to be made to flight time, the adjusted flight time obtained at the chip pad can
be assumed to have been observed at the package pin, usually with a small timing error penalty.
5.4.1
V
Guardband
REF
To account for noise sources that may affect the way an AGTL+ signal becomes valid at a receiver,
is shifted by ∆V
V
REF
Guardband region is bounded by V
which accounts for the following noise sources:
•
Motherboard coupling
•
V
TT
•
V
REF
5.4.2
Ringback Levels
The example topology covered in this guideline assumes ringback tolerance allowed to within
200 mV of 2/3 V
2/3 V
(V
TT
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
®
5.4.3
Overdrive Region
The overdrive region is the voltage range, at a receiver, from V
high going signal and V
regions encompass the V
measurements, the overdrive region does not shift by ∆V
Corrections for edge rate and ringback are documented in the Intel
Developer's Manual. However, there is an exception to the documented correction method. The
®
Intel
Pentium
the last crossing of the overdrive region back to V
should extrapolate back to the appropriate V
maximum rising edge correction, extrapolate back to V
corrections, extrapolate back to V
5-18
for measuring minimum and maximum flight times. The V
REF
noise
noise
. Since V
is specified with approximate total ±11% tolerance, this implies a
TT
TT
) range from approximately 0.89 V to 1.11 V. This places the absolute ringback
REF
II Processor Developer's Manual.
to V
REF
REF
Guardband. So, when V
REF
II Processor Developer's Manual states that extrapolations should be made from
®
REF
-∆V
and V
+∆V
REF
REF
REF
- 200 mV for a high-to-low going signal. The overdrive
is shifted by ∆V
REF
.
REF
. Simulations performed on this topology
REF
Guardband boundary, and not V
REF
+ ∆V
REF
- ∆V
.
REF
REF
∆V
has a value of 100 mV,
REF.
REF
to V
+ 200 mV for a low-to-
REF
REF
for timing
REF
Figure 5-10
depicts this relationship.
®
Pentium
II Processor
®
. So, for
REF
. For maximum falling edge
REF
®
Intel
810A3 Chipset Design Guide
®
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