Intel 820E Benutzerhandbuch

Design Guide 

 

161 

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 

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

III Processor Developer’s Manual. 

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

 II Processor Developer’s Manual. However, there is 

an exception to the documented correction method: The Intel

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

. 

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)