Advanced Micro Devices - Moderated (AMD) Message Board

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Technology Stocks : Advanced Micro Devices - Moderated (AMD)

AMD 203.14

-0.8%

Jan 9 9:30 AM EST

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To: fyodor_ who wrote (65667)	12/14/2001 9:04:33 AM
From: combjelly	Read Replies (1) of 275872

"The Tbirds were "supposed" to have a 40-ish Ohm CPU bus driver impedance, but this value was upped to the aforementioned 56 Ohm with the introduction of the AthlonXP. " The most likely problem is that AthlonXPs expect a faster rise time on the signals. A 40 ohm impedance would cause the edges to be more rounded than a 56 ohm impedance. And that would result in the slope of the signals to be greater. It sounds like the XP has tighter timing parameters, which implies that the transistors are faster. Which is interesting, seeing as how the two processors are on the same process... The major reason for terminating a bus is because it reduces reflections of the signal and ringing (where the voltage goes above and below the nominal high or low voltage levels). Reflections interfere with the signals, as will ringing. Ringing also contributes to RF emissions. If the XPs can use a higher impedance, it also means that there is less current being used on the signal lines, in addition to the greater speed of the transistors as mentioned above.

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