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*To*: stds-802-3-hssg@ieee.org, t11_2@dpt.org*Subject*: Re: XAUI IO specs*From*: kdemsky@us.ibm.com*Date*: Thu, 27 Apr 2000 18:49:49 -0500*Sender*: owner-stds-802-3-hssg@ieee.org

Ali, By your own statement below, a static Logic 1 is when Vo+ is 400 mV above Vcm, and Vo- is 400 mV below Vcm, then the differential voltage (Vo+) - (Vo-) is 800 mV (positive peak). By your own statement below, a static Logic 0 is when Vo+ is 400 mV below Vcm, and Vo- is 400 mV above Vcm, then the differential voltage is (Vo+) - (Vo-) is -800 mV (negative peak). Then peak to peak is (positive peak - negative peak) = 1600 mV, and not 800 mV as you have in your response. So, I agree with Mike Dudek, Ed Grivna, that your statement below is confusing. I am agreeable to any way of defining logic levels with or without common mode, single ended or differential. I agree with Ed Grivna that Vcm is not necessary to define a differential signal, and I am only responding to a thread that is using Vcm. Given all this confusion, it may be prudent to include pictures of logic signals, and define the swings in an already standardized way. May I recommend that future proposals define signals like Figure 3-3 on page 10 of the IEEE LVDS spec, document # (IEEE Std 1596.3-1996)? A picture would eliminate ALL confusion. In the absence of a picture, pointing to a standardized way of measurement would also be less confusing. Perhaps we could use |Vod| = Output Differential Voltage like on page 8 and 9 of the same IEEE document. Notice the absolute value sign that eliminates the polarity issue, which eliminates the factor of 2 that is biting us. We all understand that |Vod| = 1/2 * (V differential peak-to-peak). I actually prefer differential peak-to-peak, but being that there is lots of confusion, we all need to agree. Use whatever you like, as long as we can all understand. From Ali's Post: >o. Based on my understanding and everyone who participated during conference call the Vp-p >differential max defined as 800 mV. > - This means each side of the driver has a swing of 400 mV. > - Logic 1 would defined as Vo+ at (VCM+400 mV) and V0- at (VCM-400mV) > - Logic 0 would defined as Vo+ at (VCM-400 mV) and V0- at (VCM+400mV) > - If the driver output VDp-p <85 mV you got noise. In your follow up response to this thread, you wrote: >When I say each side has a swing of 400mV, I refere to each half of the differential driver. >Vo(diff)p-p = (Vo+) - (Vo-) = (400) - (-400) = 800 mV. The two sentences contradict each other. If you have static voltages, Vo+ = Vcm+400 mV, and Vo- = Vcm-400mV, the difference is 800mV, but is only ONE peak, and NOT peak to peak. For any peak-to-peak measurement you need to know what the other peak is. On 4/18 Tim Mathey posted an e-mail with Howard Johnson's recollection of peak differential amplitude vs differential peak-to-peak. He even has an ASCII waveform that makes it crystal clear. Darn those factors of 2! :)) Again, if we have pictures you can call it whatever you want and we all understand. Kevin Demsky Mixed Signal and VLSI Development IBM Corp. 3605 Hwy 52 N Dept. QXS Bldg. 050-2 Rochester, MN 55901 Internal E-mail: kdemsky@ibmusm07 External E-mail: kdemsky@us.ibm.com Phone: 507-253-5799 Fax: 507-253-4966

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