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IF Compliance Updates

发布: 2017-11-28 00:00 | 来源:www.jptranslate.com | 查 看:

There has always been a problem accurately measuring rise and fall times, especially on high speed devices. The measurement of interest is the edge rate, or slew rate, during the state change time.  To help improve accuracy of the measurement, the USB-IF is standardizing on one .

Aside from the fixturing and probes used to take the measurements, major contributors to the inaccuracies in these measurements are the shape of the edge, noise on the signal and the method of calculating the 10% and 90% points as defined in Sections 7.1.2.1 and 7.1.2.2 of the USB 2.0 Specification.

A waveform with slow corners (see sample eye diagram below) will result in a measured rise time that is slower than the actual edge rate would indicate. Also a small change in the position of the 10% and 90% points due to noise on the signal, etc., can cause a relatively large change in the measured rise time.  

IF Compliance Updates

In order to optimize the repeatability of this measurement a slew rate, or edge rate, will be used. The results are expressed in terms of volts per micro-second ( V/us ).  The conversion from rise time to slew rate uses the specified rise time over 80% of the nominal peak to peak signal amplitudes for each of the signaling rates.

High Speed:
Edge Rate(max-nominal) = (0.8V * 0.8)/300e-12 = 2133V/us (Volts per nano-second)
Edge Rate(max-warning) = (0.8V * 0.8)/100e-12 = 6400V/us (Volts per nano-second)

Full Speed:
Edge Rate(min) = (3.3V * 0.8)/20e-9 = 130V/us
Edge Rate(max) = (3.3V * 0.8)/4e-9 = 660V/us

Low Speed:
Edge Rate(min) = (3.3V * 0.8)/300e-9 = 8.8V/us
Edge Rate(max) = (3.3V * 0.8)/75e-9 = 35.2V/us

The actual slew rates are measured within the limits of the inner vertical eye of the far-end template for each test type. This method eliminates the subjective location of the vertical measurement points of the 10/90 method since a fixed voltage range is used. 

Note that the calculations use the of 300ps and 100ps for high-speed signaling.  Calculating slew rates in USBET20 provides a common measurement method and should result in more consistent results between test equipment manufacturers, test facilities and USB-IF compliance workshops.

This measurement continues to be informational for FS and LS. However, this measurement is required for HS.

The current version of the USBET20 tool converts the slew rate into an equivalent rise and fall time.