I2C Interface

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I2C Interface

Post by Sam » Sun Jan 29, 2012 1:34 pm

Hi All,

I have posted the schematic and PCB layout for the I2C interface board, which would control the Auto-Cal Attenuator:

I used different buffers than I originally anticipated, and these don't have explicit slew-rate control, so I have some concern about reflections. I2C rise times are generally fairly slow (due to the pull-up mechanism) but fall times can be fast, so my solution was to use series 75-ohm resistors, which should present about the right impedance on the fast falling side but not on the slow rising side. It seems that should do the trick. In addition, at least some I2C devices (like the PCF8574 latch) have considerable immunity to fast glitches on the bus (up to 100 nS for PCF8574), so it doesn't seem that a few feet of bus cable (S-Video) should present any problem. Any comments from people with I2C experience would be welcome. Or any other comments, for that matter.

I don't anticipate that there would be many devices simultaneously attached to the external I2C bus. In fact, I can really only envision one or two at a time. For example, the Auto-Cal Attenuator would be attached for calibration and then removed. Or maybe it would be used occasionally for some other special purpose. I can envision the internal bus controlling an input attenuator, a TG attenuator and one or more temperature monitors. The interface board also has 8 new "latch bits", which might be used to control internal attenuators directly.

The connector for the external bus might also be a simple way to provide power to a probe, since it has regulated +10V.

Sam W.

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