Reef Angel Forum

Would filling the USB ports with dielectric grease help mitigate issues with corrosion causing bus locking? Haven't thought through the marine/reef safe issues but it seems like a simple preventative option.

Additionally spraying the pcb's with a protective coating (like a spray urethane/varnish) to protect from internal corrosion.

Roberto - after all the years of Bus lock issues are the common trends connections and corrosion or does it ever boil down to hardware issues (component tolerances or failures)?

It seems to be a resistance issue caused by corrosion.

So in the scenario where a dimmer expansion is causing bus locks, looking into the schematic there is a 10k pair of pullup resistors on the bus, plus 2.2k's on the expansion hub. If the module is causing a bus lock options are 1. determine the source of corrosion adding to the bus resistance ( ie 10k + 2.2k + unknown corrosion source) and clean it. Or 2. Change the 10k's for something like 8.2k or 7.5k to provide a little extra resistance buffer? Are there any major design concerns with this approach? I see the cable lengths are another variable (capacitance on the bus) which can influence the bus stability. I am interested in your bus watchdog circuit you implemented in the Star series.

NOTALINK//electronics.stackexchange.NOTALINK//questions/1849/is-there-a-correct-resistance-value-for-i2c-pull-up-resistors
NOTALINK//www.ti.NOTALINK//lit/an/slva689/slva689.pdf

Also would shielding the cables could help from spurious noise on longer cables? (poor man's solution would be wrapping tun foil around the cable as a test)

Yes, the cable length and the resistance from corrosion are two things to watch for.

Anyone come across a good solution for preventing corrosion from occurring on the usb headers and connectors?

Thanks

The 10k resistors are I2C pull-up resistors. 10k is shown on the datasheet for the I2C level shifter that the RA uses, but it's probably too high. Search for TI's "Application Report SLVA689: I2C Bus Pullup Resistor Calculation." (I can't post links.) Lower values give a stronger signal, at the expense of a little bit of current. The minimum is a little bit below 2k. It's the capacitance that's an issue. It takes some current to bring the entire bus back up to 5V, and a 10k resistor just doesn't pass enough. If you add some extra pull-up resistors you may see improvement. It doesn't really matter where the pull-up resistors are located.

(The actual resistance of the cable "shouldn't" be a problem. That just causes a constant voltage drop across the cable; I2C is designed to work between devices running at different voltages, so that should be OK. The I2C spec is more concerned about the bus capacitance.)