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Differences in BBB and RBBB

Posted: Sat Nov 08, 2008 10:28 am
by droops
Hey I am learning, so stick with me. What is the purpose of the resistor between the pins of the ceramic resonator on the BBB? Also the 10k resistor on the tx line?

Re: Differences in BBB and RBBB

Posted: Mon Nov 10, 2008 9:51 pm
by paul
The 1M resistor on older versions of the BBB is a load resistor for the resonator. It shows up in a lot of schematics but the reality is that most contemporary microcontrollers have this built into the internal crystal oscillator pins. I had to go to some work to track this down and finally got an answer from an engineer at a resonator manufacturer.

The 10k resistor on the TX line (chip RX line really) prevents an interesting "screw-up mode" - probably more formally called a "fault mode". If the microcontroller isn't powered, (say by the BBB having it's power jumper set to external - but no external power in place) the cable TX line (which is hooked up to the chip's RX) has enough current to power the Atmega 168 chip through the protection diodes on the TX line. This was a source of some annoying problems on the NG and earlier Arduinos that was eventually tracked down. The 10K line limits the current to a level too small to actually power the chip - but downloads still work fine. I don't have it on the RBBB because I figure that if the cable is connected to the TX pin, the power is connected too, so no problem there. Someone will eventually prove me wrong I predict. (Engineers really should be pessimists. They're always worrying about how things are going to fail, but most of them I know are just kind of even-tempered optimists that just want things to work better)

Besides I was trying to make the RBBB as small as through-hole parts would allow so I jetisoned the TX-RX resistor, knowing that while there is a "fault mode" lurking out there, it isn't anything fatal (to the RBBB), and is not likely to occur very often.

The RBBB could malfunction in the "fault mode" though and I guess I should have one of those warnings about not using the RBBB for any use on which human life or NASA missions depend but somehow it seems self-evident. Maybe don't hook it up to the full sized remote control Volkswagen Beetle robot you are building.

Hope that helps,

Paul

Re: Differences in BBB and RBBB

Posted: Mon Nov 10, 2008 11:37 pm
by droops
thanks paul, very helpful. so neither is required and the 1m resistor is just on old bbb's, but is done in the chip.

Re: Differences in BBB and RBBB

Posted: Sat Feb 13, 2010 5:54 pm
by TchnclFl
The 10k resistor on the TX line (chip RX line really) prevents an interesting "screw-up mode" - probably more formally called a "fault mode".
I stumbled across this "fault mode" the other day! Really threw me off at first. Nothing particularly bad happened, but it did confuse me when I had the RBBB not powered, but connect RX and TX lines from a Duemilanove to it and the power LED came on.

Nice to know that's why.

Like I said, not a big deal, but I did stumble across it ;D!

Differences in BBB and RBBB

Posted: Sat May 18, 2019 11:11 am
by PacDup
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