Robot Control Board MSR1 Fatal Design Flaw

John Abshier

I was going to use the MSR1 for a robot designed for the Home Brew Robotics Club tabletop competition.· For motors I am going to use the· Faulhaber gear motor and its HES164A encoder.· Using the attached program I got all zeros for encoder reading and delta encoder reading.· I then wired the motor up to a Professional Development Board and everything worked as expected.· Looking at the datasheet for the level translators I found:


The TXB0108 is designed to drive capacitive loads of up to 70 pF. The output drivers of the TXB0108 have low

dc drive strength. If pullup or pulldown resistors are connected externally to the data I/Os, their values must be

kept higher than 50 kΩ to ensure that they do not contend with the output drivers of the TXB0108.

For the same reason, the TXB0108 should not be used in applications such as I2C or 1-Wire where an

open-drain driver is connected on the bidirectional data I/O. For these applications, use a device from the TI

TXS01xx series of level translators.

I guess I will solder up a motor driver on a Protoboard.

John Abshier

Mike Green

"Fatal Design Flaw" is a bit strong. TI recommends the TXB01xx translators for open-drain type applications. They recommend their TXS01xx for push-pull type applications. Neither one is good for all applications and Parallax had to choose between them. You can also bypass the translators on a 8-bit port basis and there are lots of easy ways to interface 5V logic to the Propeller's 3.3V I/O pins and most of this is documented in the sticky thread on interfacing between 3.3V logic and 5V logic.

John Abshier

Perhaps I was a bit strong. But this is the board for the Stingray which has extended shafts for encoders on its motors. However, I haven't tested other encoders. Perhaps some will work. Not being able to plug and play with I2C sensors also hurts for robotics. I didn't want to add a second board to solder up interface circuitry. That is why I will go to a Protoboard; I can fit everything I need on it. I think if I were redesigning the MSR1 I would replace the translators with a socket for each 8 pin group to plug in a 220 ohm (for 3.3 volt) or 2k (for 5 vol50 ohm resistor pack. I would also have the power swith at OFF turn off power to the Vin lines.

John Abshier

Whit

@John,
I don't think it is even a flaw. I'd rather save the Prop. More advanced users can easily fix this and I am going to add the translator by-pass pins per the Stingray Documentation (as Mike mentioned) for jumpers when I get the chance. I just didn't want to do it when the board was brand new.

I will probably use "red" jumpers to get my attention and I have also been thinking of getting something to put over the I/O pins (maybe some anti-static foam or empy 3 pin connectors or jumpers down the ground pins) to remind me when I have the translators disabled - Just so I don't accidentally forget and fry my Prop. I could also just disable just the P16 to P22 group (and keep it disabled to avoid confussion)

Any other ideas for this? I just want to make it as fool-safe as possible. As the old saying goes though - "It is hard to make anything fool-proof, because fools are so ingenius."

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Whit+


"We keep moving forward, opening new doors, and doing new things, because we're curious and curiosity keeps leading us down new paths." - Walt Disney

Post Edited (Whit) : 5/23/2010 2:14:39 AM GMT

erco

If everything X not being compatible with everything Y constitutes a "fatal design flaw", then the Microsoft Windows family and a whole bunch of PC hardware is rife with fatal design flaws! In my robotics experience, drop-in modularity is a long way off, so you gotta be a Marine: adapt & overcome. Thank goodness for this Forum resource and guys like Mike & Whit, who are always available and eager to help with knowledge and enthusiasm. They are worth their weight in voltage translators and extra hardware!

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·"If you build it, they will come."