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Topic: Would 10K pullups be okay for servos controlled by a TLC5940? (Read 2257 times) previous topic - next topic

scswift

I've seen in a various places resistors between 2K-5K being recommended for pulling up the servo's signal pin, but is there some reason the standard 10K pullup resistor would not be suitable here?

johnwasser

Depends on the input impedance of the servo inputs.  When the TLC5940 is "HIGH" the output is floating.  You need the resistor to be low enough to pull the signal pin to at least the logic threshold of the servo input (maybe 3.0V?).  If the input impedance is low, like 10K Ohms, using a 10K pull-up resistor would only bring the input to 2.5V.  The 2K or 5K choices (2.5 mA or 1mA) would cover a greater range of input impedance.

Connect the servo input to 5V through your 10K resistor.  If you get close to 5V at the signal pin then you should be fine.  If it is below 4V you should try a lower value pull-up.
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scswift

I don't have a chip to test with yet, but I googled servo input impedance and so far what I've found indicates there are two chips used to control servos:

http://www.seattlerobotics.org/encoder/200009/Servos.html

http://www.datasheetcatalog.org/datasheets/320/373808_DS.pdf
http://www.datasheetcatalog.org/datasheet/MitsubishiElectricCorporation/mXyyzzzr.pdf

The first, which is the older of the two, if I understand correctly, appears to read < 1.3v as low, and > 1.4v as high. 
The second, which is probably what modern servos use, only has one bit at the end which indicates anything over 3v on the input is considered high.

Huh.  I thought 10K was generally an accepted safe value to use for pullup resistors. :/

Btw, the reason I asked about this is I'm thinking about getting some surface mount boards assembled and I've discovered that it'll cost me like $1.50 per part to have them placed.  That makes using a bussed resistor array a very tempting option, because even though the part itself is like 60 cents, that's 60 cents for a part which replaces 19 resistors that would add around $20-$30 to the cost of each board.  It would be nice if I could use 10K resistors in this array because then I'd have three 10K's to use for other things on the board.  I guess I can pull other pins up with less than 10K though.

johnwasser

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