3.3 volt compatibility

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ahoeben
Posts: 20
Joined: Fri Jun 20, 2008 4:14 pm

3.3 volt compatibility

Post by ahoeben » Sat Mar 26, 2011 2:09 pm

I'm considering using a 3.3v microcontroller to control my openservos.

Given that the openservo boards (V3) are powered with 6V+ (and a regulator on-board to get that down to a stable 5V), would it work if I used 3.3V signals for SDC and SDA, or would I really need to levelshift these lines?

Cliff
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Joined: Tue Jan 23, 2007 1:25 pm
Location: Saratoga, CA

Post by Cliff » Sun Mar 27, 2011 2:38 am

Hi ahoeben,

It depends on the specifications of the microprocessor you are using - if it has 5V tolerant I/O (for Reset and INTn signals), it will work without level shifting. However, you will have better noise margin if you level shift.

The SCK and SDA signals are open drain (no pull-ups on OS board), so they can be pulled-up to 3.3V, but pull-ups to 5V will give better noise margin.

Cliff

ahoeben
Posts: 20
Joined: Fri Jun 20, 2008 4:14 pm

Post by ahoeben » Sun Mar 27, 2011 7:39 pm

Thanks for your response. I'll be using an Arduino Pro Mini 3.3v:
http://www.arduino.cc/en/Main/ArduinoBoardProMini

Why would I need 5v tolerance onthe reset line of my mc? It is not connected to the i2c port, is it?

Cliff
Posts: 150
Joined: Tue Jan 23, 2007 1:25 pm
Location: Saratoga, CA

Post by Cliff » Sun Mar 27, 2011 10:17 pm

ahoeben,

The reset line is brought out on the OS connector and can be used to get one or more OS's into a known state without power cycling (e.g. in the case of mind loss after a brown-out). The reset line is pulled-up (10K) to 5V on the OS board.

Cliff

ahoeben
Posts: 20
Joined: Fri Jun 20, 2008 4:14 pm

Post by ahoeben » Mon Mar 28, 2011 12:12 pm

Ah, right. Thanks for the explanation. I'm not connecting the reset-line, so I guess I'm not going to blow up my os boards by talking to them with 3.3v (apart from possibly increased noise issues).

servet
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Joined: Mon Jun 24, 2013 5:40 pm

Post by servet » Mon Jun 24, 2013 5:41 pm

You need to use the level shifters or use a 555timer in monostable mode to decrease load from your controller in case you’re using a lot of servos. The servos are mechanical parts and since I don’t know the exact specification of your servos, I’m going to assume that they require a lot of current for operation like normal servos, when you try to make them operate at a lower voltage, they draw more current, hence endangering your controller circuitry. So to be on the safer side, always use a level translator or an external buffer.

technotronix
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Post by technotronix » Wed Feb 18, 2015 1:00 pm

Thanks for your response cliff

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