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Using an level shifter to convert 3.3V TTL Serial to 5V TTL serial (and back)

by SameerP on Mon Nov 26, 2012 10:17 am

Hi all. I have a beaglebone, as previously noted, and I am trying to connect it to an arduino and a Pololu Servo controller over TTL Serial. The Beagle wants 3.3V logic level and the arduino and pololu run on 5V. I picked up a TXB0108 breakout from Adafruit (http://www.adafruit.com/products/395) but I'm having trouble getting it to work. Does anyone here have experience with this?

I've connected it I think correctly, with 3.3V to VCCA, and 5V to VCCB, and OE connected to VCCA. When I connect 3.3V to one of the input pins I get 5V on the output pins. The problem arises when I start sending serial TTL signals.

Here's scope output of my 3.3V serial TTL Tx signal coming out of the beagle, while I'm sending data over the line

https://www.dropbox.com/s/0ya065hzw6dd7 ... .23.24.jpg

Here's scope output of both the 3.3V TTL Tx signal from the beagle and the output pin of the level shifter, while I am not sending data over the line... everything looks copacetic:

https://www.dropbox.com/s/grndixk3xs69h ... .23.40.jpg

(the yellow line is the output pin of the level shifter, the blue line is the Tx signal from the beagle)

However, when I start sending bits over the serial line from the beagle, everything goes haywire. Even the signal on the line from the beagle looks wonky:

https://www.dropbox.com/s/r7a7ff7xfid43 ... .23.53.jpg

Anyone have ideas on this one?

Thanks,
-s
SameerP
 
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Re: Using an level shifter to convert 3.3V TTL Serial to 5V TTL serial (and back)

by adafruit_support_rick on Mon Nov 26, 2012 11:09 am

How are you attached to GND?
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adafruit_support_rick
 
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Re: Using an level shifter to convert 3.3V TTL Serial to 5V TTL serial (and back)

by SameerP on Tue Nov 27, 2012 10:55 am

I have all my grounds tied together to the ground pin on the beagle.

Thanks
-s
SameerP
 
Posts: 2
Joined: Mon Nov 26, 2012 10:17 am