Teensy3 I2C pull-up mystery with LED backpacks

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fridata
 
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Teensy3 I2C pull-up mystery with LED backpacks

Post by fridata »

After some troubleshooting, I finally figured out that the Teensy 3.0 and 3.1 only works with my 1.2" 7-segment LED backpack when the I2C 4.7k pull-up resistors were connected to 5V. At first i had them connected to 3.3V along with the V_IO / Vi2c on the backpack.
Since the Teensy has 3.3V GPIO, I figured that would be the same voltage for the pull-ups.

Because my initial testing was only with the Teensy 3.0 that doesn't have 5V tolerant GPIO, I did not dare connect the pull-ups or V_IO to 5V.

1) Is there an explanation why the display only works with pull-ups set to 5V or do I have some damaged LED backpacks?

2) If it is in fact ok to have the pull-ups to 5V, can I just have the V_IO / Vi2c connected to 5V on the Teensy 3.1 as well?
It works and it seems like a logic consequence of using 5V pull-ups.

Any feedback would be appreciated. I don't really know if I am doing things right or risking an unstable I2C bus.

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adafruit_support_rick
 
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Re: Teensy3 I2C pull-up mystery with LED backpacks

Post by adafruit_support_rick »

post some pictures showing your wiring. Are you driving the display logic with 3.3V? You should not use pullups to 5V.

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michaelmeissner
 
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Re: Teensy3 I2C pull-up mystery with LED backpacks

Post by michaelmeissner »

I always connect the Teensy 3.0/3.1 pull-ups to 3.3v (I use 4.7K resistors). If you need to drive 5v devices, you need to use an i2c-safe level shifter like: https://www.adafruit.com/products/1875.

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fridata
 
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Re: Teensy3 I2C pull-up mystery with LED backpacks

Post by fridata »

Thanks for the input! It turns out slightly smaller resistors of 3.9k did the trick. Though, when I redid the circuit on a breadboard it worked fine with 4.7k. But not with 10 cm jumper wires to the LED backpack.

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michaelmeissner
 
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Re: Teensy3 I2C pull-up mystery with LED backpacks

Post by michaelmeissner »

This article talks about varying i2c pull-up resistances depending on the line qualities and number of i2c devices: http://www.dsscircuits.com/articles/47- ... -resistors.

I personally only have a few i2c devices, and I tend to use them 1 or 2 at a time, and close to the Teensy. So I haven't needed to change the 4.7K resistors I currently use.

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