Teensy 4.0 I2c Pullup resistors

T

TonyAme

Active member
I am using the Teensy 4.0 with two external I2c components. One is the Adafruit MPR121 capacitive touch sensor board and the other is an LCD. I am using Adafruit's library for the MPR121 cap touch board, which uses the "Wire" library which automatically uses the internal pullups?.

Both the MPR121 board and the LCD are being powered by 5V.

*Is it ok to only use the internal pullups for the sda/scl? Or do I need to add external pullups in addition to the internal pullups?

Thank you,
TonyAm
 

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I would add external 3.3V pull-ups for any more than one device, assuming none of them incorporate their own pullups (and if they did, that they were not connected to 5V because the Teensy's not going to like being directly connected to those...)
 
jmarsh said:
I would add external 3.3V pull-ups for any more than one device, assuming none of them incorporate their own pullups (and if they did, that they were not connected to 5V because the Teensy's not going to like being directly connected to those...)
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thank you. right, i would connect the pullup resistors from sda/scl to the 3.3V.
is it ok that the i2c parts are powered by 5V while also using pullups connected to 3.3V?
 
It depends on the specific device. Most should be fine with it. For safety I would power the device without connecting SCL/SDA to anything and measure them, to ensure the device does not have its own internal pull-ups to VCC (5V). If it doesn't, then test it with the Teensy. If either test fails use a bidirectional voltage shifter.
 
In case anyone else has questions about using external pullup resistors for the sad/scl lines when using the Adaruit MPR121 capacitive touch board, I have discovered that they already have built-in 10k pullup resistors, as described here: https://forums.adafruit.com/viewtopic.php?t=133614

*Edit I forgot to mention that I have changed powering to the MPR121 to 3V instead of 5V as seen in my above schematic.

Thanks again, the info from the thread here helped me.
TonyAm
 
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