I2C display backpack pull-ups? #1014
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Hoping anyone can confirm my intended approach here. I bought a Geeekpi LCD1602 display which claims to be compatible with 5V Vcc and the RPi I2C bus. I'm not so sure. When powering up the display but not connecting it to the Pi4, the SDA and SCL pins show 5V, so according to the wiki it's unsafe to connect to the Pi GPIO pins. The display will not initialize when powered with 3.3V. The board looks identical to mine (photo attached). If I read the text correctly, R8 and R9 are the suspicious pull-ups (they are connected to the SDA and SCL lines on the chip PCF8574T). Am I correct in that I need to remove them in order not to risk damage to the Pi? 
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Replies: 3 comments 2 replies
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Ideally you should remove them. Lots don't (me included) and haven't killed a pi (yet!). IIRC the PI I2C pins have 1k8 pullups to the 3v3 rail. |
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Your maths work out for me - I actually measured the voltage on the I2C pins and came up with exactly 3.7VDC. So I'll de-solder them and make sure it still works reliably. |
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And, after removing them I get a nice 3.3VDC on the I2C lines and the display is OK. Going to let it run for a few hours to be sure everything works as desired. |
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All good I presume? :) |
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Yes, it works great. I also took the time to "upgrade" my backup 3B+ minidexed to make sure it won't have any issues. Now to install the DAC that just came in the mail today, and see about having someone print me a nice case! |
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Ideally you should remove them. Lots don't (me included) and haven't killed a pi (yet!).
IIRC the PI I2C pins have 1k8 pullups to the 3v3 rail.
If my maths is good, the 4k7 to 5v and the 1k8 to 3v3 should yield an idle voltage on the pin on 3v7.