@Katak255
Thank you for your investigation and suggestion. I have since gone back and probed the 2004 LCD Display and its PCF8574 pigiback board vs the PFC8574 datasheet and like you I found two 4.7K resisters as pull-up to VCC as marked on the pigiback board.
I also ran a test to scan and report for attached I2C devices with the VCC of the LCD board connected to the Pico 3.3v(OUT) pin. As I found with the 24C256 eeprom board, if I use the 3.3v for the VCC of the board then No Devices are found suggesting that the LCD display like the eeprom boards requires 5v VCC supply to function and I tested it without any jumper to force the backlight to draw power.
As neither the LCD display module or the eeprom module would function without 5v VCC applied to it, which if I am right, means that the Pico SDA and SCL pins would be draining the 5v from the pull-ups to ground for the devices to communicate over those lines. I did look at my LCD pigiback board with the PCF8574 converter chip and it has 4.7Kohm pull-up resisters on it.
That left me with an issue to fix, how to ensure both devices could operate with their need for 5v VCC and not overload the Pico. I have read the Pi Pico hardware design pages but at no point did I find reference to what the Pico could handle on the SDA SCL lines. The fact that it operates and offers a 3.3v (Out) wasn't sufficient info. What I was trying to determine was if the Pico Pins are limited for 3.3V when used to drive and output but could accommodate pulling 5v to ground? Clearly the pins have been working pulling 5v to ground, but that is not to say that doing this is within the specification of the Pico.
Based on these findings I am back looking for a way to interface the 5v requirement devices safely to the Pico hence I was considering the PCA9306 chip as an idea to bridging the connection I2C connection using 3.3v Pico side and 5v on device side. The matter is mode more complex using the devices with pull-up resisters onboard for the SDA and SCL lines as if both devices were connected to an SDA/SCL pair then I think the pull-up resistances would be in parallel which for 4.7K and 10K would equate to C. 3.2Kohm pull-ups on the lines but I am not sure if this would work yet.
While I am here, can anyone explain why the eeprom module returned 2 addresses when scanned whereas the Lcd device only returned one?
It a learning game here.....
Thank you for your investigation and suggestion. I have since gone back and probed the 2004 LCD Display and its PCF8574 pigiback board vs the PFC8574 datasheet and like you I found two 4.7K resisters as pull-up to VCC as marked on the pigiback board.
I also ran a test to scan and report for attached I2C devices with the VCC of the LCD board connected to the Pico 3.3v(OUT) pin. As I found with the 24C256 eeprom board, if I use the 3.3v for the VCC of the board then No Devices are found suggesting that the LCD display like the eeprom boards requires 5v VCC supply to function and I tested it without any jumper to force the backlight to draw power.
As neither the LCD display module or the eeprom module would function without 5v VCC applied to it, which if I am right, means that the Pico SDA and SCL pins would be draining the 5v from the pull-ups to ground for the devices to communicate over those lines. I did look at my LCD pigiback board with the PCF8574 converter chip and it has 4.7Kohm pull-up resisters on it.
That left me with an issue to fix, how to ensure both devices could operate with their need for 5v VCC and not overload the Pico. I have read the Pi Pico hardware design pages but at no point did I find reference to what the Pico could handle on the SDA SCL lines. The fact that it operates and offers a 3.3v (Out) wasn't sufficient info. What I was trying to determine was if the Pico Pins are limited for 3.3V when used to drive and output but could accommodate pulling 5v to ground? Clearly the pins have been working pulling 5v to ground, but that is not to say that doing this is within the specification of the Pico.
Based on these findings I am back looking for a way to interface the 5v requirement devices safely to the Pico hence I was considering the PCA9306 chip as an idea to bridging the connection I2C connection using 3.3v Pico side and 5v on device side. The matter is mode more complex using the devices with pull-up resisters onboard for the SDA and SCL lines as if both devices were connected to an SDA/SCL pair then I think the pull-up resistances would be in parallel which for 4.7K and 10K would equate to C. 3.2Kohm pull-ups on the lines but I am not sure if this would work yet.
While I am here, can anyone explain why the eeprom module returned 2 addresses when scanned whereas the Lcd device only returned one?
It a learning game here.....
Statistics: Posted by Stratcat — Mon Aug 25, 2025 6:16 pm