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Setting the I2C address - Funduino LCM1602 I2C Backpack

Discussion in 'Microcontrollers, Programming and IoT' started by bigone5500, Aug 22, 2015.

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  1. bigone5500

    bigone5500

    712
    120
    Apr 9, 2014
    I looked all over the web for information pertaining to this board as to if I could change the I2C address on it. I could not find my specific one so I loaded an I2C scanner sketch and tested what I thought were address jumpers on the back of the module. There are 6 holes in a 3x2 pattern which can accept a pin header or you can simply use a section of 22ga wire to jumper it.

    Here is my board:

    2015-08-22 08.49.56.jpg

    I will represent the jumpers by brackets. The jumpers are installed in columns. For example: to set 0x24, you will jumper column 1 and column 2.

    0x27
    [] [] []
    [] [] []

    0x26
    [/] [] []
    [/] [] []

    0x25
    [] [/] []
    [] [/] []

    0x24
    [/] [/] []
    [/] [/] []

    0x23
    [] [] [/]
    [] [] [/]

    0x22
    [/] [] [/]
    [/] [] [/]

    0x21
    [] [/] [/]
    [] [/] [/]

    0x20
    [/] [/] [/]
    [/] [/] [/]

    I hope this can help someone else.
     
  2. AnalogKid

    AnalogKid

    2,425
    690
    Jun 10, 2015
    If you look at the traces on the pc board, you see that they go to pins 1, 2, and 3 of a PCF8574. This is an I2C I/O expander. The datasheet shows that it has three address pins (1, 2, and 3), and how they fit into the I2C address space. The datasheet does not mention internal pull up or pull down resistance, so an unconnected pin might be a problem. Please show a photo of the bottom side of the board, because there might be resistors there that are an important part of your question.

    ak
     
  3. bigone5500

    bigone5500

    712
    120
    Apr 9, 2014
    I cannot take a photo of the bottom as it is soldered in. However, a quick look underneath shows what appear to be 3 SMD resistors mounted just to the left of the 6 holes. The bottom 3 holes are all common and the top ones go to pins. The common trace goes to the backside through a via and is connected to a resistor. Whether or not the single resistor takes care of the pull up or pull down, I am unsure.

    *edit*

    After more investigation I am certain that the single resistor takes care of the pull up/down. The top 3 connections go straight to pins on the expander. I came to this conclusion by holding the board to a light and shining through from the underside so as to see the top traces.
     
    Last edited: Aug 22, 2015
  4. AnalogKid

    AnalogKid

    2,425
    690
    Jun 10, 2015
    What probably is the case is that the three underside components are either pull up or pull down resistors for the three address pins, and the jumpers pull the pins to the opposite rail. This is important because the direction the jumpers pull the pins sets the logic polarity.

    ak
     
  5. bigone5500

    bigone5500

    712
    120
    Apr 9, 2014
    It could be. I tried once to desolder this board so I could solder female pin headers on it and use it on other boards but could not get the solder to give up. Otherwise I could have gotten a photo of it.
     
  6. bingolotto

    bingolotto

    1
    0
    Nov 25, 2015
    This is the backside and front-side. Resistor seems 10k,Maybe it helps a bit.
     

    Attached Files:

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