ESP8266 SPI - LCD1602 and the PFC8574 Backpack

Discussion in 'Embedded Systems and Microcontrollers' started by AgentSmithers, Oct 18, 2017.

  1. AgentSmithers

    Thread Starter Active Member

    Jan 14, 2011
    62
    0
    https://www.nxp.com/docs/en/data-sheet/PCF8574_PCF8574A.pdf
    https://images-na.ssl-images-amazon.com/images/I/51fDBZV-ebL._SY355_.jpg

    Hi Everyone,
    I don't know how well acquainted you are with the backpack PFC8574 but it seems I connected to the device via SPI.
    I then created a FOR LOOP sending predictable data to it.

    PSUDO CODE
    for (i=0x0; i<FF;i+=2)
    {
    START SPI
    Send LCD ADDRESS
    Send Data(i)
    STOP SPI
    }

    Before I ran the code I hooked a bunch of LED's to the Pin Extender and saw what order they flashed with the Hex codes and it appears that with a Pinout that soldiers to the LCD screen it looks like this (The pin outs match of course)
    http://wiki.friendlyarm.com/wiki/images/f/f1/Lcdpcb.png

    Now what I noticed is that the last 4 bits of the Data control the D4,D5,D6,D7 on the pinout to the LCD and values 9-15 (In binary) control the other three pins connect to the RS,E and RW pins on the LCD1602.

    Does anyone know any Pseudo code or very basic easy to read code that will handle just the basic functions of this translation?

    Correct me if I'm wrong but at my ESP it seems like I am going to take a string, Break it up then remove the bottom 4 bits using an AND operation, then apply the RS,E,RW pins using an or operation then send that value to the Backpack to Toggle the correct Pin's then bitshift the last 4 its of the char rinse and repeat (Of course not to forget to toggle the 'E' bit along with the last 4 bits of the char to be sent to tell the LCD to process it).

    I think I'm pretty warm but wanted to make sure if I had it right. Is it possible the orientation of the 4 bits on the High Side are backwards (MSB vs LSB)?

    Has anyone traveled down this road before?

    One more thing to add, As I was bruteforcing with the Pseudo code above it seems that the hex values don't seem to match at all from what the Datasheet says.
    https://www.nxp.com/docs/en/data-sheet/PCF8574_PCF8574A.pdf

    After reading here it seems that the hex values are all within 16 values either in the 0x4? or 0x7? range, but with the code above I am seeing that
    the LED's are turning on just like the sample values below.
    The 4 bits on the left are D4,D5,D6,D7 and the right on the right are RS,E,R/W and the 8th bit seems that it is a on or off flag to send RS,E,RW at all.
    Like the values of 1-8(DEC) in binary are moot. In order to turn on RS,E,R/W you must send values 9-15(DEC) to toggle the three LED's in any combination.
    [EXAMPLE]
    0x19 Binary 00011001
    0x29 Binary 00011001
    0x39 Binary 00011001
    0x49 Binary 00011001
    0x59 Binary 00011001
    0x69 Binary 00011001
    0x79 Binary 00011001
    0x89 Binary 00011001
    0x99 Binary 00011001
    0xA9 Binary 00011001
    0xB9 Binary 00011001
    0xC9 Binary 00011001
    0xD9 Binary 00011001
    0xE9 Binary 00011001
    0xF9 Binary 00011001

    Any help would be appreciated.
    Thank you.
     
  2. paulfjujo

    New Member

    Mar 6, 2014
    23
    4
    no chance to dialogue with your PCF8754 in SPI mode ..
    it is an I2C dialogue !
    an example of I2C LCD 4x20 with PCF8754
    check if you have a PCF8754 or a PCF8754A , they have not the same I2C adresse !
     
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