Python I2C LCD Driver displays weird characters

Question

I have a 2 x 16 LCD display with the I2C piggy back adapter. As a simple start I'm just displaying the encoder value. However, when I turn the encoder I get weird text written on the lcd in different areas. Also sometime the display recovers and just displays "Encoder: X" with an empty second line:

The code

#!/usr/bin/python3 -u

import time

import RPi.GPIO as GPIO

import lcd_i2c
from encoder import Encoder

GPIO.setmode(GPIO.BCM)

switch_pin = 13
encoder_down_pin = 6
encoder_up_pin = 5

GPIO.setup(switch_pin, GPIO.IN, pull_up_down=GPIO.PUD_UP)

encoder_value = 0

mylcd = lcd_i2c.lcd()


def valueChanged(value):
    encoder_value = value
    print(encoder_value)

    mylcd.lcd_clear()
    mylcd.lcd_display_string(f"Encoder: {encoder_value}", 1)


e1 = Encoder(encoder_down_pin, encoder_up_pin, callback=valueChanged)
while True:
    time.sleep(60)

# lcd_i2c.py
# -*- coding: utf-8 -*-
# Original code found at:
# https://gist.github.com/DenisFromHR/cc863375a6e19dce359d

"""
Compiled, mashed and generally mutilated 2014-2015 by Denis Pleic
Made available under GNU GENERAL PUBLIC LICENSE

# Modified Python I2C library for Raspberry Pi
# as found on http://www.recantha.co.uk/blog/?p=4849
# Joined existing 'i2c_lib.py' and 'lcddriver.py' into a single library
# added bits and pieces from various sources
# By DenisFromHR (Denis Pleic)
# 2015-02-10, ver 0.1

"""

# i2c bus (0 -- original Pi, 1 -- Rev 2 Pi)
I2CBUS = 1

# LCD Address
ADDRESS = 0x27

import smbus
from time import sleep

class i2c_device:
   def __init__(self, addr, port=I2CBUS):
      self.addr = addr
      self.bus = smbus.SMBus(port)

# Write a single command
   def write_cmd(self, cmd):
      self.bus.write_byte(self.addr, cmd)
      sleep(0.0001)

# Write a command and argument
   def write_cmd_arg(self, cmd, data):
      self.bus.write_byte_data(self.addr, cmd, data)
      sleep(0.0001)

# Write a block of data
   def write_block_data(self, cmd, data):
      self.bus.write_block_data(self.addr, cmd, data)
      sleep(0.0001)

# Read a single byte
   def read(self):
      return self.bus.read_byte(self.addr)

# Read
   def read_data(self, cmd):
      return self.bus.read_byte_data(self.addr, cmd)

# Read a block of data
   def read_block_data(self, cmd):
      return self.bus.read_block_data(self.addr, cmd)


# commands
LCD_CLEARDISPLAY = 0x01
LCD_RETURNHOME = 0x02
LCD_ENTRYMODESET = 0x04
LCD_DISPLAYCONTROL = 0x08
LCD_CURSORSHIFT = 0x10
LCD_FUNCTIONSET = 0x20
LCD_SETCGRAMADDR = 0x40
LCD_SETDDRAMADDR = 0x80

# flags for display entry mode
LCD_ENTRYRIGHT = 0x00
LCD_ENTRYLEFT = 0x02
LCD_ENTRYSHIFTINCREMENT = 0x01
LCD_ENTRYSHIFTDECREMENT = 0x00

# flags for display on/off control
LCD_DISPLAYON = 0x04
LCD_DISPLAYOFF = 0x00
LCD_CURSORON = 0x02
LCD_CURSOROFF = 0x00
LCD_BLINKON = 0x01
LCD_BLINKOFF = 0x00

# flags for display/cursor shift
LCD_DISPLAYMOVE = 0x08
LCD_CURSORMOVE = 0x00
LCD_MOVERIGHT = 0x04
LCD_MOVELEFT = 0x00

# flags for function set
LCD_8BITMODE = 0x10
LCD_4BITMODE = 0x00
LCD_2LINE = 0x08
LCD_1LINE = 0x00
LCD_5x10DOTS = 0x04
LCD_5x8DOTS = 0x00

# flags for backlight control
LCD_BACKLIGHT = 0x08
LCD_NOBACKLIGHT = 0x00

En = 0b00000100 # Enable bit
Rw = 0b00000010 # Read/Write bit
Rs = 0b00000001 # Register select bit

class lcd:
   #initializes objects and lcd
   def __init__(self):
      self.lcd_device = i2c_device(ADDRESS)

      self.lcd_write(0x03)
      self.lcd_write(0x03)
      self.lcd_write(0x03)
      self.lcd_write(0x02)

      self.lcd_write(LCD_FUNCTIONSET | LCD_2LINE | LCD_5x8DOTS | LCD_4BITMODE)
      self.lcd_write(LCD_DISPLAYCONTROL | LCD_DISPLAYON)
      self.lcd_write(LCD_CLEARDISPLAY)
      self.lcd_write(LCD_ENTRYMODESET | LCD_ENTRYLEFT)
      sleep(0.2)


   # clocks EN to latch command
   def lcd_strobe(self, data):
      self.lcd_device.write_cmd(data | En | LCD_BACKLIGHT)
      sleep(.0005)
      self.lcd_device.write_cmd(((data & ~En) | LCD_BACKLIGHT))
      sleep(.0001)

   def lcd_write_four_bits(self, data):
      self.lcd_device.write_cmd(data | LCD_BACKLIGHT)
      self.lcd_strobe(data)

   # write a command to lcd
   def lcd_write(self, cmd, mode=0):
      self.lcd_write_four_bits(mode | (cmd & 0xF0))
      self.lcd_write_four_bits(mode | ((cmd << 4) & 0xF0))

   # write a character to lcd (or character rom) 0x09: backlight | RS=DR<
   # works!
   def lcd_write_char(self, charvalue, mode=1):
      self.lcd_write_four_bits(mode | (charvalue & 0xF0))
      self.lcd_write_four_bits(mode | ((charvalue << 4) & 0xF0))

   # put string function with optional char positioning
   def lcd_display_string(self, string, line=1, pos=0):
    if line == 1:
      pos_new = pos
    elif line == 2:
      pos_new = 0x40 + pos
    elif line == 3:
      pos_new = 0x14 + pos
    elif line == 4:
      pos_new = 0x54 + pos

    self.lcd_write(0x80 + pos_new)

    for char in string:
      self.lcd_write(ord(char), Rs)

   # clear lcd and set to home
   def lcd_clear(self):
      self.lcd_write(LCD_CLEARDISPLAY)
      self.lcd_write(LCD_RETURNHOME)

   # define backlight on/off (lcd.backlight(1); off= lcd.backlight(0)
   def backlight(self, state): # for state, 1 = on, 0 = off
      if state == 1:
         self.lcd_device.write_cmd(LCD_BACKLIGHT)
      elif state == 0:
         self.lcd_device.write_cmd(LCD_NOBACKLIGHT)

   # add custom characters (0 - 7)
   def lcd_load_custom_chars(self, fontdata):
      self.lcd_write(0x40);
      for char in fontdata:
         for line in char:
            self.lcd_write_char(line)

# encoder.py
# Class to monitor a rotary encoder and update a value.  You can either read the value when you need it, by calling getValue(), or
# you can configure a callback which will be called whenever the value changes.

import RPi.GPIO as GPIO

class Encoder:

    def __init__(self, leftPin, rightPin, callback=None):
        self.leftPin = leftPin
        self.rightPin = rightPin
        self.value = 0
        self.state = '00'
        self.direction = None
        self.callback = callback
        GPIO.setup(self.leftPin, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)
        GPIO.setup(self.rightPin, GPIO.IN, pull_up_down=GPIO.PUD_DOWN)
        GPIO.add_event_detect(self.leftPin, GPIO.BOTH, callback=self.transitionOccurred)
        GPIO.add_event_detect(self.rightPin, GPIO.BOTH, callback=self.transitionOccurred)

    def transitionOccurred(self, channel):
        p1 = GPIO.input(self.leftPin)
        p2 = GPIO.input(self.rightPin)
        newState = "{}{}".format(p1, p2)

        if self.state == "00": # Resting position
            if newState == "01": # Turned right 1
                self.direction = "R"
            elif newState == "10": # Turned left 1
                self.direction = "L"

        elif self.state == "01": # R1 or L3 position
            if newState == "11": # Turned right 1
                self.direction = "R"
            elif newState == "00": # Turned left 1
                if self.direction == "L":
                    self.value = self.value - 1
                    if self.callback is not None:
                        self.callback(self.value)

        elif self.state == "10": # R3 or L1
            if newState == "11": # Turned left 1
                self.direction = "L"
            elif newState == "00": # Turned right 1
                if self.direction == "R":
                    self.value = self.value + 1
                    if self.callback is not None:
                        self.callback(self.value)

        else: # self.state == "11"
            if newState == "01": # Turned left 1
                self.direction = "L"
            elif newState == "10": # Turned right 1
                self.direction = "R"
            elif newState == "00": # Skipped an intermediate 01 or 10 state, but if we know direction then a turn is complete
                if self.direction == "L":
                    self.value = self.value - 1
                    if self.callback is not None:
                        self.callback(self.value)
                elif self.direction == "R":
                    self.value = self.value + 1
                    if self.callback is not None:
                        self.callback(self.value)

        self.state = newState

    def getValue(self):
        return self.value

Credits for the i2c lcd script go to its author DenisFromHR

Answer 1

Your LCD module is internally using a 4-bit interface mode; each byte of command or data is written in two 4-bit chunks. There's no way to tell which chunk is expected next, and no way to force the start of a new byte other than completely reinitializing the LCD - you just have to be careful to always send the chunks in pairs. If you ever lose one of the chunks, the LCD is going to display gibberish from that point onward (until another chunk gets lost), since the bytes it's receiving would consist of halves from two different bytes that were sent.

And that's exactly what's going wrong here. This is the first few bytes of the "Encoder:" message, in hex:

45 6E 63 6F 64 65 72 3A

And this is the start of the repeated garbage string you're getting, again in hex:

56 E6 36 F6 46 57 23 A2

Note that it's exactly the same data, just shifted over by one hex digit.

I see no obvious reason why this would be happening, but the first thing I'd try is increasing those sleep() s in lcd_strobe() by a factor of 100 or so, in case they're not long enough for the chunk to be reliably read by the LCD module.

Answer 2

I did get rid of the issue. It seems to be a problem to update the lcd from within a callback. So how I solved the issue is by merely changing the state of the program and to regularly check for updates in that state and then display the results accordingly:

#!/usr/bin/python3 -u

import os
import time

import RPi.GPIO as GPIO

import lcd_i2c
from encoder import Encoder

GPIO.setmode(GPIO.BCM)

os.system('say start')

switch_pin = 13
encoder_down_pin = 6
encoder_up_pin = 5
encoder_value = 0
dirty = True
pressed = True
mylcd = lcd_i2c.lcd()


def switch_pressed(v):
    global pressed, dirty
    print("OK")
    pressed = True
    dirty = True


GPIO.setup(switch_pin, GPIO.IN, pull_up_down=GPIO.PUD_UP)
GPIO.add_event_detect(switch_pin, GPIO.FALLING)
GPIO.add_event_callback(switch_pin, switch_pressed)


def update():
    global encoder_value, dirty, pressed

    if not dirty: return
    mylcd.lcd_clear()
    mylcd.lcd_display_string(f"Encoder: {encoder_value}", 1)

    if pressed:
        mylcd.lcd_display_string("Pressed", 2)
        pressed = False
    dirty = False


def valueChanged(value):
    global encoder_value
    global dirty

    dirty = True
    encoder_value = max(0, value)
    print(encoder_value)

e1 = Encoder(encoder_down_pin, encoder_up_pin, callback=valueChanged)
update()

try:
    while True:
        update()
        time.sleep(.5)

finally:
    print("Cleanup")
    GPIO.cleanup()

I'm using a variable called dirty to determine if the display should be updated. This prevents the display from flickering.

Answer 3

Does that piggyback use 5v logic levels or 3.3v? This looks to me like either

• a floating clock/data line(s) (maybe check with a scope whether they are correctly pulled up by the Pi's pullup resistors)
• mismatching logic levels. It the device expects "high" to be 5v, but the RPi pulls it up to 3.3v, strange behaviour can occur when the bus is idle, as the device may interpret the 3.3v as if the master pulled the bus low.

This document describes what you should expect electrically