strange strange problem with LCD - reset the arduino

Hello ,
I have connected my LCD that work on SCL\SDA
to A4\A5 on my Pro-Mini card.

even if it doesn’t have power , jsut A4 - connedted to SDA (LCD) and A5 connected to SCL
the card won’t start
when I disconnect it it start

I ahve try with 3 differnet (working) LCD
and aslo 3 new Pro-Mini
when I disconnect the A4\A5 is start O.K
also I have try many wires

this is the setup of the code
it wont even start it

#include <SoftwareSerial.h>
#include <Wire.h>
#include <LiquidCrystal_I2C.h>
SoftwareSerial RBSerial(7, 8);

String rx_str = "";
char rx_byte = 0;

///which words I can use for comparing
char OK[] = "OK";
char YES[] = "YES";
char NO [] = "NO";
boolean cmpr = false;



LiquidCrystal_I2C lcd(0x27, 16, 2); // set the LCD address to 0x20 for a 16 chars and 2 line display

static byte ndx = 0;
const byte numChars = 32;
char receivedChars[numChars];  // an array to store the received data
char printChars[numChars];

boolean newData = false;
boolean printData = false;
void setup()
{
  pinMode(5, OUTPUT);
  RBSerial.begin(9600);
  Serial.begin(9600);
  lcd.begin();
  Serial.println("Starting.....");
  digitalWrite(5, HIGH);
  lcd.setCursor(0, 0);
  lcd.print("Starting.....");
  for (int i = 0; i < 10; i++)
  {
    Serial.println("**");
    delay (100);
  }
  for (int i = 0; i < 3; i++)
  {
    lcd.backlight();
    delay(250);
    lcd.noBacklight();
    delay(250);
  }
  lcd.backlight(); // finish with backlight on
  lcd.setCursor(0, 1);
  lcd.print("Done!");
  Serial.println(F(__FILE__)); //print the file name
  Serial.println( "Done!");
  digitalWrite(5, LOW);
  delay (2000);
  lcd.clear();
}

what could it be ?

Hi,
Link to the specs for the LCD please.

even if it doesn't have power , jsut A4 - connedted to SDA (LCD) and A5 connected to SCL
the card won't start
when I disconnect it it start

If what doesn't have power, the LCD will not work without power.
When you disconnect what, what starts.

Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Thanks... Tom.. :slight_smile:

5V or 3.3V ProMini? 5V or 3.3V LCD display (link to product / datasheet can be useful).

No experience with I2C so can say if you're doing something wrong. How do you know that it does not start? Not seeing anything on the display does not necessarily mean that it does not start. Maybe add a simple blink-a-led in the beginning of setup.

even if it doesn't have power

If what does not have power?

http://www.ebay.com/itm/5V-Serial-IIC-I2C-TWI-1602-LCD-Module-Display-Arduino-UNO-R3-MEGA2560-Nano-Due-/351348064265?hash=item51cdfa0009

http://www.ebay.com/itm/Pro-Mini-Atmega328-5V-16M-Micro-controller-Board-for-Arduino-Compatible-Nano-new-/221481619677?hash=item33915550dd

as you can see I have put hte setup\loop a blink to the LCD - so I know it doesn't work .

when I turn on the power - the LCD goes blue (So it's working )
simple connection
A4 - SDA
A5 - SCL
VCC - 5V
GND - GND

this is he connection I have ,
now if I disconnect A4 \ A5 - the Pro-Mini is working (I can see on the serial that startup)
but when I put the cable between A4\A5 to the LCD and reset the Arduino - it won't start

:expressionless:

You need a 4K7 pull up resistor on each of the I2C lines, that is SDA and SCL.

Never connect an active signal to anything that is unpowered, that is one of the best ways of damaging things.

the LCD goes blue (So it's working )

No that is no indication it is working. Just that you are getting power to it.

You need to post a schematic.

but up until now , I didn't use any pullup resistors - and everything was working
isn't the device was inside allready the pullup inside?

Thanks ,

david1234:
but up until now , I didn't use any pullup resistors - and everything was working
isn't the device was inside allready the pullup inside?

Yes, the backpack module you cited already has pull-up resistors.

What can cause problems, is connecting a number of modules which each have pull-up resistors, in common, in which case you would actually need to remove the pull-ups from most of them.

Maybe I don't understand - so easy :slight_smile:

I only connect the LCD no other models are connected to the ARD

the schematic is very simple :

Arduino mini LCD
Pin A4 - - SDA
Pin A5 - - SCL

VCC - 5V (to external power supply + 5V )
GND - GND (to external power supply GND)

Hi,
Can you post a picture of your project for us please?

Thanks.. Tom.. :slight_smile: