LCD-Display showing wrong numbers

Question

Hy guys,

I am having an issue with my new LCD16x2 display. I connected a potentiometer and read the values. The values are properly read and send to Serial. I also display the values on the display. I am having no issues when increasing the values, but when I go down again at a certain point the displays doesn't display the correct value anymore, but on the Serial I can still see the right readings.

I suppose this has something to do with the display still storing previous values for this digit in the buffer and when I go down the values are not properly reset. But I don't wan't to clear the whole buffer, since then the whole thing just blinks every iteration.

Here's my code

// ESC_Calibration.ino
#include <Wire.h>
#include <Adafruit_MCP23017.h>
#include <Adafruit_RGBLCDShield.h>
#include <Servo.h>

#define WHITE 0x7

Servo firstESC;
Servo secondESC;
int throttlePin = 2;
int firstESCPin = 2;
int secondESCPin = 3;

Adafruit_RGBLCDShield lcd = Adafruit_RGBLCDShield();

void setup() {
    Serial.begin(9600);

    lcd.begin(16, 2);
    lcd.setBacklight(WHITE);
    lcd.setCursor(0, 0);
    lcd.print("throttle:");

    firstESC.attach(firstESCPin);
    secondESC.attach(secondESCPin);
}

void loop() {
    int throttle = analogRead(throttlePin);
    throttle = map(throttle, 0, 1023, 0, 179);

    lcd.setCursor(0, 1);
        Serial.println(throttle);
        lcd.setCursor(0, 1);
    lcd.print(throttle);

    firstESC.write(throttle);
    secondESC.write(throttle);

    delay(500);
}

Does anyone have a suggestion how to improve this and fix it, so it'll display the right readings, but doesn't blink?

Thanks for your help in advance! :-)

Answer 1

As you have surmised, the problem is that the LCD retains what was there before and you need to clear it away.

That means, as the numbers increase you get, say,

4
10
48
89
194
309

etc.

When they go down though, because the LCD remembers what was there, only the characters you print will be replaced:

309 (309)
194 (194)
894 (89 - the 4 remains from before)
484 (48 - the 4 remains from before)
104 (10 - the 4 remains from before)
404 (4 - the 04 remains from before)

Also, as you have noticed, clearing the whole screen is not very nice since it makes it flash.

To combat that it is important that you only overwrite the characters that you need to overwrite and leave those that want to stay. There's two common ways of doing this:

1. Append enough spaces after the number to force an overwrite: lcd.print(throttle); lcd.print(" "); // 3 spaces should be enough for 3 extra digits

2. Format the data with snprintf(): char temp[5]; // enough room for 4 numbers and one NULL character snprintf(temp, 5, "%4d", throttle); // %4d = 4 digits, right aligned lcd.print(temp);

The latter method, though it is more complex and uses more resources, can be preferred because it creates a much nicer output with the numbers properly right-aligned. Also arbitrarily adding spaces after the number could overwrite things you want to keep.

You could also achieve a similar result doing it manually:

if (throttle < 10) lcd.print(" "); // 0-9 add one space
if (throttle < 100) lcd.print(" "); // 0-99 add one space
if (throttle < 1000) lcd.print(" "); // 0-999 add one space
lcd.print(throttle);

Just like with cats, there's many ways to skin this problem...