Two versions of a screen showing depending on criteria value

[ZivojinM]

As a newbie I feel very proud that I managed to make my GPS device show all the relevant data and everything pixel perfect, the part of the sketch I posted here for help is just a very simplified excerpt of it.

This is the obstacle I encountered - when GPS signal is normal, the screen shows "interface" that I drew and the appropriate values from the signal. However, every now and then the signal gets lost completely.

In this situation, I would like to erase the entire screen and just print "No signal" on the screen - and then as soon as the signal is picked back up again - revert back to the default screen. Sorry if this is a basic problem but I'm very new at this so hence my asking. Also, bonus question - not crucial but if anyone has the time - how would I make a hello message appear for 3 seconds only once, when I power up the Arduino? Thank you for your time :)

void loop()
{

	if (gps.location.isValid())
	...
	
	u8g2.firstPage();
	do { 
	
		if (gps.location > x)
		{
		
		u8g2.setFont(u8g2_font_UnnamedDOSFontIV_tr);
		u8g2.setCursor(40, 30);
		u8g2.print("gps.value");
                u8g2.drawFrame(...);
		
		}
	
	} 
	while ( u8g2.nextPage() );
	
	if (gps.sentencesWithFix() < 1)
	Serial.println(F("No signal."));
	
}

[olikraus]

You need to use a state machine, see for example here: https://www.embedded.com/implementing-finite-state-machines-in-embedded-systems/ or also here: https://www.embedded.com/programming-embedded-systems-the-easy-way-with-state-machines/

State machines are the key to solve your problems. Try to draw a suitable state diagram first, then implement the diagram by using the approaches mentioned in the above web pages.

Taking your above description, I assume you will have three states:

• Intro Screen
• GPS available
• GPS no signal

Probably you will need to number your states, like this (the number itself is not important):

State	Internal Number
Intro Screen	5
GPS available	10
GPS no signal	15

Your code will look like this:

int state = 5;   // initial state: Intro Screen

void loop(void) {
  switch(state) {
    case 5: // Intro Screen
      break;
    case 10: // GPS available
      break;
    case 15: // GPS no signal
      break;
    default: // unknown state, reset state to inital state:
     state = 5;
     break;
  }
}

Now, you need to do some actions here, like displaying the GPS signal or the "Hello" message.
Let's have a look into the "Hello" Message. Always use a sub procedure to draw this:

void drawHello(void) {
	u8g2.setFont(u8g2_font_UnnamedDOSFontIV_tr);
	u8g2.firstPage();
	do { 
		u8g2.setCursor(40, 30);
		u8g2.print("Hello");
	} 
	while ( u8g2.nextPage() );
}

Then obviously you will call this function:

int state = 5;   // initial state: Intro Screen

void loop(void) {
  switch(state) {
    case 5: // Intro Screen
      drawHello();
      break;
    case 10: // GPS available
      break;
    case 15: // GPS no signal
      break;
    default: // unknown state, reset state to inital state:
     state = 5;
     break;
  }
}

Now the question is, how to display your screen only for 3 seconds. Let me split the "Intro Screen" activity into two steps:

1. Draw "Hello"
2. Wait for 3 seconds

We will need one more state:

State	Internal Number	Description
Draw Intro Screen	5	Assign millis() to "t", goto state 6
Wait Intro Screen	6	if t+3000 > millis() goto 10 or 15
GPS available	10
GPS no signal	15

Here is a code example for this:

int state = 5;   // initial state: Intro Screen
long t = 0;

void loop(void) {
  switch(state) {
    case 5: // Draw Intro Screen
      drawHello();
      t = millis();    // remember the current time
      state = 6:     // jump to the Wait Intro Screen 
      break;
    case 6:  // Wait Intro Screen
     if ( t + 3000 > millis() ) {   // if target time is larger then the current time, then jump to the next state
      if ( isGPSSignalAvailable() )
        state = 10;  // jump to GPS available
      else
        state = 15; // jump to GPS no signal
     } // else: stay in state 6 and wait until target time is reached
     break;
    case 10: // GPS available
      break;
    case 15: // GPS no signal
      break;
    default: // unknown state, reset state to inital state:
     state = 5;
     break;
  }
}

Implementation for state 10 and 15 are missing, but I hope you got the idea here. Feel free to post your suggestion on the remaining states.

[ZivojinM]

I'll study this response - many thanks for such a detailed response!