Same program runs on Uno but not Mega

I have been working on a program on an Uno, but moved to a Mega because I outgrew the Uno.

Everything works except for the SD card reader and the RTC, both of which are on a Velleman VMA202 shield.

If I upload a simple program, like this example sketch below, it works on the Uno, but not the Mega. It isn’t using the A0IN pin, which I understand isn’t connected. What else am I missing?

// Date and time functions using a DS3231 RTC connected via I2C and Wire lib
#include <Wire.h>
#include "RTClib.h"

RTC_DS3231 rtc;

char daysOfTheWeek[7][12] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};

void setup () {

#ifndef ESP8266
  while (!Serial); // for Leonardo/Micro/Zero
#endif

  Serial.begin(9600);

  delay(3000); // wait for console opening

  if (! rtc.begin()) {
    Serial.println("Couldn't find RTC");
    while (1);
  }

  if (rtc.lostPower()) {
    Serial.println("RTC lost power, lets set the time!");
    // following line sets the RTC to the date & time this sketch was compiled
    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
    // This line sets the RTC with an explicit date & time, for example to set
    // January 21, 2014 at 3am you would call:
    // rtc.adjust(DateTime(2014, 1, 21, 3, 0, 0));
  }
}

void loop () {
    DateTime now = rtc.now();
    
    Serial.print(now.year(), DEC);
    Serial.print('/');
    Serial.print(now.month(), DEC);
    Serial.print('/');
    Serial.print(now.day(), DEC);
    Serial.print(" (");
    Serial.print(daysOfTheWeek[now.dayOfTheWeek()]);
    Serial.print(") ");
    Serial.print(now.hour(), DEC);
    Serial.print(':');
    Serial.print(now.minute(), DEC);
    Serial.print(':');
    Serial.print(now.second(), DEC);
    Serial.println();
    
    Serial.print(" since midnight 1/1/1970 = ");
    Serial.print(now.unixtime());
    Serial.print("s = ");
    Serial.print(now.unixtime() / 86400L);
    Serial.println("d");
    
    // calculate a date which is 7 days and 30 seconds into the future
    DateTime future (now + TimeSpan(7,12,30,6));
    
    Serial.print(" now + 7d + 30s: ");
    Serial.print(future.year(), DEC);
    Serial.print('/');
    Serial.print(future.month(), DEC);
    Serial.print('/');
    Serial.print(future.day(), DEC);
    Serial.print(' ');
    Serial.print(future.hour(), DEC);
    Serial.print(':');
    Serial.print(future.minute(), DEC);
    Serial.print(':');
    Serial.print(future.second(), DEC);
    Serial.println();
    
    Serial.println();
    delay(3000);
}

The shield probably uses pins A4 and A5 for I2C and 11, 12, 13 for SPI (+1 for CS). The Mega uses different pins for those buses (20, 21 for I2C and 50, 51, 52 for SPI) +1 for CS).

There is no SD card code in the posted sketch.

groundFungus:
The shield probably uses pins A4 and A5 for I2C and 11, 12, 13 for SPI (+1 for CS). The Mega uses different pins for those buses (20, 21 for I2C and 50, 51, 52 for SPI) +1 for CS).

There is no SD card code in the posted sketch.

That code was just an example. I have attached my actual code. I’ll check out your suggestion. When I examined the pinouts I didn’t see any difference, but I’ll check again. Thanks!

Indoor_Temp_Humidity_v4.ino (9.11 KB)

When I examined the pinouts I didn't see any difference

What, you don't see any difference between Uno and Mega pinouts? Look again at the locations of SCL, SDA, SCK, MOSI, MISO and CS.

groundFungus:
What, you don't see any difference between Uno and Mega pinouts? Look again at the locations of SCL, SDA, SCK, MOSI, MISO and CS.

OK, now that I do a deeper look I see the difference. When comparing the boards side-by-side there is no labeled difference. Thanks for making me aware of this!