Arduino EEPROM seems flaky, should I write twice?

Question

I am used to programming PC's and smartphones using high level languages, microcontrollers are a new territory for me. Are they somehow different, more untrustworthy, requiring different techniques? Here is bit of code to write and read to EEPROM running on a Arduino Mega: (there is an Ethernet Shield attached, not used here)

#include <EEPROM.h>
int addr = 0;
int val;
byte value;
void setup()
{
  Serial.begin(9600);
}
void loop()
{
  val = 9;
  EEPROM.write(addr, val);
  delay(500);
  addr = addr + 1;
  if (addr == 20) addr = 0;
  value = EEPROM.read(addr);
  Serial.print(addr);
  Serial.print("\t");
  Serial.print(value);
  Serial.println();  
}

Heres what comes out:

1   91
2   91
3   9
4   9
5   9
6   9
7   9
8   9
9   9
10  9
11  9
12  202
13  202
14  202
15  202
16  202
17  202
18  202
19  202
0   9
1   89
2   91
3   9
4   9
5   9
6   9
7   9
8   9
9   9
10  9
11  9
12  9
13  9
14  9
15  9
16  9

..... In general address 1 and 2 are always flaky and it takes two writes to change memory locations above ~10.

I can switch out another board and still get similar oddities.

How can I adapt my programming to this seemingly flaky performance?

Answer 1

Simply enough, your code is wrong.

Logically step through it. You are writing to an EEPROM at address addr. You then wait 500ms, increment addr, and then read from the new addr. The addr you read from is therefore not the addr you write to.