Chris, thank you for your help. The Adafruit DS1307 shield does have two pullup resistors, 2.2 kOhm each. I just double checked it. So yes, I was hooking up the DS1307 shield to Due with the 2.2 kOhm external pullup resistors and it worked, but yes, I don't know how long the Due would have lasted, and I want it to last for a long time..
In the Raspberry Pi pdf, page 4, I found this information, apparently Pi is also 3.3 volt system:
To make the job really easy, we'll use the the Adafruit
DS1307 RTC Breakout Board Kit (http://adafru.it/264)-
it comes with all the parts you need,
even a coin battery!
The Kit does require a little light soldering. In theory you could use all the parts and build them
onto a breadboard, but the coin holder is a little difficult since its not breadboard-friendly, so
please go ahead and build the kit. (http://adafru.it/aPf
When building the kit, leave o ut the 2.2K&o hm; resisto rs - by leaving them o ut,
we fo rce the RTC to communicate at 3.3V instead o f 5V, which is better fo r the
Wiring is simple:
1. Connect VCC on the breakout board to the 5.0V pin of the Pi
2. Connect GND on the breakout board to the GND pin of the Pi
3. Connect SDA on the breakout board to the SDA0 pin of the Pi
4. Connect SCL on the breakout board to the SCL0 pin of the Pi
DONT FORGET TO POWER IT FROM 5V - THIS IS FINE AND SAFE FOR THE PI - JUST
MAKE SURE TO REMOVE THE 2.2K PULLUPS AS MENTIONED IN THE LARGE RED
BOX UP ABOVE TO KEEP THE I2C PINS AT 3.3V LOGIC LEVELS
The diagrams below show the 2.2KΩ resistors in place - but plleasse rremove tthem
eiittherr by nott ssollderriing tthem iin orr cclliippiing tthem outt iiff you diid ssollderr tthem iin!! Thiiss
way,, we''llll usse tthe Pii''ss 1..8K pullll--up rressiissttorrss tto 3..3V
This is the clearest explanation I have found so far of what to remove and where. So I am going to remove the two 2.2 kOhm resistors from the Adafruit DS1307 shield.