So I am attempting to tie a real time clock (DS1302) into an Arduino and then eventually a AVR chip directly. From all of the schematics I have seen, it appears a crystal is required. I read through the datasheet (above) and did not get the feeling you can run the chip without one.
Am I mistaken? I hope so as I would just like to hook this up to the Arduino and also have a coin cell battery back it up. Does this IC require a crystal or does it have its own built on board?
Yep, it needs a 32768 Hz crystal clock.
The DS1302 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or capacitors to operate.
App. Note 58: Crystal Considerations with Dallas Real-Time Clocks (RTCs)
It pays off to read a datasheet thoroughly. On page 2 this typical application circuit is shown:
This schematic shows no load capacitors, and indeed, the block schematic on page 3 shows that the capacitors are integrated.
Most RTC ICs don't have their crystal on board. I used to think that this had to do with the required size for such low frequency crystals; larger crystals typically resonate at lower frequencies. Case in point: the Maxim DS32kHz, a 32.768 kHz oscillator with integrated crystal comes in a 16-pin package, despite that only 4 pins are used.
However, recently I discovered the Abracon ABS06 32kHz crystal, which is just 2mm x 1.2mm, that's 0805 size!
The real reason for not integrating these small crystals in the RTC's package is probably cost. This crystal costs 10 times what the cylindrical can costs.
