Dica:
Capacitores = < 10 pF tem Coeficiente NP0 (zero) , pinta preta
Alguns Cristais tem Capacitores Internos.
Da Microchip:
Real Time Clock IC : 32kHz oscillator
Surprisingly making an accurate 32kHz oscillator is a difficult task (much more than a high speed oscillator e.g. a Mhz crystal oscillator). This is because low speed oscillator drivers are designed for low power operation. That means high impedance and therefore low current which makes the driver extremely sensitive to noise (or any nearby signals which can capacitively couple to the crystal wire).
Using the DS1307 lets you put the crystal in the least noisy part of the board. In addition it sets the crystal load capacitance which is critical in making the crystal oscillate at exactly 32kHz - controlling its initial error i.e. for the specified ppm error value the load capacitance must be exact.
Note: A common way of calibrating a crystal (not in this project) is crystal pulling or changing the capacitance at one crystal pin relative to the other - so load capacitance is crucial.
The DS1307 loads the crystal with 12.7pF so you need to buy a crystal that is defined to use this load capacitance. Circuit layout also affects the capacitance at the crystal pins so you must keep the crystal as close as possible to the chip and the tracks from crystal to chip must be short.
To ensure the crystal oscillates correctly you must ensure that :
Crystal uses 12.7pf load capacitance (correct crystal type).
The crystal is close to the IC.
The tracks are short.
The chip supply has lots of decoupling (capacitors from +5V to GND). e.g. A 100n and a 10n
There are no signal tracks near to the crystal.
For a pcb: It has a guard ring and a ground plane and away from digital signals.
If you are doing a board layout there is good advice in the PIC 16F88 datasheet (Timer 1 section) on crystal pcb guard rings. Dallas recommends Application note 58 which I have not read yet.
Abraço e não percam a hora.
