I've seen that a lot of clocks use a 32.768KHz crystal. I'm curious why that is exactly. I am working on a binary clock project with some 4520 ICs. I have some 555 timers too, but I want to use a crystal if I can.
I just don't know what would be the most accurate. I also don't know anything about the configuration. For instance, does the 32.768KHz need to be used with resistors or capacitors to operate?
a lot of clocks use a 32.768KHz crystal. I'm curious why that is exactly.
This particular crystal is optimized for use in clocks. The low frequency operation consumes less power than oscillators that run in the Megahertz. The small size, unusual for a low-frequency crystal, allows it to be used in watches. And the chosen frequency can be repeatedly divided by 2 until an exact 1-second time period is obtained.
The extra load capacitance isn't required to make it oscillate. It's required to bring the crystal exactly on frequency.
When divided by 2^15 it gives 1s.
Most 32.768 kHz oscillators on MCUs have built-in capacitors. You just have to select a crystal with the correct load capacitor value, which will be in the MCU data sheet, to ensure accurate timekeeping.
The smallest PIC with a suitable timer for a 32.768 kHz oscillator appears to be the 8 pin 12F675.
$$32768=2^{15}$$
Some PICs of the PIC16 series have an 8-bit timer which gives an interrupt on an overflow. The minimum prescale value is 1:128 and \$128=2^7\$. So with a 1:128 prescaler, you'll get an interrupt in every \$128\cdot2^8=2^7\cdot2^8=2^{15}=32768\$ pulses. At 32.768kHz, that's exactly once per second. I suspect other chips work with similar values.
You absolutely should use a crystal when designing a clock. A 555 isn't stable at all. Little temperature variations will cause huge timing variations.
It depends on the chip you're using how you should implement the crystal. Most PICs will want you to connect it directly to the OSC pins, with small capacitors to ground. In case you just want an oscillating signal, use a crystal oscillator like here.
I've designed a pcb for a clock once. I used an RTC chip there which used a 32khz crystal. I drew pads for capacitors but it turned out that I never needed it. You'll have to look for the needed capacitance in the datasheet of the RTC you are using. I used the MCP79410 RTC mainly because I could get it for less than 1$ and it has an alert open drain interrupt and I liked the small package but there is nothing very special about this specific RTC. Use any that can do whatever you need to do.
I chose the RTC because a uC only version would have drawn more current. the RTC chips are optimized vor very low power consumption.
You should definitely use a crystal since everything else will run out of sync quite fast (especially 555 circuits which are good for some applications but not as stable clock sources)
