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I am planning on using a 32.768KHz crystal as a real-time clock and connecting this to a pic microcontroller via the SOSCI and SOSCO The Crystal is a Seiko SSP‑T6 32.768KHz watch crystal in 8 × 2.1 × 1.6mm surface‑mount package. 20ppm accuracy. 7pF load capacitance. 55KΩ series resistance.

SSP-T6 Crystal
Here's a similar datasheet link

Now The datasheet recommends the following setup with a resistor R of 100K (see image below): Dspic 32.768 Khz Crystal recommendation

On the Internet I have seen designs whereby the resistor value is in the Mega Ohms range. Can anyone answer the following questions:
A) Whether a watch crystal is appropriate in this design.
B) What the purpose of R is here and whether a value of 100K is recommended as opposed to say using a 20M Ohm resistor?
C) What would happen if the resistor was left out as is the case for higher value crystals?

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The watch crystal is fine.

The resistor limits the power to the crystal. If you look at the datasheet you'll see maximum drive level is 1 µW, for other crystals it's even 0.5 µW. So I wouldn't decrease the value or you may destroy the crystal. The drive level is different for each manufacturer, so there's no solid rule for the resistor's value. I would follow Microchip's advice.

A larger value may look even safer, but then the power will be too low, and there's the chance that the oscillator won't start. At 20 MΩ that will definitely be the case.

Isn't it possible that the MΩ resistors you saw were between output and input of the oscillator?

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  • \$\begingroup\$ Isn't it possible that the MΩ resistors you saw were between output and input of the oscillator?....Well I've been doing a lot of research and reading on Pierce oscillators recently so it may well be the case...Just did a quick browse on the web... saw a design with R=510K not 20M Ohm... \$\endgroup\$ – Peter H Aug 16 '12 at 12:08
  • \$\begingroup\$ @Peter - That resistor is for DC feedback, which biases the input DC level. A value of 1 to a few M\$\Omega\$ is typical for CMOS oscillators, for TTL it has to be much lower (a few k\$\Omega). \$\endgroup\$ – stevenvh Aug 16 '12 at 12:13
  • \$\begingroup\$ Thanks Steve..will stick with the standard 100K resistor as per your recommendation and Microchip's. \$\endgroup\$ – Peter H Aug 16 '12 at 12:55

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