Caps values for 25MHz crystal?

I need help to reverse engineer the crystal part of a router (ASUS WL-520gU). A little history. I use a router with openwrt on it to do some linux stuff. The clock was getting off by 10 seconds a minute so... I decided that the crystal is bad (I was wrong, looks like the problem is in OpenWrt code). Last night I wanted to replace the crystal and with it a trace came off... and then trying to fix it I lost a small smd capacitor.

I want to replace the whole crystal part of the router with a small pcb with through hole parts. Please help me find out what the values of those two capacitor are? Here are the pictures:

https://imgur.com/a/eaFTF

Here is the board before the damage:
**broken link removed**


Here is the schematic I came up with:
**broken link removed**

You need to look at the datasheet for the crystal on its manufacturer's website and hope that its Chinese people talk in English.

In the datasheet for the TXC 3.2 ~ 90 MHz crystals I found this:

Shunt Capacitance (C0): 7 pF Max
Load Capacitance: Series, 16 pF, 20 pF, 30 pF, 32 pF, or specify

Should I try all those values?

fixed!

I used 2 x 12pF caps and a 10M resistor (2x20M in parallel, I didn't have one 10M). The crystal is the original one.

**broken link removed**

Nice fix.

JimB

Well done.

At high frequencies (anything that isn't 32.768 kHz) the resistor in parallel with a fundamental crystal does not need to be a precise value.

It is only there to bias the amplifier to a mid point for start-up. Without the resistor, the input will float and it is unlikely to start, but once it is running the resistor is not needed.

As long as the impedance of the resistor is large compared to the capacitors at the operating frequency, and small compared to the leakage current, it will work.

At 25 MHz, the impedance of a 10 pF capacitor is about 600 Ω. The leakage on a CMOS gate is hard to measure, but obviously the 10 MΩ resistor works. However I think that anything in the 10 kΩ to 10 MΩ range would be fine.

If the original problem was an error of seconds per minute, that is far to far to be adjusted for by altering the capacitors on a crystal. That would only change by 0.01 % or so. Errors of a second every few hours can be adjusted by altering the capacitors.

Diver300 said:

If the original problem was an error of seconds per minute, that is far to far to be adjusted for by altering the capacitors on a crystal. That would only change by 0.01 % or so. Errors of a second every few hours can be adjusted by altering the capacitors.

Click to expand...

The problem was a bug in OpenWrt (the router being underclocked at 200MHz instead of the 240MHz). After applying the patch and recompiling the firmware, the clock is (almost) perfect. In few days after the patch, the time is only a second off and that's not a big problem (I'm syncing the time with the ntp server once a day).