Why Zero ohm resistor in series with the XTAL oscillator
The below image is taken from this refrence design.
May I know why the two zero ohm resistors are used.
1 answer
The following users marked this post as Works for me:
| User | Comment | Date |
|---|---|---|
| newbie | (no comment) | Aug 6, 2025 at 19:53 |
This seems like them covering their butt. Most crystals work with most crystal drivers without any series resistors being necessary. In fact, series resistance together with inevitable parasitic capacitance can cause a phase shift, which causes the crystal to be run a little off its design point.
In some cases you need a resistor between the crystal driver output and the crystal when the crystal if very low power. The cases I've seen that being an issue have all been very small low-frequency crystals. The kinds of crystals meant for wrist watches, for example, could be overdriven by a 5 V microcontroller. This is very unlikely to be an issue with a 16 MHz crystal as shown in your schematic.
Some microcontrollers have two or three different crystal driver power levels for this reason. Instead of adding a series resistor, you use a lower power setting in the crystal driver.
Even in the case of preventing crystal over-drive, a resistor between the crystal and the crystal driver input makes no sense. The schematic you show seems to be from someone that is designing more by "rules of thumb" instead of actual understanding.
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