Currently I'm trying to make a circuit for the ESP8266EX Chip and I've stumbled across the "Crystal Oscillator" - section.

It's said that the ESP8266EX supports 40MHz, 26MHz and 24MHz crystal oscillators, with +-10PPM. In the schematic on page 9/30 there are 4 pins on the oscillator. XIN; XOUT; GND; GND. When I try to select an oscillator using the easyEDA software I can never find 2x GND outputs. It's always one ground and one VDD pin.

Basically I think I'm just getting confused by the labeling. What I would've done is, connect GND to GND and VDD to GND.

This is the oscillator I'm talking about in particular. When implementing him to easyEDA the pin no.4 is not GND but rather VDD.

So, what does the VDD stand for? According to my research it stands vor Voltage Drain Drain. VSS stands for Voltage Source Source and is 0V, thus VDD being different from 0V.

But how come there is an oscillator in the datasheet with 2xGND and no VDD, while I can only find them with 1xGND and 1xVDD?

Please correct me on my misunderstanding.

  • \$\begingroup\$ "But how come there is an oscillator in the datasheet with 2xGND and no VDD" I can't find that in the datasheet you have provided the link for. Where is that supposed to be? Note that the 'test circuit' in there has one ground and one control pin which can be connected to ground. It clearly has pin 4 which is marked "Vdd" in the table on page 1 as going to the supply. \$\endgroup\$
    – Oldfart
    Aug 17, 2019 at 11:09

1 Answer 1


There are two different products that come in similar packages. You can get either a bare crystal — which is what the schematic is showing — or you can get a complete, self-contained oscillator module (the one with the Vdd pin).

Both products are available in tiny SMT packages, which have four pads, one on each corner. For the crystal, only two of the pads are required, so the other two pads are typically connected to ground, for shielding. The oscillator requires three pads — power (Vdd), ground, and the output signal. The fourth pad is sometimes used as an output enable or frequency control voltage connection, but otherwise it is either not connected or connected to ground.

  • \$\begingroup\$ Oh, so basically I was looking for SMD Oscillator (XO) but I had to look for SMD Crystal Resonators ._., my mistake sorry! \$\endgroup\$ Aug 17, 2019 at 13:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.