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I have to connect an oscillator (RTV-104EF13P-S-10.000-TR) to an MCU (STM32G051F8P6) in order to have a more precise clock.

The datasheet provides very little information about the Vc pin function. I guess it is a power control but I could be wrong.

The MCU datasheet also does not provide information about how the OSC_EN pin function works. I assume it should be driven LOW to disable the clock output but, again, could be wrong.

Will my circuit (shown below) be able to disable the oscillator to save power?

The oscillator only draws 1.5 mA but that is still significant in my application.

schematic

simulate this circuit – Schematic created using CircuitLab

Edit: Accuracy should be less then or equal to +-10 ppm I assume IC like LTC6930IMS8 will be the best choice: It consume less then 1mA so it could be powered from OSC_EN pin

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  • \$\begingroup\$ Yes correct I put other socilator by mistake it should be RTV-104EF13P-S-10.000-TR \$\endgroup\$
    – Harry
    Commented Mar 5 at 8:07
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    \$\begingroup\$ What is your accuracy requirement for the master clock? \$\endgroup\$ Commented Mar 5 at 20:58
  • \$\begingroup\$ A very warm welcome to the site :-) Can you edit your question to add new info like this, please don't post it in comments. Otherwise, readers have to piece together the full question from scattered fragments. Comments can be deleted over time during site clean-ups. Thanks. \$\endgroup\$
    – TonyM
    Commented Mar 6 at 14:09

3 Answers 3

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Vc is a control voltage, 1.65V +/- 1V and it pulls the frequency by 5ppm over that range. Since this is a 10Mhz oscillator, you will get 10Mhz +/- 50hz +/- the other drifts and offsets (about 5ppm + 1ppm / year worst case).

This pin would typically be driven from a DAC for a GPS (or other accurate timebase) disciplined oscillator.

OSC_EN will not save power in this configuration, you need to configure the clock source for an external clock input, rather then an external crystal. If you want to save the power, you will have to switch VCC on the oscillator.

This may all be a poor solution, depending on the clock accuracy you need. Consider using a precision oscillator/crystal instead of a VCO if you are looking for something simple. enter image description here

Depending on what you need the accurate clock for, you might consider using a slower clock on a timer input pin, or using the OSC_32 oscillator with a 32khz crystal (the latter being very low power). Use HSI_TRIM to discipline the internal RC oscillator does not have the resolution to get to 10ppm ultimate accurate so disciplining that to an external clock doesn't appear to be an option for you.

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  • \$\begingroup\$ The oscillator output is 0.8Vpp clipped sine wave, so the MCU would likely not be able to use this when configured for external logic level clock. \$\endgroup\$
    – Justme
    Commented Mar 5 at 9:01
  • \$\begingroup\$ would Precision µPower Oscillators like LTC6930 work in my case if I will connect V+ to OSC_EN pin? Power consumption of this oscillator is very low and I guess MCU pin could supply sufficient amount of current. \$\endgroup\$
    – Harry
    Commented Mar 5 at 9:53
  • \$\begingroup\$ @Harry Why would you want to use LTC6930 and power it from MCU? You will get more precision and less current consumption by using MCU internal oscillator with 8 MHz quartz crystal and be able to turn it on/off in software. \$\endgroup\$
    – Justme
    Commented Mar 5 at 10:50
  • \$\begingroup\$ @Justme Internal oscillator (16 MHz) precision is not sufficient. I'm building application which is very sensitive to frequency error and it also need to be low power \$\endgroup\$
    – Harry
    Commented Mar 5 at 11:16
  • \$\begingroup\$ @Harry I did not ask you to use the inaccurate internal oscillator. I am asking why can't you use an external 8MHz quartz crystal with the HSE oscillator meant for external crystals. \$\endgroup\$
    – Justme
    Commented Mar 5 at 11:23
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(sorry for my poor english)

The (RH100-8.000-10-1010-EXT-TR) is a Crystal oscillator.

From the datasheet of your MCU there is the application schematic for a crystal. enter image description here

Here is the datasheet of the crystal, from where you can find that only 2 pins (1 & 3) are used, the two other pins are only more mecanincs. The datasheet gives olso the value of CL1 & CL2 (Load capacitance). you can put in your design a 0Ohm 0402 resistor in case if you need the Rext (neved used it from my side)

enter image description here

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  • \$\begingroup\$ yes I did use osther oscilator just wrong part number. It should be RTV-104EF13P-S-10.000-TR \$\endgroup\$
    – Harry
    Commented Mar 5 at 8:08
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The oscillator might not be a good choise.

The oscillator output is not a logic level square wave, but a 0.8Vpp Clipped Sine Wave. The data sheet for the whole Raltron RTV-104 series says that the oscillator output should be used with an AC coupling capacitor to remove DC bias.

As there are no other details about the output DC specs so it may be incompatible with the MCU input.

The MCU can be fed with logic level clock by bypassing the oscillator, but if the oscillator is enabled, the input voltage AC and DC specs are unknown, as it is assumed to connect directly to a crystal. It may work if the MCU is in crystal mode and the oscillator output is AC coupled to MCU crystal input pin.

And like others have mentioned, the oscillator does not have an enable pin, the Vc is an analog control voltage input to fine-tune the output frequency.

There might be ways to convert the oscillator output into logic level square wave with various methods but it will just be simpler and more reliable to choose another clock oscillator with logic level output.

Especially if you don't need the accuracy, stability, and tunability of a VCTCXO, since you did not want to adjust it anyway.

Edit: If you just want a stable and low-power solution, use a 8 MHz crystal with the MCU crystal pins and HSE. Better accuracy, lower power and cheaper than the LTC oscillator you asked about.

Edit2: As the MCU package you will be using does not support HSE crystals, use a 32768 Hz LSE crystal to get a low power clock reference and use it to trim the internal HSI16 oscillator in software.

Or, change the MCU to one that supports crystals directly.

In general, ready made crystal oscillator modules that output square wave tend to consume several if not tens of milliamps so they should not be powered from a GPIO pin. Some of them have a shutdown pin to stop their operation to conserve power. Some of them have an output enable which simply disables the output, but the crystal oscillator will still consume same amount of current. But they are quite precise.

Lower power and less precise oscillator modules do exist.

So the balance of low cost, low power and high precision defines which solution you should pick.

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