Installing a 16MHz 0.1 ppm TCXO on a Atmega2560 chip

Hello,

I am stuck with my frequency counter hardware design. I am trying to change the clock source of a custom made PCB with the atmega2560 chip. The idea is to use this 16MHz 0.1 ppm TCXO (the link is at the end) and connect the output pin to the XTAl1 pin and set the fuses accordingly (-U lfuse:w:0x20:m -U hfuse:w:0xD8:m -U efuse:w:0xFD:m). However, I have seen that the output signal is a sine wave. Do I have to convert the signal to a square wave before feeding it into the atmega 2560?

Thank you!

Where are you seeing this alleged sine wave?

I didn't find the exact TCXO in the link. One I did find specifies TTL/CMOS output, but offers to have sine wave output units made up in a hurry on request if that's what you need.

So you may have the wrong kind of TCXO for this application. Can you post a better link to a data sheet or make and model?

a7

They gave me this datasheet. The frequency in the datasheet is diferent from my TCXO but the rest of the information is correct.

116MHz.doc (52.5 KB)

Do I have to convert the signal to a square wave before feeding it into the atmega 2560?

No, as long as it has sufficient amplitude and is positive. 3-4V peak to peak should be fine for a 5V MCU.

jremington:
No, as long as it has sufficient amplitude and is positive. 3-4V peak to peak should be fine for a 5V MCU.

Ok, so I understand that this is my problem. Since by looking at the data sheet where it says "output symmetry = 1V Vpp" the signal output doesn't have enough amplitude. This means I have to buy a different TCXO or use an Op-amp to amplify the signal?

Tip: It is often possible to remove everything after the .hmtl : 20MHz 20.000MHz TCXO 0.1ppm compensación de temperatura oscilador de cristal TCXO 20MHZ|crystal oscillator|tcxo 0.1ppmtcxo oscillator - AliExpress.

Koepel:
Tip: It is often possible to remove everything after the .hmtl : 20MHz 20.000MHz TCXO 0.1ppm compensación de temperatura oscilador de cristal TCXO 20MHZ|crystal oscillator|tcxo 0.1ppmtcxo oscillator - AliExpress.

Thanks, good to know!

You might be able to trick the processor into taking a 1V p-p signal by setting the fuses to LP XTAL and connect to the input pin of the oscillator. The signal MUST be DC coupled, and above ground.

What exactly is the output of that oscillator? Have you looked at the signal using an oscilloscope? If so, post a photo.

This thursday I will receive an oscilloscope and will be able to post a picture of the output signal.

Hello, I finally connected the TCXO to the oscilloscope with the x1 probe and a 5V volt supply from an arduino nano. The output obtained doesn't look like a 1V p-p as the data sheet said. However, I have changed to a x10 probe and the output signal is quite different. Am I doing something wrong? If not which one should I trust more?Why does the V p-p change so much? Sorry if these are really obvious questions I am new with all this equipment.

Triple check and post a picture of your probe attachment, especially the ground connection.

Probe artifacts are common, especially with beginners. Always use the 10X probe until you are more familiar with the instrument.

You will find some useful guides on the web, such as this one.

Ok, I dind't have the ground crocodile clip of the probe connected I hope now everything is correct. The new signal obtained is the following. For some reason the link you have posted just opens and closes a tab immediately, could you provide the link please?

Proper ground is pretty important, and those long clip leads for power and ground to the can, may be a problem. Straight wires have inductance.

For some reason the link you have posted just opens and closes a tab immediately

Check for the downloaded PDF file.

Ok, I read the PDF and changed some things in order to get a better measurement following your recommendations. This are the results.

That is starting to look pretty reasonable, and will work fine as an external clock. Ignore the earlier suggestion to try the low power crystal fuse setting.

What was the basis for this comment in your first post?

However, I have seen that the output signal is a sine wave.

evansg124:
They gave me this datasheet. The frequency in the datasheet is diferent from my TCXO but the rest of the information is correct.

Here you can find the data sheet given to me where it says it is supposed to output a 1 V p-p sine wave output. I just placed the TCXO on my prototype board and probed the output at the CLK0 pin and obtained the 16MHz signal, so I guess everything is working properly! Thank you for all the help!

it is supposed to output a 1 V p-p sine wave output

That is clearly wrong.

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