Can I replace this oscillator circuit with oscillator IC?

Question

Disclaimer: I am programmer and when it comes to analog circuits I have no idea what I am doing.

I have a board with STM32F103 that uses external oscillator made by crystal and two capacitors:

The crystal is CD05M008000RD1 / 49SMD-8-20-20 described as 8Mhz, 20pF, 20ppm, ESR 60Ohm.

Everything seems to be working fine, but I am trying to reduce the board size and the crystal is rather big. I am looking for either smaller crystal or a complete oscillator in small package.

I found for example this: O 8,0-JO32-B-1V3-1-T1-LF described as XTAL OSC XO 8MHZ HCMOS SMD, 50ppm.

Can I use it so that it will look like this?

I see that it is less precise (20ppm vs 50ppm), but for my use-case it should be well within the limit. It is still way better than the internal oscillator. Are there any other differences that I am not seeing?

Answer 1

From the datasheet both types can be used. Just select the type in FW.

Answer 2

You can find 3.2 x 2.5mm crystals https://www.digikey.com/products/en/crystals-oscillators-resonators/crystals/171?k=crystal&k=&pkeyword=crystal&sv=0&pv46=10964&sf=1&FV=1989%7C0%2Cmu8MHz%7C2150%2C-8%7C171%2C69%7C409393&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25

Surely that and two 0402 caps are smaller than an oscillator that you also have to run power to? I stand corrected - you can find 8 MHz oscillators that are 1.6 x 1.2mm! https://www.digikey.com/products/en/crystals-oscillators-resonators/oscillators/172?k=oscillator&k=&pkeyword=oscillator&sv=0&pv46=7836&sf=1&FV=1989%7C0%2Cmu8MHz%7C2150%2C-8%7C172%2C276%7C174115&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25

Are those small enough for you?

Answer 3

Are you prepared to learn all the details of Xtal Osc (XO) tuning and uW power limitations and Q and Xtal temperature curves ,range and PPM for temp, PPM for initial tolerance and PPM for C variations and design error?

If not use the SMD XO (Standard). It will probably be more accurate, very tiny and can be tunable(VCXO), temperature compensated (TCXO) or both (TCVCXO) or programmable, etc. and low cost but maybe not cheapest in high volume.

Your discrete design could have more tolerance stackup:

• 20 ppm for ΔTmax=?, or more?
• 20 ppm @ 25'C
• 20~50ppm for C load design error & other tolerances.

Since all AT cut Xtals have different 3rd order temp curves , the ppm temp tolerance depends greatly on ΔTmax e.g. -25 ~40'C vs -40~70'C

You have a choice of temp range and ppm tolerance. vs cost vs volume.

Answer 4

I came across this and just wanted to give some feedback on what I came across while dealing with this same problem. The MCU I am using the STM32G031. What I discovered is, don't go by only what is said in the datasheet. You have to look at the reference manual. The datasheet for the STM32G031 did not specify that the STM32G031 could take an external clock source as depicted in Aaron's answer.

Almost all STM32 MCUs allow for the HSE and sometimes the LSE to be selectable as either Crystal/Ceramic Oscillator or BYPASS Clock Source. For the Crystal/Ceramic Oscillator, you have both an input and an output from the MCU as OSC_IN and OSC_OUT. With BYPASS Clock Source you get only OSC_IN, which, for example, would get the square wave CMOS output from a TCXO. You can do this right from CubeMX or do it manually. Here is the section in the reference manual that describes it.

External source (LSE bypass) In this mode, an external clock source must be provided. It can have a frequency of up to 1 MHz. You select this mode by setting the LSEBYP and LSEON bits in the Backup domain control register (RCC_BDCR). The external clock signal (square, sinus or triangle) with ~50% duty cycle has to drive the OSC32_IN pin while the OSC32_OUT pin should be left Hi-Z.

I checked this on another MCU I had used and it was the same thing. I had seen the BYPASS Clock Source in CubeMX many times, but never knew what it mean't until now that I actually have to use. I believe the language they are using here for this option is something like this: The STM32 has a built in op amp style circuit that, when combined with your supplied oscillator (eg. crystal), makes a clock. You may also BYPASS the "MCUs clock" and provide your own in the form of, for example, an external TCXO (that has it's own built in mems or crystal resonator) with a square wave output.