Do I have to use an external resistor for the HSE oscillator on STM32WB55 MCU?

[Batuhan A.]

Hello,

Depending on the datasheet of the STM32WB55 MCU, there is an external resistor for the oscillator circuit. In many Nucleo board schematics, it is not used. The datasheet says that the resistor is to avoid overdriving current.

How can I decide that overdriving or do I have to limit the current? Does not the MCU limit the current?

The schematic in the attachment is from the STM32WB Nucleo Board.

Thank you,

Batuhan

[Javier1]

@Batuhan A.​ 

Could you tell us what XTAL(oscillator) are you planning to use in your design?

Once we know that, we can take a look at the XTAL datasheet and figure out how much driving current and capacitance (impedance) is expecting from the microcontroller HSE pins.

They dont use Rext in the STM32WB Nucleo Board design because they chose a XTAL whose current requirements were met by the microcontroller pins. (same with every XTAL ive seen in STM nucleo/discovery boards)

[Batuhan A.]

Hello Javier,

Thank you for the comment.

The XTAL that I want to use is "ABM8W-32.0000MHZ-7-D1X-T3". Its link is that.

Its specifications are below

 

[Javier1]

I see your problem now, all STM datasheets say every STM HSE circuit has a builtin RF resistor.

They dont say the value at all.

I dont have an answer for you

Well... what about trying your design without the Rext? Or playing safe and use the XTAL they use in the nucleo board?

[Batuhan A.]

Yes, it is the gap.

The playing-safe option pretty much makes sense. I'm looking for the XTAL.

Thank you for your help.

Best regards,

[Amel NASRI · Accepted solution]

Hi @Batuhan A.​ ,

HSE accuracy is mandatory for RF applications. Unlike other STM32 products where we provide some recommended components in AN2867 (Oscillator design guide for ST microcontrollers), you need to refer to AN5042 Precise HSE frequency and startup time tuning for STM32 wireless MCUs.

This application note describes some methods to apply in order to reach the expected level of accuracy for the selected HSE.

-Amel

[Batuhan A.]

Hi @Amel NASRI​ ,

Thank you for describing the application notes. I'll study on it.

Best regards,