Question about choosing 32MHz HSE quartz crystal

[Dmitriy Dmitriy]

Hello I am going to use STM32WB55Vx for my aplication and have a question about choosing 32MHz HSE quartz crystal.

The document "AN5042 HSE trimming for RF applications using the STM32WB Series"  determines Gm ( the transfer conductance of the HSE amplifier ) as vlaue from 0.18 to  2.84 mA/V. For stable operation, I have to chose 32MHz quartz crystal with Gmcrit ( minimal transconductance of an oscillator) is 5 (GAINmargin)  times smaller (acording to AN2867 Oscillator design guide for STM8AF/AL/S and STM32 microcontrollers).  The crystal NX2016SA 32 MHz EXS00A-CS06654 (from AN5042) has parametres: Cl =8pF, Co~ 1pF, ESR = 60 Ohm so Gmcrit ~= 0.6366 and GAINmargin =4.46 for the greatest Gm 2.84 mA/V.  

I think it is difficult to find oscillator for GAINmargin>= 5 taking into account embedded capacity bank 12-16pF HSETUNE[5:0]. 

Сan confirm operation for GAINmargin values below 5? What is mimum GAINmargin for STM32WB?

[Marc Laury]

Hello,

For the NX2016SA 32 MHz EXS00A-CS06654, C0 = 0.6pF

To assure no startup problem, for STM32WB, gain margin for HSE > 5 and for LSE > 3

Best Regards.

[Dmitriy Dmitriy]

According to AN5042: Gmcrit = 4*ESR* (2*pi*F)^2*(Co+Cl)^2

For NX2016SA 32 MHz EXS00A-CS06654 C0 = 0 .6pF Cl =8pF ESR= 60 Ohm F =32MHz Gmcrit = 0.7176 mA/V Gm

For STM32WB (AN5042 Application note HSE trimming for RF applications using the STM32WB Series )  transfer conductance of the amplifier HSEGMC[2:0]  G =  0.18 -2.84 mA/V

Gain margin for HSE (AN5042) Gm = G/Gmcrit . For STM32WB and NX2016SA 32 MHz EXS00A-CS06654 Gm=2.5 - 3.96 Gm<5 !

Can you confirm operation with Gm < 5? 

Maybe there is a resonator with Gmcrit< 0.568 ( Gm=5) Cl < 7pF but it isn't suitable fot internal load capacitance: HSETUNE[5:0] 12-16pF , Pin capacitance Cio = 5pF (datasheet STM32WB55xx  p125 Table 69. I/O static characteristics ) and mounting capacity 0.5- 1pF.

Could you advise a suitable HSE resonator?

[Marc Laury]

Hello,

The HSE recommanded for the STM32WB is the NX2016SA 32 MHz from NDK.

Best Regards.

[Adam Santamaria]

Hello Dmitriy Dmitriy,

The fact is that the procedure to choose a crystal exposed in AN2867 (part 3.4) is covering STM32 products for which the datasheet gives the gm oscillator transconductance of the STM32 oscillator. See STM32F103 datasheet for an example : https://www.st.com/resource/en/datasheet/stm32f103rb.pdf.

In this case, to choose the crystal you need to follow the part 3.4 of AN2867 and make sure that the gain_margin=gm/gm_crit (with gm_crit determined thanks to the AN2867 formula and the used crystal specifications) is strictly above 5.

For STM32WB and some other products, the Gm maximum critical crystal transconductance is given instead of the gm oscillator transconductance. See STM32G081 datasheet for an example : https://www.st.com/resource/en/datasheet/stm32g081cb.pdf.

For STM32WB, Gm corresponds to HSEGMC.

In this case, the method to choose a crystal is not indicated in AN2867 but here it is: you need to operate the gm_crit of this crystal (see AN2867 formula) and to make sure that it is below the Gm (or HSEGMC chosen for WB) of the STM32 datasheet.

We are aware of the ambiguity that can create AN2867 and AN5042 and this two documents will be updated soon.

Sorry for the late answer.

Best regards,

Adam 

[Dmitriy Dmitriy]

Thank you, Adam