Few parameters needs to be considered when choosing a HSE for STM32WL3x products
This page helps to easily check if the user HSE source is fully compliant with STM32WL3x requirements
1. Frequency choice
STM32WL3x can be used with either 48 MHz or 50 MHz HSE's.
The main rule of that the RF frequency to be used must be > 1.5MHz away from a harmonic frequency of the HSE.
Example 1: Is it possible to use a 48 MHz HSE for an operation @ 433 MHz?
48*9 = 432MHz ==> This is below 1.5 MHz ==> For 433 MHz operation, a 50 MHz HSE should be used
Example 2: Is it possible to use a 50 MHz HSE for an operation @ 915 MHz?
50*18 = 900 MHz and 50*19 = 950MHz ==> both HSE harmonics are > 1.5 MHz from 915 MHz, so this is fine.
2. Load capacitor
STM32WL3x embeds a capacitor bank enabling to control center HSE resonance frequency.
The widest tuning range is achieved by using an external 8 pF HSE, but other load capacitance HSE can be used (thus reducing the tuning range)
It is recommended to use a 8 pF HSE.
But this is not critical requirement: It is still possible to place external loading capacitors on the PCB to fine tune this centering frequency of the HSE.
Note that when the internal capacitor bank is said to be "bypassed", it still exhibits a equivalent loading capacitor of about 4 pF.
3. Transconductance
3.1. Critical transconductance
This parameter can be calculated from the HSE parameters given in its datasheet.
This is the minimum value that ensures that the HSE block will start correctly and maintain a stable oscillation
To do so, the critical transconductance value is calculated as per the following formula
3.2. Gain margin
To consider some additional margin, apply a gain margin to this critical transconductance such as:
Example of transconductance calculation from HSE datasheet
4. How to relate transconductance to STM32WL3x ?
STM32WL3x has two main parameters to be set as per HSE specifications
4.1. Start_up current (ISTARTUP):
Use the transconductance value obtained with gain margin
In the last example, this is 11 mS ==> Choose the closest value in the following table ("01" value is just fine)
4.2. Steady-state current (GMC[6:0])
At that point, the nominal transconductance can be considered as oscillations has already started.
In the last example, this value is 2.1 mS, so choose the values GMC[6:5] and GMC[4:0] that give the lowest XO current control value (this lowers the noise and therefore optimize both current consumption and sensitivity is steady mode)
Here, GMC[6:5] = 00 and GMC[4:0] = 16 gives us a Gm = 3.26mS for a current of only 160 µA
5. Conclusion
Three parameters need to be checked to validate the compatibility of an HSE with STM32LW3x devices.
1- Frequency
2- Load capacitance
3- Transconductance for both start-up and steady mode