Calculating Rext after increasing LSE Drive Level on STM32H7

[jmh299]

Hello,

A design I am working on utilizes a LSE crystal with the following parameters:

f = 32.768kHz
CL = 12.5pF
Co = 0.9pF
ESR = 50kR Max
DL = 1uW MAX

I am calculating gmcrit ~ 1.52uA/V.  According to table 5 in AN2867, the drive level should be set to Medium High, will REXT required?  The design is already produced without REXT so adding it is not advised if avoidable.

I don't have the instrumentation to measure the actual drive level as it requires ~0.1pF probes for proper measurement.  There is no indication of LSE max drive current for the H7, the datasheet only specifies 550nA IDD at medium high.

Should I be concerned that Medium High may violate 1uW?

Thanks

[mƎALLEm]

Hello @jmh299 and welcome to the ST community,

Rext is required only if If DL > DLcrystal.

Please refer to the application note AN2867 "Guidelines for oscillator design on STM8AF/AL/S and STM32 MCUs/MPUs":

Please also refer to the section 3.5 Drive level and external resistor calculation in the same application note

Hope that helps.

[jmh299]

I have read AN2867, unfortunately I do not have the instrumentation to perform these measurements accurately.  I was not sure if there was any rule of thumb regarding the drive level setting and the drive current.

Thanks

[mƎALLEm]

You need at least to measure VPP with an oscilloscope, otherwise you cannot continue:

[jmh299]

I do not have a 0.1pF probe to make an accurate measurement, I can attempt to but I believe a standard probe will affect the measurement as it will greatly impact the load capacitance.

[mƎALLEm]

The probe should not exceed 1pF of capacitance not 0.1pF:

[mƎALLEm]

Ok in the note it indicates it should not exceed 0.1pF (for the voltage measurement)

I need to escalate that internally for the verification (for the current measurement)

[jmh299]

Just as an update, I measured with my lab equipment albeit not 0.1pF-1pF per suggested as we don't have that kind of instrumentation.  
Used a standard 18.5-22.5pF passive probe.

Measured 224mV Pk-Pk on Medium High.

If using the combined capacitance of the passive probe(18.5-22.5pF) and the oscilloscope (18pF) itself ~0.0373uW seems awfully low and well within spec but not sure how much my measurement method is affecting this.

Thanks