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HSI Accuracy on the Value line STM32L

Question asked by Yevpator on Feb 21, 2014
Latest reply on Feb 21, 2014 by waclawek.jan

The following question I have applied to the support team,on yet another forum and still awaiting an answer. Once I get an answer,I will share it here.

The Value Line series STM32L datasheet claims the HSI accuracy is +-10%, which is without calibration. Since my product is defined as outdoor and HSI frequency is affected by temperarture, the accuracy issue is important for me. To simulate the temperature changes, I've connected a signal generator to LSE and have increased/decreased the frequency by 10%. I have implemented HSI calibration vs LSE in the software. I have expected to fix 10% drift , but, in fact, I've managed to to fix only 5%,which is absolutely unacceptable. Apparently ,the reason for that was the trimming step of 0.4%. There is a 5-bit calibration register that defines the multiplier to the trimming step.
From among these bits the MSB is a sign. Thus, the maximal value I should have reached was 0.4 x 16= 6.4%, which does not cover the mentioned 10% . Inthedatasheet you can find that there 2 optional trimming step: 0.4 or 1.5%. With the latter, I think, it should have worked properly, but it is not clear for me who decides, according to what criteria, which trimming step to put into device in fab ?

Thank you in advance for your opinions,