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AN2979 Datasheet(PDF) 9 Page - STMicroelectronics |
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AN2979 Datasheet(HTML) 9 Page - STMicroelectronics |
9 / 12 page AN2979 Measurements and calibration Doc ID 15651 Rev 1 9/12 The following figure shows the impact of the capacitor and resistor accuracy. Figure 7. Charge equation depending on RC accuracy 1. In blue: Vmeasacc1 ---- In red: Vmeas ---- In green: Vmeasacc2 2. Condition: VDD=3.3 V Note: The results are not only impacted by the marging error due to hardware components but also by the accuracy of the MCU clock. In this example, the device works at a HSI frequency of 16 MHz at 25 °C, that is, with a clock accuracy of 1% (refer to the datasheet). The temperature variation also impacts the accuracy of the measured value as the microcontroller, resistors and capacitors are temperature dependent. 4.3 How to get a better accuracy 4.3.1 Hardware solution The first solution is to choose components with a high accuracy value (this may imply a higher cost). 4.3.2 Software solution The second solution is to calibrate the RC constant in the software using VDD as a reference. Implementation description In order to perform the calibration, you need to uncomment the “#define calibration” in the “param.h” file. y = 3.3(1-exp(-x/0.882)) y = 3.3(1-exp(-x)) y = 3.3(1-exp(-x/1.122)) 0.5 1 1.5 2 2.5 3 0.5 1 1.5 2 2.5 V ms Vmeas_max = 2.05 tmax tacc1max tacc2max tacc2max = 1.09 ms tacc1max = 0.856ms tmax = 0.970ms |
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