May 2013 DocID18304 Rev 1 1/38

AN3322

Application note

Watt-hour meter based on the STM32F101 microcontroller

By Ludek Holoubek

Introduction

This document describes, in detail, the hardware and software implementation of a watt-

hour meter using the STM32F101 microcontroller. This cost effective watt-hour meter uses

shunt with an operational amplifier as a current sensor, an embedded 12-bit ADC for current

and voltage measurement, GPIO for LCD management, and a lot of other peripherals for

communication, tamper detection, keyboard, and power disconnection. Powerful

architecture of the STM32™ microcontroller allows sampling at 1 Msps. The high sampling

rate makes it possible to use methods for ADC resolution enhancement.

Main features

 Metrological parts

– Microcontroller and external op amp only

– Compliant with EN 50470-1:2006 Class B, precision better than 1%

– Starting current 20 mA, nominal current 5 A, maximum current 100 A

– Reads the voltage and the current signal up to the 21

harmonic

– Shunt as a current sensor. CT and Rogowski coil is optional

 Additional parts included in reference design

– LCD driven directly by microcontroller

– Keyboard driven directly by microcontroller

– An extra external EEPROM for user data

– Bi-stable relay, IR communication, serial communication

– Backup battery, complete power meter power management

www.st.com

Contents AN3322

2/38 DocID18304 Rev 1

Contents

1 Watt-hour meter using the STM32 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.1 STM32 ADC parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

1.2 Requirements for meter in Class B . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Definitions of the power meter parameters (according to EN 50407-1:2006) . . . . . 7

1.3 STM32 ADC dynamic measurement range . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3.1 Voltage measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.3.2 There are two current ranges in the meter . . . . . . . . . . . . . . . . . . . . . . . 9

1.4 Error sources on the STM32 meter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

1.5 Linearity of STM32 ADC and its INL reduction . . . . . . . . . . . . . . . . . . . . . 10

1.6 Thermal noise present on STM32 meter platform . . . . . . . . . . . . . . . . . . 10

1.7 Crosstalk between subsequent multiplexed channels . . . . . . . . . . . . . . . .11

1.8 Power and energy measurement by STM32 embedded ADC . . . . . . . . . 13

1.9 ADC enhancement by oversampling technique . . . . . . . . . . . . . . . . . . . . 15

1.10 Output values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Some reference values and constants are defined . . . . . . . . . . . . . . . . . . . . . . . . 16

1.10.1 RMS voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

1.10.2 RMS current - high gained . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

1.10.3 RMS current - low gained . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.10.4 Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

1.10.5 Energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

1.10.6 Pulse output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2 Computational issues and compensation algorithms . . . . . . . . . . . . . 22

2.1 Non-coherent sampling issues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

2.2 Timer evaluation for coherent sampling triggering . . . . . . . . . . . . . . . . . . 24

2.3 Timer setting constraints for coherent sampling triggering . . . . . . . . . . . . 25

2.4 Precise timer setting for coherent sampling . . . . . . . . . . . . . . . . . . . . . . . 26

2.5 Frequency measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.6 Filtering of input signal for proper zero crossing detection . . . . . . . . . . . . 29