How to develop ultra-low-power and battery less demo with STM32U0 MCUs

This article provides explanations on how the STM32U0 battery less demo works and describes step-by-step how to reproduce it.

Information

The bill of material to reproduce the demo can be found here.

1. Presentation of the demo

1.1. Purpose

The aim of this demo is to illustrate the STM32U0's best-in-class ultra-low-power capabilities. It is running without battery thanks to Dracula technologies inkjet Printed Organic Photovoltaic module, which harvests energy for the STM32U0 MCU. This is a luxmeter and thermometer demo with data being displayed on an LCD segment display, and with the possibility to share measured data wirelessly.

1.2. Configurations

This demo can be configured in two versions.

• The first one is referred as the Data logger only configuration. It consists of one STM32U083C-Discovery kit powered by one Layer photovoltaic module. Every second, the measured temperature or illuminance is displayed on the LCD segment display. By pressing the joystick blue hat of the discovery kit, the user can change which data is being displayed.
  
• The second one is referred as the Data logger & RF configuration. It consists of two modules, the Data logger & Transmitter module, and the Receiver & Display module.
  
  • The first one is based on the first configuration of the demo, on top of which an X-Nucleo-S2868A2, an RF expansion board, is plugged. In addition to measuring temperature and illuminance, and displaying one of the two measures on the LDC segment display, it sends each second the two measures thanks to the RF module.
  • The second one is a module which receives the data and displays it on an LCD screen. It is based on a Nucleo-U083RC on top of which two expansion kits, an X-Nucleo-S2868A2, to receive the data, and an X-Nucleo-GFX01M2, with the LCD screen, are plugged. The two received measures are displayed in real time on the LCD screen, and variations are shown live on a graph chart. This module needs external power supply, such as USB.
    

Information

The software of the data logger works for both configurations. During initialization, it detects whether the X-Nucleo-S2868A2 is present or not, and adapts behavior depending on that.

2. How the demo works

2.1. Schematics

The figure below shows schematics of the Data logger only configuration on the left, and of the Data logger & RF configuration on the right.

Schematics of the two configurations of the demo.

2.2. Demo principle

The following flowchart and graph describe the three main phases of the Data logger module: the boot phase, the initialization phase, and the measurement phase.

Flowchart describing the principle of the demo, and related graph showing the evolution of MCU's VDD over time.

After connecting the panel, the capacitor starts to charge (1). When voltage first reaches the POR voltage (1.65 V) (2), only the minimum instructions are done before entering stop 2 mode, to consume as little energy as possible. Hence, in this time frame, only PVD is set to level 6 to wake up the STM32U0 when voltage reach 3.0 V (3). This is the critical part of the initialization.

Once the voltage reaches 3.0 V for the first time (4), the initialization of the peripherals can start (5). GPIOs, LCD, ADC, communication with the temperature sensor, and, if present, communication with the RF module is initialized. PVD is set to level 0 (2.0 V). It is used to check that voltage is high enough before starting the measurement and sending process. It is set to this level to have the largest working range. RTC is also configured to wake-up the MCU every second. Then, the MCU enters stop 2 mode until wake-up from RTC (6).

At wake-up, and after checking voltage is high enough (7), the measurement phase is entered (8). Temperature is measured by the STT22H temperature sensor included in the discovery kit. Light is measured by measuring voltage provided by the panel, as explained in the following part. If the RF module is present, measured data is sent. The last step is to enter stop 2 again to harvest energy from the photovoltaic module (9).

2.3. Illuminance measurement

The figure below illustrates the flow of current outside of the illuminance measurement phase. PA0 is in open drain output high configuration, so current flows to the capacitor and the STM32U0.

First phase: charge of the capacitor.

The illuminance is deduced from the current supplied by the photovoltaic module as both are proportional. The current supplied by the photovoltaic module is deduced by measuring the voltage across the photovoltaic cell when its current is flowing through a resistor only. This voltage is measured by one of the ADC of the STM32U0.

So, the following formula is used to deduce illuminance from the measured ADC voltage:

$L_{PV}=I_{PV}*K=V_{ADC}/R_{1}*K$ Where:

$L_{PV}$ is the ambient illuminance.

$I_{PV}$ is the current provided by the photovoltaic module.

$V_{ADC}$ is the voltage measured by the ADC.

$K$ is a proportionality constant.

$R_{1}$ is the value of resistor R1.

Information

$K$ is determined by measuring the current supplied by the photovoltaic module at a given illuminance.

To have the current of the photovoltaic module to flow through $R_{1}$ only, PA0 is configured in open drain output low, and a diode isolates the photovoltaic cell and the resistor from the capacitor. The figure below illustrates the flow of current during the illuminance measurement phase.

Second phase: measurment of illuminance.

3. How to reproduce

3.1. Bill of material

Information

To reproduce the Data logger only configuration, only material in bold is needed.

Data logger & Transmission module

• 1x STM32U083C Discovery kit
• 1x Layer® inkjet Printed Organic photovoltaic product from Dracula Technologies
• 1x diode
• 1x 100 µF capacitor
• 1x 2.2 kΩ resistor
• 1x X-Nucleo-S2868A2
• 1x 300 kΩ resistor

Receiver & Display module

• 1x Nucleo-U083RC
• 1x X-Nucleo-S2868A2
• 1x X-Nucleo-GFX01M2
• 2x SSQ-119-03-T-D

3.2. Reproduction of Data logger & Transmission module

Information

To reproduce the Data logger only configuration, only steps 1 to 5 must be followed

The figure below shows the connections to be done on the bottom side of STM32U083-DK.

Connections on bottom side of STM32U083-DK

• 1. Connect PV- to battery– pin.
  
• 2. Connect PV+ to CN7-38 (PC1).
  
• 3. Connect a diode between CN7-38 (PC1) and battery+ pin.
  

Information

For steps 2 and 3, connections on the battery pins can be done by plugging on the pins on the front side of the DK, or wires can be soldered directly on the bottom side of the DK.

The figure below shows the modifications to be done on the front side of STM32U083-DK and X-Nucleo-S2868A2.

Modifications and connections to do on top of STM32U0 DK and RF module

• 4. Connect a 100 µF capacitor in the dedicated slot of the DK.
  
• 5. Connect a 2.2 kΩ resistor between A4 (PC1) and A0 (PA0).
  
• 6. Connect a 300 kΩ pull-up resistor between VREF (CN5-3) and D7 (CN9-8).
  
• 7. Remove JP1.
  

The figure below shows the modifications to be done on the bottom side of X-Nucleo-S2868A2.

Modifications and connections to do on bottom side of X-Nucleo-S2868A2.

• 8. Cut CN9-9 and CN9-10.
  
• 9. Connect CN5-8 to JP1-1.
  
• 10. Remove R12.
  

Picture of the Data logger & Transmitter module.

3.3. Reproduction of Receiver & Display module

The figure below shows modifications to be done on the X-Nucleo-S2868A2.

Modifications to do on bottom side of RF module X-Nucleo-S2868A2 of the Receiver and display module.

The following solder bridges must be opened.

• A1. R10 - Disconnect SDN from D7.
  
• A2. R11 - Disconnect SPI SCLK from D3.
  
• A3. R12 - Disconnect GPIO0 from A0.
  
• A4. R13 - Disconnect CSN from A1.
  
• A5. R14 - Disconnect GPIO1 from A2.
  
• A6. R15 - Disconnect GPIO2 from A3.
  
• A7. R16 - Disconnect GPIO3 from A5.
  

The following solder bridges must be closed.

• B1. R6 - Connect SPI SCLK to D13.
  
• B2. R9 - Connect CSN to D10.
  
• B3. R18 - Connect GPIO0 to A4.
  
• B4. CN9-3 on R19 (or R10) - Connect SDN to D2.
  

The figure below shows the connections and the modification to be done on Nucleo-U083RC:

Connections and modification to be done on Nucleo-U083RC

• C1. Cut CN7-36(conflict on this pin between the 2 extension kits).
• C2. Plug the 2 SSQ-119-03-T-D 19x2 connectors on Nucleo-connectors.
• C3. Plug X-Nucleo-S2868A2 on Arduino-connectors.
• C4. Plug X-Nucleo-GFX01M2 on SSQ-119-03-T-D connectors.

Picture of the Receiver & Display module.

3.4. Software

Coming soon

4. References

• STM32U0 new product for entry level battery operated devices, video in which you can see running demo.
  
• STM32U0 ULP Lux demo, video presenting the principle of this demo.