The idea to design and manufacture our own development board came from the company’s area of interest: low-power wireless communication and sensors. The hardware that is optimized for these kinds of applications has to follow several requirements:

  • the central piece should be a micro-controller designed especially for low-power applications
  • a radio transceiver with best-in-class power consumption for transmitting and receiving data frames
  • adjustable wireless transmit power
  • crystals and oscillators with a high frequency accuracy to power the micro-controller
  • jumpers for measuring the current consumption for the board’s main components
  • low-power sensors for various data acquisition

The Andustria Hydrogen development board is meant to fulfill all the above mentioned requirements and to be used as a starting point for any kind of low-power wireless application. How did we achieve this ?

The micro-controller

After careful consideration and taking into account our experience with low-power micro-controllers, we have selected the STM32L462RE from STMicroelectronics to power up the development board as it was designed for these types of applications and has a best-in-class power consumption vs. available features.

STMicroelectronics offers four families of low-power micro-controllers depending on the amount of processing power needed starting with the low-end STM32L0 family based on the Arm Cortex-M0 core and ending with the high-end and most recently released STM32L5 family based on the Arm Cortex-M33 core.

The STM32L5 family of micro-controllers is still in preview, but we will consider upgrading the boards to use one from this family, once it’s fully available.

The STM32L462RE micro-controller is based on the Arm Cortex-M4F core running at maximum 80 MHz, featuring 512 KB of flash memory for the code and 160 KB of RAM for the variables. On the peripheral side, it has 6 GPIO ports, 8 timers (advanced timers, low-power timers), 3 SPI interfaces, 4 I2C interfaces, 4 UART interfaces, 1 ADC and 1 DAC. Many wireless communication protocols, if not all, require frames to be encrypted for security reasons, so another reason why we have chosen this micro-controller is that it featured an AES engine.

Since the transceiver and the micro-controller are not sourced from the same crystal/oscillator, the speed of the SPI interface is limited to 7.5 MHz also limiting the operating frequency of the micro-controller to 60 MHz. With this configuration, we will have a current consumption of around 5 mA for the run mode and 2 uA for the low-power (STOP2) mode.

Wireless section

The wireless part of the development board features the Microchip AT86RF212B sub-GHz transceiver, the Skyworks SE2435-L power amplifier, the filters and two antenna options: on-board SMD and an external one using the SMA connector. 

Originally manufactured by Atmel, the AT86RF212B is a IEEE 802.15.4-2011 compliant sub-GHz transceiver supporting O-QPSK and BSK modulations at variable data rates ranging from 50 kbps to 1000 kbps. It supports multiple frequency bands, we will consider just the European 868 MHz band and optimized the rest of the board as such.

The current consumption during frame transmission depends on the power setting and it’s shown in the figure below.

As default, we are using the O-QPSK modulation with the IEEE-compliant data rate of 250 kbps and a maximum 127-byte frame. The overall transmit power (transceiver + amplifier) is set to follow the ETSI EN 300 220 standard regarding short range devices operating in the 25 – 1000 MHz frequency range.

An external power amplifier, the SE2435-L, is used to boost and filter the output of the transceiver. By default, the maximum gain for the 868 MHz band is around 28 dBm. Using this amplifier, we can also use the development board in the ETSI-defined P band which allows up to 27 dBm of effective radiated power.

The current consumption of the power amplifier with various power inputs is found in the figure below.

There are two antenna options: a SMD antenna and an external antenna via the SMA connector. Both antennas can be used by making use of the antenna diversity feature offered by the transceiver.

ETSI considerations

For compliance with the European ETSI EN 300 220 standard regarding short range devices, the applications that uses the wireless components must follow the mandatory requirements:

  • the operating frequency must be in the defined bands
  • the effective radiated power must be under the threshold as defined for each band
  • the channel access and occupation rules must be under the threshold as defined for each band
BandFrequency rangeEffective radiated powerChannel access
K863 – 865 MHz25 mW or 14 dBm0.1% or polite
L863 – 868 MHz 25 mW or 14 dBm 1% or polite
M868.0 – 868.6 MHz 25 mW or 14 dBm 1% or polite
N868.7 – 869.2 MHz 25 mW or 14 dBm 0.1% or polite
O869.4 – 869.65 MHz500 mW or 27 dBm10% or polite
P869.7 – 870 MHz5 mW or 7 dBm
Q869.7 – 870 MHz 25 mW or 14 dBm 1% or polite

When accessing the channel (transmitting frames), the device must follow the channel access and occupation rules as above. The duty cycle method means that for a specific time interval, the device must transmit a maximum percentage of that interval (e.g. for band M, the duty cycle is 1% meaning the device can transmit for a maximum of 10 milliseconds in a second). When using the polite spectrum access method, the device is allowed to transmit for maximum 100 second in an hour meaning a maximum duty cycle of 27.77%. The firmware implements the listen-before-talk algorithm by using the clear channel assessment (CCA) feature of the transceiver.

Debugging

To ease the firmware debugging, we have added several ways to help: a JTAG interface for programming and debugging code, two LEDs to signal different application states, one user push-button to simulate an asynchronous event and access to a micro-controllers’ UART interface via a FTDI chip and a micro USB port.

The firmware uses the USB port to log messages when it reaches certain states. This is achieved by re-mapping the printf function to use the UART interface instead of the console. The default configuration for the UART interface is 921600 baud, 8-1-N-1.

Two connectors for measuring the frequencies of the crystal and the oscillator are also available. The frequency accuracy must be as high as possible, as the timing in low-power applications are of extreme importance.

Sensors

We have includes two sensors on the development board, for the user to test the communication interfaces and read different air quality parameters.

Temperature and humidity parameters are read from the Sensirion SHT21 sensor connected on one if the I2C interfaces. It provides a temperature range from -40 to 125 °C and a humidity range from 0 to 100 RH.

The CO2 and TVOC sensor, the CCS811 from ams implements a new technology in terms of air quality measurements which allows a low power consumption when in measurement mode. The measurement range goes from 400 to 8192 ppm for CO2 and from 0 to 1187 ppb for TVOC.

The ams CC811 sensors has four operating modes, depending on the measurement interval. The current consumption for each mode is found in the table below.

Measurement modeAverage current consumption
MODE1 (every 1 s)17.68 mA
MODE2 (every 10 s)3.54 mA
MODE3 (every 60 s)590 uA
MODE4 (every 250 ms)17.68 mA

Power

There are several ways the user can power up the board, one way is to use the micro USB connector. The original 5V are converted to 3v3 by the FTDI chip after the pairing has succeeded with the USB host. A second option is to power the board via the 5 pin TTL cable from FTDI:

Cable datasheet: https://www.ftdichip.com/Support/Documents/DataSheets/Cables/DS_TTL-232R_CABLES.pdf

The third option, is to power it with 3 AA batteries mounted on the back in their respective connectors.

Contact us if you require more info!