Weather station part 2: The radio link


As mentioned in the previous post I’m hoping to put together a weather monitoring system. It will consist of an outstation in the garden which will be solar powered. The outstation will send data over a radio link to a base station located indoors. This post outlines the radio link code and setup.

The outstation radio wiring

The figure above shows the connections between the outstation MCU, the BMP180 pressure/temperature sensor and the NRF905 radio module. There are lots of connections to the radio module and I’m not sure they are all strictly necessary at the moment but for now they stay.
The NRF905 can transmit at a number of different frequencies. After some trial and error I decided to go with a frequency in the 433 ISM band as it seemed to work best (the NRF905 is on a breakout board which already has inductors and capacitors in the antenna circuit which I suspect were tuned for this band). The actual module is supplied from (link). Overall I found this module much easier to use than the NRF24L01.
The code for the radio link is available on github over here. This code transmits an counter value once per second at maximum power (10mW). This current consumption while transmittiing is quite high (about 29mA) but this current burst is pretty short lived. Early indications are that the range is sufficient for my purposes though I may fiddle with the antenna later to see how far it can be pushed.
A (messy) prototype is shown in the photo below
You may notice the ST-Link debugger salvaged from a Nucleo board attached to the target. This debugger was reflashed with the Segger/J-Link firmware and behaved very well.

The base station

The base station consists of an STM32L011 Nucleo board attached to an NRF905 module. A photo is shown below (wiring details in the code only for the moment)
The Nucleo outputs any data it receives over its built-in USB/Serial converter. This is displayed on the host PC.
One problem arose with the STM32L011’s SPI interface which is covered in its errata sheet. There is a timing problem with the SPI pins and ST provide a simple fix for this (which I found out about after two days of head scratching :). Code for the base station is available here

Next post

The next post will deal with the issue of power consumption on the outstation. This needs to be kept as low as possible if the solar power cells I salvaged from cheap garden LED lights are to work out.

STM32L011 serial, analog and low power examples

Here are some further examples for the STM32L011 Nucleo board. The first is a simple interrupt driven serial communications demonstration. It makes use of the built in serial interface in the Nucleo board and so no additional hardware is required for your host PC to exchange data with the MCU.
The second example builds on the first. It reads ADC channel 0 and outputs the result to the host PC over the serial port.
The third example is a little more elaborate and is aimed at those wishing to develop a very long life battery application. It uses the built-in real-time-clock (RTC) to wake the MCU up about once every second. While awake, the MCU reads the ADC, outputs data to the host PC over the serial port and blinks the LED on the Nucleo board. Current consumption is minimized during sleep by turning off
peripherals, reducing AHB clock speed and putting the Cortex M0+ into a deep sleep. I found it difficult to measure the current flow during this time but it seems to be around 3 to 4 microamps. One thing I did notice is that the CPU will not go to sleep with interrupts disabled – a good way of preventing you from shooting yourself in the foot.

Links to code.

Serial communications over the nucleo serial interface to the host PC
Read the ADC and output the result over the serial port to the host PC
Minimize power consumption and wake using the internal RTC every second

Blinky on the STM32L011 Nucleo

The STM32L011 Nucleo board is an mbed enabled development kit featuring the STM32Lo11K4 MCU. This is a Cortex M0+ device with some interesting features such as hardware accelerated AES encryption (Correction: AES not present on the STM32L011) and a “Firewall” which seems to prevent firmware and other data being offloaded from the device. The board is meant to be programmed with a large IDE or using the mbed online development environment. I was curious to see could it be done from the command line with all code contained in a single directory. Normally I would tackle this with OpenOCD and GDB but this is such a new board that OpenOCD/GDB can’t interface with it properly (yet). All is not lost however! The mbed feature of this board means that it appears as a removable storage device to the operating system. Programming the board is simply a matter of dropping a correctly formatted executable (binary) file into this removable storage device. The utility objcopy can be used to prepare the executable file.
Assuming you have compiled your code to an “elf” file called main.elf, you can generate a binary file as follows:
objcopy -O binary main.elf main.bin
The “bin” file can be dropped on the Nucleo and should run fine.
Code for blinky is available here