{"id":625,"date":"2015-08-01T20:55:47","date_gmt":"2015-08-01T19:55:47","guid":{"rendered":"https:\/\/axotron.se\/blog\/?p=625"},"modified":"2026-10-24T09:24:09","modified_gmt":"2026-10-24T08:24:09","slug":"frdm-based-zigbee-humidity-sensor-node-prototype","status":"publish","type":"post","link":"https:\/\/axotron.se\/blog\/frdm-based-zigbee-humidity-sensor-node-prototype\/","title":{"rendered":"FRDM-based ZigBee Humidity Sensor Node Prototype"},"content":{"rendered":"\n

I have previously written about how to interface an XBee ZigBee module to a FRDM-KL25Z board<\/a> and also how to talk to an XBee ZigBee module from a Python program on a PC<\/a>. In this article I build on these foundations and show how to make a somewhat more complex system where a Python program running on a PC sends commands to – and receives replies from – a sensor node. The sensor node has an SHT21 humidity (and temperature) sensor which is connected via I2C to the FRDM board.<\/p>\n\n\n\n

Here I use the API mode of the XBee module on the sensor node, instead of AT mode that I used in the previous blog post<\/a>. There are a few XBee libraries for e.g. Arduino and mbed available. It might be possible to use one of those, but I looked into some of them briefly and it seemed like more work to understand and adapt one of those than to write the code necessary to take care of the somewhat limited communication with the XBee module required for this system. On the Python side, I continue to use the XBee library I used before.<\/p>\n\n\n\n

Preparing the XBee Modules<\/h2>\n\n\n\n

The XBee ZigBee modules need to be properly configured before they can be used in this system. The one connected to the PC shall be programmed with the ZigBee API-mode Coordinator firmware and the one used in the sensor node shall have the API-mode Router firmware. A 64-bit PAN ID must be selected and both modules should be configured with this.<\/p>\n\n\n\n

Humidity Sensor SHT21 and I2C<\/h2>\n\n\n\n

The combined humidity and temperature sensor I selected for this project is called SHT21<\/a> and is made by Sensirion<\/a>. This is a reasonably priced nice little sensor (3×3 mm2<\/sup>) with I2C interface and a tolerance on the relative humidity of at most 3% (2% typical) between 20% and 80% RH while the temperature is accurate to 0.4 \u00b0C (0.3 \u00b0C typical) between 5 and 60 \u00b0C.<\/p>\n\n\n\n

A minor problem is that it comes in a 6-pad DFN-package (dual flat, no leads) with a pitch of 1 mm. I connected to the chip by soldering very thin wires (a single strand from a relatively thin multi-stranded wire) to the four pads that are used. I then soldered these wires to a small piece of prototype board and added a decoupling capacitor and I2C pull-up resistors (I happened to use 2.2 k\u03a9 and 4.7 k\u03a9 because I had them easily available in suitable form factors). The top side of the package needs to be away from the board to expose the opening of the sensor to the environment. This could potentially lead to short circuits between the board and the bare thin wires, but I put a piece of insulating tape under the sensor to prevent this. See photo below.<\/p>\n\n\n\n

\"SHT21<\/a>
SHT21 sensor (central black square) on a prototype board with a few auxiliary components.<\/figcaption><\/figure>\n\n\n\n

I ended up using the pins PTC11 (J1 pin 15, SDA1) and PTC10 (J1 pin 13, SCL1) as the I2C bus on the FRDM-KL25Z. I first tried A4 (J10 pin 10, SDA1) and A5 (J10 pin 12, SCL1), but it turned out that there is a square wave on A5 that obviously ruins the I2C communication. Maybe this could be turned off, but I moved to other pins instead of investigating what caused the square ware.<\/p>\n\n\n\n

The SHT21 has a bit of a quirky requirement on the I2C communication, namely that a delay of 20 \u00b5s is recommended between the ACK bit after some command and before issuing the stop condition. This turned out to be straightforward to implement using the mbed I2C library routines, see the file sht21.cpp.<\/p>\n\n\n\n

I did not implement a test of the checksum that the SHT21 provides. That can be added later and the easiest is probably to use code from the SHT21 sample code application note<\/a> for this.<\/p>\n\n\n\n

A photo of the complete sensor node prototype is shown below.<\/p>\n\n\n\n

\"ZigBee<\/a>
ZigBee sensor node prototype based on a FRDM board, an XBee module and an SHT21 humidity sensor.<\/figcaption><\/figure>\n\n\n\n

Communication Protocol<\/h2>\n\n\n\n

To enable the PC to control the sensor node, some form of protocol needs to be devised on top of ZigBee that allows the PC to send requests to the sensor and also allows the sensor to respond.<\/p>\n\n\n\n

To send data between the two XBee modules, the “Transmit Request” API command is used. This allows messages containing about 80 bytes of payload data to be sent. At the receiving end the messages show up as “RX Packet” frames.<\/p>\n\n\n\n

I defined the request data packet to just contain two payload bytes, namely an ASCII acronym for the command to be executed. Currently I have two commands defined:<\/p>\n\n\n\n