This is a decoder for IKEA SPARSNÄS.

It uses RTL-SDR running on a Raspberry Pi to demodulate the FSK signal, and decodes the packet.

The packet data is encrypted using your sender's ID. The sender ID is the last 6 digits of the serial number located under the battery.

How to build:

First edit sparsnas_decode.cpp to setup your sender id. Then: g++ -o sparsnas_decode -O3 sparsnas_decode.cpp

How to calibrate to your sender frequency:

rtl_sdr -f 868000000 -s 1024000 -g 40 - > sparsnas.raw

Wait about 20 seconds. Press Ctrl-C.

./sparsnas_decode sparsnas.raw --find-frequencies

Wait about a minute for it to finish. It prints something like:

#define FREQUENCIES {65050.000000, 105050.000000}

Edit your sparsnas_decode.cpp with those values and rebuild it according to the How to build step above.

How to run:

rtl_sdr -f 868000000 -s 1024000 -g 40 - | ./sparsnas_decode

Technical details:

SPARSNÄS uses a CC1101 chip configured for GFSK modulation at 868MHz. The two FSK symbol frequencies once downconverted to baseband are roughly 67kHz and 105kHz. (EDIT: It seems like with a different RTL-SDR the baseband frequencies are 12.5khz and 50khz. Maybe my first RTL-SDR was inaccurate). The packet length is 18 bytes, including the length byte, and a 16-bit CRC is appended to the end. The CC1101 sync word is 0xD201. The packet payload byte 5 to 17 are encrypted using a key derived from the sender device's ID.

The encryption key is a repeating XOR key:

      const uint32_t sensor_id_sub = SENSOR_ID - 0x5D38E8CB;
      enc_key[0] = (uint8_t)(sensor_id_sub >> 24);
      enc_key[1] = (uint8_t)(sensor_id_sub);
      enc_key[2] = (uint8_t)(sensor_id_sub >> 8);
      enc_key[3] = 0x47;
      enc_key[4] = (uint8_t)(sensor_id_sub >> 16);

Packet format (big endian):

0: uint8_t length;        // Always 0x11
1: uint8_t sender_id_lo;  // Lowest byte of sender ID
2: uint8_t unknown;       // Not sure
3: uint8_t major_version; // Always 0x07 - the major version number of the sender.
4: uint8_t minor_version; // Always 0x0E - the minor version number of the sender.
5: uint32_t sender_id;    // ID of sender
9: uint16_t time;         // Time in units of 15 seconds.
11:uint16_t effect;       // Current effect usage
13:uint32_t pulses;       // Total number of pulses
17:uint8_t battery;       // Battery level, 0-100.

This is how to convert the 'effect' field into Watt:

float watt =  (float)((3600000 / PULSES_PER_KWH) * 1024) / (effect);

Note: Due to how the encryption works, it's possible for two different SPARSNÄS with unique sender_id to confuse each other. When decrypting a packet from sender B with the A key, it's possible that the resulting packet gets a sender_id equal to A, which means A will display incorrectly decrypted data from the B device.

Example usage