AVRRFM

About

Experimental project to drive an RFM69HCW radio module with plain avr-libc and an Atmega328p MCU.

This is work in progress. Simple Tx-Rx is working so far.

To do something really extraordinary, the temperature reading of an MCP9808 sensor is periodically transmitted to the receiver.
To save battery power, the controller, radio module and temperature sensor are put to power down/sleep mode in between transmissions. The idle current is ~57 uA, which is still quite a lot (< 10 uA should be possible), but already better than 8 mA :-)

The receiver currently just converts the raw temperature reading to °C and displays it with the RSSI value on a nice IPS TFT display.

Range

With the default ouput power setting (register RegPaLevel (0x11) set to 0x9f), which should give +13 dBm with PA0, the range was very bad; just up to 10 m indoor plain sight. Setting RegPaLevel to 0x5f, which also gives +13 dBm but with PA1, changes it all: very good RSSI everywhere and in an actual "field" test, at 1.6 km, packet reception was still reliable with an RSSI of about -100 dBm.

Fun Stuff

Looking at the payload in the transmitted signal in URH (with an RTL-SDR Blog V4):

The four selected payload bytes are:

• 0b00000011 Payload length (address byte + 2 byte temperature value)
• 0b01000010 Address (0x42)
• 0b11000001 Upper byte of raw temperature value from MCP9808
• 0b01010011 Lower byte of raw temperature value from MCP9808

Calculating the temperature (assuming >= 0°C):

jshell> (0b11000001 & 0x1f) * 16 + 0b01010011 / 16f
$1 ==> 21.1875

So, 21.2 °C 🙂

The first 15 0b10101010 bytes are the preamble, then there are 4 sync word bytes. After the 4 payload bytes, there are 2 CRC bytes as described in the datasheet of the RFM69HCW: