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 "screensaver style" with a tiny log on a nice IPS TFT display.

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

The temperature calculated (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: