avrrfm

About

Experimental project around RFM radio modules using an ATmega328P MCU
and librfm69 (FSK)/librfm95 (FSK + LoRa).

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 ~75µA, which is still quite a lot, but already better than 8mA 🙂
MCU, radio and temp sensor take about 5µA, so the TC1262 3.3V regulator seems
to account for ~70µA.
There is basic SD card support
that might be useful for something like a data logger.

The receiver currently converts the raw temperature reading to °C and displays
it with the RSSI value, CRC result and transmitter output power on a nice IPS
TFT display. It responds to the transmitter as kind of ack with the RSSI, which
is used for some very basic power management in the transmitter, significantly
reducing the supply current by reducing the output power i.e. on short distance.
The transmitter waits for this response with a timeout so it won't be blocked
and consumes a lot of power just because there is no response coming back.

Fun Stuff

Looking at the payload in the FSK modulated signal from the transmitter 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:

LoRa

With LoRa, the signal of course looks a bit different:

And it doesn't look like URH can decode LoRa for the time being.