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 with response is working so far.
I'm impressed how well these radio modules work; the range achieved with
simple wire antennas as well as the reliable packet transmission.

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 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, reducing the
supply current from 45 mA down to ~15 mA. The transmitter waits for this
response with a timeout so it won't be blocked and comsume a lot of power just
because there is no response coming back.

Range

Setting RegPaLevel to 0x5f, which gives +13 dBm with PA1, indoor range is very good
and in an actual "field" test, packet reception was still reliable with an RSSI of about -90 dBm
at about 2.2 km distance - with simple wire antennas. What would be the range with +20 dBm and
decent antennas?

Susceptibility to Temperature Changes

With the default Frequency Deviation of 5 kHz and Receiver Bandwidth of 10.4 kHz, packet transmission
is very unreliable and fails completely for me, when the temperature of the transmitter is below
10°C and above 40°C, while the receiver temperature is at 20°C. The receiver does not seem to be
prone to temperature changes.
Increasing Frequency Deviation to 10 kHz and Receiver Bandwidth to 20.8 kHz, temperature susceptibility
is eliminated; when testing with transmitter temperature from -20°C to 50°C, packet transmission is
perfectly reliable.

Frequency Deviation = 10 kHz (transmitter)
RegFdevMsb = 0x00
RegFdevLsb = 0xa4

Receiver Bandwidth = 20.8 kHz
RegRxBw = 0x54

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: