rbaron d8aea1d4ef Introduces version-specific config & condition BLE lux encoding

In the BLE service data payload, one of the reserved bits is now used to
indicate whether or not the ambient lux value is encoded in the packet.
Bit 0 of the first byte is now the `has_lux` bit. If it is set, bytes
16-17 in the service data will contain the ambient lux. If `has_lux` is
not set, bytes 16-17 may not exist (for compatibility with older
firmwares) or may hold meaningless values.

This commit also introduces version-specific configuration in
prst_config.h. Photoresistor config, for example, is only meaninful for
version 1.1.x.

2021-09-22 22:39:11 +02:00

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Overview

This is the b-parasite formware based on Nordic's nRF5 SDK.

It uses Nordic's SoftDevice, which should additionally be flashed to the chip before running our firmware.

I use a JLink probe for flashing and debugging.

Configuration

The b-parasite specific configuration, such as active/sleep time and transmitting power are defined in config/prst_config.h.

Flashing SoftDevice and Firmware

# Flash softdevice
$ SDK_ROOT=~/dev/nrf52/sdk/nRF5_SDK_17.0.2_d674dde make flash_softdevice
# Compile annd flash our firmware
$ SDK_ROOT=~/dev/nrf52/sdk/nRF5_SDK_17.0.2_d674dde make flash

Debugging

Calls to NRF_LOG will be readable on the console using JLinkRTTLogger. This is the handy one-liner I use for pulling log messages:

$ echo "\n\n\n\n0\n/dev/stdout" | JLinkRTTLogger | sed 's/^.*app: //'

Bluetooth Low Energy Advertisement Data Encoding

Sensor data is encoded in the BLE advertisement packet as Service Data for the Environmental Sensing Service profile (UUID 0x181a).

Sensor data is encoded in unsigned 16 bits (2 bytes), and whenever multiple bytes are used to represent a single value, the encoding is big-endian.

Byte index	Description
0	Protocol version (4 bits) + reserved (3 bits) + has_lux* (1 bit)
1	Reserved (4 bits) + increasing, wrap-around counter (4 bits)
2-3	Battery voltage in millivolts
4-5	Temperature in millidegrees Celcius
6-7	Relative air humidity, scaled from 0 (0%) to 0xffff (100%)
8-9	Soil moisture, scaled from from 0 (0%) to 0xffff (100%)
10-15	b-parasite's own MAC address
16-17*	Ambient light in lux

* If the has_lux bit is set, bytes 16-17 shall contain the ambient light in lux. If the has_lux bit is not set, bytes 16-17 may not exist or may contain meaningless data. The reasons for this behavior are:

b-parasite version 1.0.x has no light sensor and its advertisement data may have only 16 bytes if its using an older firmware. In this case, has_lux shall never be set;
b-parasite version 1.1.x has space for an optional LDR. Users can configure whether or not they have added the LDR by setting the PRST_HAS_LDR to 1 in prst_config.h.

Supported Modules

This code supports two E73 modules:

E73-2G4M08S1C (nrf2480, default)
E73-2G4M08S1E (nrf2833)

To choose for which one you want to compile, just pass PLATFORM as an env variable to make, and set it to the platform you want to use. For example, to compile for E73-2G4M08S1E:

SDK_ROOT=<...> PLATFORM=E73_2G4M08S1E make

and vice-versa for E73-2G4M08S1C, although that platform will be chosen as default anyways:

SDK_ROOT=<...> PLATFORM=E73_2G4M08S1C make

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