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Updates docs for data encoding in the BLE advertisement packet

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rbaron 2021-05-02 20:20:39 +02:00
parent 837a30ba03
commit 795fc46849
4 changed files with 34 additions and 2 deletions
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3
README.md
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@ -40,6 +40,9 @@ Another possibility is running [parasite-scanner](https://github.com/rbaron/para
This is the quickest way to collect and visualize data from b-parasites, and I personally use it a lot for testing and debugging.
## Protocol and Data Encoding
Sensor data is transmitted via BLE advertisement broadcasts. [Here](./code/b-parasite/README.md) you can find a byte-by-byte description of the data as it is encoded inside the advertisement packet.
# Battery Life
**tl;dr:** By taking readings 10 minutes apart, the battery should last for over a year.

18
code/b-parasite/README.md
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@ -22,4 +22,20 @@ Calls to `NRF_LOG` will be readable on the console using `JLinkRTTLogger`. This
```bash
$ 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](https://www.bluetooth.com/specifications/assigned-numbers/environmental-sensing-service-characteristics/) (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 | 4 bits for protocol version + 4 reserved bits |
| 1 | 4 reserved bits + 4 bits wrap-around counter for deduplication |
| 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, big-endian format |

13
code/b-parasite/src/prst/ble.c
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@ -21,6 +21,19 @@
// Sensor data payload that will go into the advertisement message.
// We have a maximum of 20 bytes to play with here.
// 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 | 4 bits for protocol version + 4 reserved bits |
| 1 | 4 reserved bits + 4 bits wrap-around counter for deduplication |
| 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 |
*/
#define SERVICE_DATA_LEN 16
static uint8_t service_data[SERVICE_DATA_LEN];

2
code/b-parasite/src/prst/shtc3.c
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@ -64,7 +64,7 @@ prst_shtc3_read_t prst_shtc3_read() {
NRF_LOG_INFO("[sht3c] Read temp: " NRF_LOG_FLOAT_MARKER " oC",
NRF_LOG_FLOAT((float)ret.temp_millicelcius / 1000.0));
NRF_LOG_INFO("[sht3c] Read humi: " NRF_LOG_FLOAT_MARKER " %%",
NRF_LOG_FLOAT(100.0 * ret.humidity / (1 << 16)));
NRF_LOG_FLOAT(100.0 * ret.humidity / 0xffff));
#endif
return ret;
}