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microReticulumTbeam/exercises/26_Bluetooth_discover/README.md

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Exercise 26: BLE Discovery

Plain BLE neighborhood discovery for LilyGO T-Beam SUPREME ESP32-S3 units. This exercise is not Reticulum and does not use LoRa.

Each unit displays Take me outside at startup, waits for GPS UTC, GPS coordinates, PPS-backed RTC discipline, and then starts BLE advertising/scanning. AMY may run without SD logging because its card reader is defective; other units should mount SD and write logs.

Build

cd /usr/local/src/microreticulum/microReticulumTbeam/exercises/26_Bluetooth_discover
pio run -e amy
pio run -e bob
pio run -e cy
pio run -e dan
pio run -e ed
pio run -e flo
pio run -e guy

Upload example:

pio run -e bob -t upload --upload-port /dev/ttytBOB
pio device monitor -b 115200 -p /dev/ttytBOB

Behavior

  • Advertises manufacturer data in this format: B2|NODE|seq|tx_payload_epoch_ms.
  • Accepts current B2 advertisements and legacy TBMSND|1|NODE|seq|uptime advertisements from known nodes in AMY, BOB, CY, DAN, ED, FLO, GUY.
  • Displays heard nodes sorted by rolling RSSI average, strongest first.
  • Drops stale entries after 20 seconds.
  • Logs valid accepted advertisements to SD as /logs/YYYYMMDD_HHMISS_NODE_ble_search.log.
  • Refreshes the receiver GPS position at 1 Hz and logs the last known receiver coordinates plus GPS fix age. Observation rows are written while the holdover clock is valid; stale GPS is recorded with gps_valid=0 instead of suppressing the row.
  • Writes periodic # STATUS heartbeat lines so a quiet BLE interval can be distinguished from a reset, hang, SD fault, or power interruption.
  • Starts a WiFi AP and HTTP web service during boot. This does not wait for GPS.

Default web addresses:

  • AMY: SSID TBEAM-AMY, http://192.168.23.1/
  • BOB: SSID TBEAM-BOB, http://192.168.24.1/
  • CY: SSID TBEAM-CY, http://192.168.25.1/
  • DAN: SSID TBEAM-DAN, http://192.168.26.1/
  • ED: SSID TBEAM-ED, http://192.168.27.1/
  • FLO: SSID TBEAM-FLO, http://192.168.28.1/
  • GUY: SSID TBEAM-GUY, http://192.168.29.1/

The T-Beam hosts the WiFi access point and web page itself. No service needs to run on the workstation. To use the page, connect the workstation WiFi interface to the unit SSID, for example TBEAM-ED, then browse to that unit address, for example http://192.168.27.1/. A Panda USB WiFi adapter is useful only as the workstation WiFi interface used to join the T-Beam AP. The root page is a lightweight status page; click Files when you want the SD directory listing.

Field Procedure

  1. Flash all units with this BLE Discovery image, each with its own environment.
  2. Start all units where they can see sky and wait until each passes GPS/RTC startup discipline.
  3. Place one unit near the intended origin, preferably central to a star topology.
  4. Carry another unit and watch the OLED.
  5. Find a position where it hears only the intended neighbor with weak or moderate RSSI.
  6. Place it there.
  7. Repeat outward to create a chain.
  8. Record the topology manually before switching to the Reticulum BLE-interface firmware.

Limitations

  • RSSI is not a precise distance measurement.
  • Body position, antenna orientation, terrain, buildings, and foliage matter.
  • BLE advertisements are not guaranteed delivery.
  • Scan and advertise duty-cycle choices affect detection latency and power draw.
  • WiFi log download can affect BLE timing; use it for retrieving logs, not while making fine placement decisions.

Bench Acceptance Test

  • With AMY and BOB on the bench, AMY displays BOB with RSSI and age, and BOB displays AMY.
  • A third unrelated Bluetooth device nearby does not appear on OLED or accepted serial records.
  • If BOB is powered off, AMY ages BOB out after the stale timeout.
  • Serial output can be captured with:
pio device monitor -b 115200 -p /dev/ttytAMY | tee logs/ble_discovery_AMY_YYYYMMDD_HHMMSS.log

Logs

Here are two logs from ED and FLO which were activated in the field.

jlpoole@jp ~/work/tbeam/logs $ ls -lath ed/*_16*
-rw-r--r-- 1 jlpoole jlpoole 1.1M May 25 09:53 ed/20260525_162217_ED_ble_search.log
jlpoole@jp ~/work/tbeam/logs $ ls -lath flo/*_16*
-rw-r--r-- 1 jlpoole jlpoole 1.1M May 25 09:53 flo/20260525_162213_FLO_ble_search.log
jlpoole@jp ~/work/tbeam/logs $

Here are start and end samples from an earlier trial. These show the previous 12-column schema; current firmware uses the 19-column schema described below.

jlpoole@jp ~/work/tbeam/logs $ cat -n ed/20260525_162217_ED_ble_search.log|head -n 3
     1  human_time,epoch_ms,receiver,lat,lon,heard,rssi,avg_rssi,age_s,count,seq,payload
     2  2026-05-25 16:22:18,1779726138897,ED,44.9364577,-123.0218702,FLO,-56,-56,0,1,1,TBMSND|1|FLO|0001|0775
     3  2026-05-25 16:22:18,1779726138938,ED,44.9364577,-123.0218702,FLO,-51,-54,0,2,1,TBMSND|1|FLO|0001|0775
jlpoole@jp ~/work/tbeam/logs $ cat -n ed/20260525_162217_ED_ble_search.log|tail -n 1
 10277  2026-05-25 16:47:42,1779727662217,ED,44.9364577,-123.0218702,FLO,-42,-42,0,10276,611,TBMSND|1|FLO|0611|2300
jlpoole@jp ~/work/tbeam/logs $ cat -n flo/20260525_162213_FLO_ble_search.log|head -n 3
     1  human_time,epoch_ms,receiver,lat,lon,heard,rssi,avg_rssi,age_s,count,seq,payload
     2  2026-05-25 16:22:16,1779726136737,FLO,44.9365132,-123.0218183,ED,-52,-52,0,1,0,TBMSND|1|ED|0000|0805
     3  2026-05-25 16:22:16,1779726136829,FLO,44.9365132,-123.0218183,ED,-51,-52,0,2,0,TBMSND|1|ED|0000|0805
jlpoole@jp ~/work/tbeam/logs $ cat -n flo/20260525_162213_FLO_ble_search.log|tail -n 1
 10121  2026-05-25 16:47:37,1779727657219,FLO,44.9365132,-123.0218183,ED,-41,-40,0,10120,608,TBMSND|1|ED|0608|2325
jlpoole@jp ~/work/tbeam/logs $

The header represents:

Column Header Explanation
1 human_time Receiver timestamp in human-readable UTC form.
2 rx_epoch_ms Receiver timestamp as Unix epoch milliseconds.
3 receiver Unit that wrote the log row.
4 rx_lat Receiver GPS latitude from the latest valid local GPS fix.
5 rx_lon Receiver GPS longitude from the latest valid local GPS fix.
6 gps_fix_age_ms Age of the receiver GPS fix in milliseconds when the row was written. If GPS becomes unavailable, the last known coordinates remain and this age grows.
7 clock_valid 1 when the startup-disciplined clock is still inside the configured holdover window.
8 gps_valid 1 when the receiver GPS fix age is inside the configured freshness window.
9 discipline_age_ms Milliseconds since the startup GPS/PPS/RTC discipline event.
10 last_discipline_epoch_ms Epoch milliseconds assigned to the PPS edge used for startup discipline.
11 heard Remote unit heard in the BLE advertisement.
12 rssi RSSI measured by the receiver for this advertisement.
13 avg_rssi Receiver-calculated arithmetic mean of the most recent RSSI measurements for this heard unit, using up to the last 5 accepted advertisements and rounded to the nearest integer. The window size is kRssiWindow = 5 in main.cpp.
14 age_s Age in seconds of the displayed/heard entry.
15 count Number of accepted advertisements from that heard unit.
16 seq Sequence number advertised by the heard unit.
17 tx_payload_epoch_ms Sender payload generation timestamp from the BLE payload as Unix epoch milliseconds, when available. Legacy v1 payloads report 0 here.
18 payload Raw BLE manufacturer-data string. See Payload Definition below.
19 vbat_mv Battery voltage reported by the PMU, in millivolts. -1 means the PMU value was unavailable.

Payload definition

payload is the exact BLE manufacturer-data string received from the other unit. Current firmware advertises a compact v2 text payload:

B2|NODE|SEQ|TX_PAYLOAD_EPOCH_MS

Example:

B2|FLO|0611|1779727662217

Meaning:

Part Example Meaning
B2 B2 Compact Exercise 26 payload prefix and version.
NODE FLO Sending unit name. Receiver accepts only known units and ignores itself.
SEQ 0611 Sender’s advertisement sequence number, zero-padded, wraps every 10,000 advertisements.
TX_PAYLOAD_EPOCH_MS 1779727662217 Sender payload generation timestamp as Unix epoch milliseconds, derived from the disciplined local clock. This is not a per-advertisement RF transmit timestamp.

The receiver also accepts the legacy v1 payload used by earlier Exercise 26 firmware:

TBMSND|1|NODE|SEQ|UPTIME

Constraints currently enforced by the receiver:

  • Payload must fit in the receive buffer, currently less than 48 bytes.
  • It must be pipe-delimited.
  • Prefix must be current B2 or legacy TBMSND version 1.
  • Node must be one of AMY, BOB, CY, DAN, ED, FLO, GUY.
  • Node must not be the receiver’s own name.
  • Current B2 payloads provide SEQ and TX_PAYLOAD_EPOCH_MS; legacy v1 payloads provide SEQ only and log tx_payload_epoch_ms as 0.

Clock Discipline and Timestamp Semantics

BLE survey advertising starts only after startup GPS UTC, GPS coordinates, and PPS-backed RTC discipline have succeeded. Before that point the OLED remains on a blocking GPS/clock status page and the unit does not enter active BLE survey mode.

Observation logging requires clock_valid=1. The clock uses a holdover model: startup discipline sets last_discipline_epoch_ms, and clock_valid remains true until discipline_age_ms exceeds the configured holdover limit. Temporary GPS loss does not immediately invalidate the clock, and it does not suppress observation rows. GPS validity is tracked separately with gps_fix_age_ms and gps_valid, so stale-coordinate rows can be filtered later.

rx_epoch_ms is the receiver's local disciplined-clock time at the moment it writes the observation row. tx_payload_epoch_ms is the sender's local disciplined-clock time when it generated and installed that BLE advertising payload. A single payload may be observed many times before the sender replaces it, so tx_payload_epoch_ms is useful for correlating logs but is not an RF transmit timestamp.

rx_epoch_ms - tx_payload_epoch_ms is diagnostic for clock alignment, scheduling, payload age, and repeated-observation behavior. It is not BLE propagation delay. The status fields that prove a row was written under valid survey conditions are clock_valid, gps_valid, gps_fix_age_ms, discipline_age_ms, and last_discipline_epoch_ms.

Event Lines

Comment-style event lines begin with # EVENT and are not normal CSV observation rows. They preserve diagnostic transitions without complicating the ordinary CSV row format. Current event names are:

  • boot
  • log_opened
  • log_closed
  • gps_stale
  • gps_fresh
  • clock_stale
  • sd_removed
  • sd_remounted
  • sd_write_short
  • sd_write_ok

Status Lines

Comment-style status lines begin with # STATUS and are written about every 10 seconds while the log is open. They are not normal CSV observation rows. They report the unit's clock/GPS state, SD/log state, rows written, heard-node counts, and vbat_mv. These heartbeat lines are intended to show whether the firmware was still alive during periods with no BLE observations.