Bluetooth Dual-Mode Power Management (ESP32)

Goal: Keep Bluetooth always on without excess drain.
How: Idle = BLE with long advertising intervals & low TX power.
On interest/traffic = tighten connection params, boost TX power, and optionally start Classic SPP for maximum robustness/throughput.
On inactivity = gracefully fall back to low-power BLE.

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Table of Contents

1. Features
2. System Design
3. Power Strategy & Defaults
4. Build & Flash
5. Usage
6. Tuning Guide
7. API: GATT Service & Control
8. Project Structure
9. Troubleshooting
10. FAQ
11. Roadmap
12. License
13. Contact

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Features

• Ultra-low idle power
  • BLE advertising ~1 s interval, −24 dBm TX power
  • CPU light sleep & dynamic frequency scaling
• Adaptive discovery
  • Button press or scan interest → probe mode (advertising ~100 ms)
• Strong active link
  • Tight BLE connection intervals 7.5–15 ms
  • +9 dBm TX power in ACTIVE
  • Optional Classic SPP server for robust/high-duty sessions
• Auto fallback
  • Inactivity (default 15 s) → relax params, stop SPP, return to idle advertising
• Activity aware
  • Promotes to ACTIVE when ≥256 B received within 3 s window

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System Design

            ┌──────────────┐
   Idle     │  BLE Adv     │  ~1s, -24 dBm
            └──────┬───────┘
                   │ Button/Scan interest
                   ▼
            ┌──────────────┐
   Probe    │  BLE Adv     │  ~100ms, -24 dBm
            └──────┬───────┘
                   │ Connection + traffic or BOOST_ON
                   ▼
            ┌────────────────────────────┐
   Active   │  BLE 7.5–15ms, +9 dBm      │
            │  + optional Classic SPP    │
            └──────┬─────────────────────┘
                   │ Inactivity timeout
                   ▼
                  Idle

Modes

• IDLE: Discoverable, minimal drain
• PROBE: Temporarily easier to find/connect
• ACTIVE: Low latency, strong signal, optional SPP

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Power Strategy & Defaults

Mode	BLE Adv Interval	BLE Conn Interval	Conn Latency	Supervision Timeout	TX Power	Classic
Idle	~1000 ms	N/A	N/A	N/A	−24 dBm	Off
Probe	~100 ms	N/A	N/A	N/A	−24 dBm	Off
Active	N/A	7.5–15 ms (6–12 ticks)	0	4 s	+9 dBm	Optional

Promotion/Demotion

• Promote: Data activity ≥ 256 B within 3 s, or explicit BOOST_ON
• Demote: No traffic for 15 s → PROBE/IDLE; BOOST_OFF stops SPP

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Build & Flash

Prereqs

• ESP-IDF 5.x (or recent 4.x with Bluedroid + SPP)
• ESP32 dev board (e.g., WROOM/WROVER)

idf.py set-target esp32
idf.py build
idf.py -p /dev/ttyUSB0 flash monitor

Logs show mode switches, advertising/connection events, and SPP status.

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Usage

BLE (default)

• Device name: ESP32-LOWPOWER-HYBRID

• GATT service UUID: 0xABF0

  • RX 0xABF1: Write to send data to ESP32 (counts toward activity)

  • TX 0xABF2: Notifications; device echoes RX for demo

  • CTL 0xABF3: Write ASCII control commands:

    • BOOST_ON → Enter ACTIVE, max TX power, start Classic SPP
    • BOOST_OFF → Stop SPP, relax back to PROBE/IDLE

Classic SPP (optional “boost”)

• Device name: ESP32-SPP-BOOST (appears after BOOST_ON)
• Standard RFCOMM socket; demo echoes received bytes

Button (GPIO0 / BOOT)

• Press to temporarily enter PROBE (faster advertising)

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Tuning Guide

All knobs are in main/main.c:

• Advertising

  • Idle: adv_params_idle.adv_int_min/max (units = 0.625 ms; 1600 ≈ 1 s)
  • Probe: adv_params_probe.adv_int_min/max (160 ≈ 100 ms)

• Connection (BLE)

  • Active: CONN_ITVL_MIN_ACTIVE=6 (7.5 ms), MAX=12 (15 ms)
  • Save: CONN_ITVL_MIN_SAVE=80 (100 ms), MAX=200 (250 ms), LATENCY=4

• TX Power

  • Idle: set_tx_power_idle() → ESP_PWR_LVL_N24
  • Active: set_tx_power_active() → ESP_PWR_LVL_P9

• Promotion/Demotion

  • Activity window: ACTIVITY_WINDOW_MS (default 3000)
  • Threshold bytes: ACTIVITY_BYTES_THRESH (default 256)
  • Inactivity timeout: INACTIVITY_MS (default 15000)

• User input

  • Button pin: GPIO_BTN (default 0)

Tips

• Latency-sensitive HID: raise Active interval upper bound slightly (12→18)
• Sensor/beacon: loosen Save intervals (e.g., 200→400) for more savings
• Range issues: keep Active +9 dBm; consider PROBE at −12 dBm instead of −24 dBm

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API: GATT Service & Control

Service: 0xABF0

Characteristic	UUID	Props	Direction	Notes
TX	0xABF2	Read, Notify	ESP32 → Central	Enable notifications on client
RX	0xABF1	Write, Write NoRsp	Central → ESP32	Payload counts toward activity
CTL	0xABF3	Write	Central → ESP32	BOOST_ON, BOOST_OFF

Behavior

• Any RX write increments activity window; TX echoes for demo.
• CTL commands switch modes and start/stop SPP.

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Project Structure

bt_dual_mode_pm/
├─ CMakeLists.txt
├─ sdkconfig.defaults
└─ main/
   ├─ CMakeLists.txt
   └─ main.c

• sdkconfig.defaults enables Bluetooth (BLE + Classic), Bluedroid, PM, and reduces log noise.
• main.c implements GAP/GATT, SPP, timers, mode state machine, and power control.

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Troubleshooting

Device not discovered

• Verify advertising started (log: “Advertising start…”).
• Temporarily switch to PROBE (button press) to shorten adv interval.
• Ensure phone/host supports BLE and is scanning.

Disconnects / weak link

• Keep ACTIVE at +9 dBm.
• Increase supervision timeout (e.g., 4 s → 6–8 s).
• Consider raising Active interval max (12→18) if collisions/congestion.

SPP doesn’t appear

• You must send BOOST_ON via CTL first (SPP starts dynamically).
• Some OSes require pairing before SPP shows; remove/forget old pairing.

High idle current

• Confirm you’re in IDLE (logs).
• Check adv_params_idle ~1 s; ensure TX power is N24.
• Make sure tickless idle and light sleep are enabled in config.

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FAQ

Q: Can I use NimBLE instead of Bluedroid? A: Yes—port the GATT parts to NimBLE. For SPP you’ll still need BR/EDR (Classic) support; keep dual-mode controller enabled.

Q: Can I do A2DP instead of SPP for audio? A: Yes, swap SPP init for A2DP sink/source. Expect higher duty cycle and power use while streaming.

Q: Can this run on nRF52/Zephyr? A: Conceptually yes. Recreate the three-state machine (IDLE/PROBE/ACTIVE), map to Zephyr’s bt_le_adv_param, connection parameter updates, and power levels.

Q: How do I make it “always on” but nearly zero drain? A: Stretch idle advertising to 2–5 s, keep −24 dBm TX, and rely on PROBE on user input or scheduled windows.

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Roadmap

• Optional RSSI-based adaptive TX power during ACTIVE
• Zephyr/NimBLE reference port
• Config over GATT (change intervals/power from client)
• Metrics characteristic (expose bytes/s, mode, RSSI)

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License

This project is licensed under the MIT License. See LICENSE.

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Contact

• Author: R&D Biotech Alaska
• Email: contact@rdbiotechalaska.com
• GitHub: https://github.com/R-D-BioTech-Alaska