Bluetooth Classic Protocol Guide: A2DP, SPP, HID & Diagnostics

Core Technical Specifications

center Freq 2.402–2.480 GHz

modulation GFSK / FHSS (79 channels, 1600 hops/sec)

data Encoding SCO (voice), ACL (data), eSCO (enhanced voice)

operating Range 10 m (Class 2) / 100 m (Class 1)

antenna PCB trace or ceramic chip

tx Power Class 1: 20 dBm, Class 2: 4 dBm, Class 3: 0 dBm

bandwidth 1 MHz per hop channel

channel Spacing 1 MHz across 79 channels

Real-World Devices & Applications

Audio & Media

Bluetooth headphones and speakers (A2DP)
Car hands-free kits (HFP)
Wireless audio receivers/transmitters

Peripherals

Keyboards and mice (HID profile)
Game controllers
Serial adapters (SPP profile) for embedded systems

Reference Circuits & Schematics

Bluetooth Classic Module — HC-05 / BC127

The HC-05 is the most common Bluetooth Classic module for hobbyist and embedded use. It presents a UART interface for SPP serial communication.

Pin	Description
`VCC`	3.3V–6V supply (onboard regulator)
`GND`	Ground
`TX`	UART transmit (module → MCU)
`RX`	UART receive (MCU → module)
`KEY/EN`	HIGH = AT command mode, LOW = data mode
`STATE`	Connection status output

Circuit Walk-Through

The HC-05 configured via AT commands (baud rate, name, PIN). When paired, it creates a virtual serial port (SPP) — data sent to TX appears on the remote device’s serial port and vice versa. For A2DP audio, more capable modules like BC127 or CSR8675 are used with I²S audio output.

Signal Structure & Waveform Analysis

Bluetooth FHSS Hopping Pattern

Bluetooth Classic uses adaptive frequency hopping across 79 channels at 1600 hops/sec to avoid interference.

Inquiry/Page

Device discovery

32 hop frequencies, 1.28s inquiry scan

Connection

ACL/SCO links

1600 hops/sec, 625 µs per slot

AFH Map

79 channels

Bad channels marked and avoided

Timing Notes: Each time slot is 625 µs (1/1600 sec). Packets can span 1, 3, or 5 slots. AFH (Adaptive Frequency Hopping) avoids Wi-Fi-occupied channels. A2DP uses SBC codec by default; aptX/AAC/LDAC negotiated if supported.

Analysis & Capture Tips

Use Ubertooth One to sniff Bluetooth Classic packets
Check HCI logs on Android (btsnoop) or Linux (btmon) for pairing issues
Verify AFH channel map to check for Wi-Fi coexistence problems
For audio: check codec negotiation in A2DP SBC/aptX headers

Bench Troubleshooting & Repair Guide

1. Pairing Failure

Device Bluetooth settings

Clear pairing history on both devices and re-pair. Check PIN code (default 1234 or 0000 for HC-05). Verify both devices are discoverable.

2. Audio Quality Issues

Bluetooth developer options (Android) / HCI logs

Check negotiated codec (SBC = basic, aptX = better). Verify no Wi-Fi interference on same 2.4 GHz band. Distance >10 m with Class 2 causes dropouts.

3. Serial SPP Drops

Logic Analyser / Serial Monitor

Check baud rate match between HC-05 and MCU. Verify RX/TX are crossed correctly. Add 1kΩ voltage divider on RX if module is 3.3V and MCU is 5V.

Bluetooth Classic