USB-BT Network Bridge (lwIP NAT Solution)

Source code path: example/cherryusb/host/usb_bt_bridge_nat

Overview

This example implements a USB-BT network bridge: the Dongle (SiFli SoC) acts as a USB RNDIS host connecting to a phone (HarmonyOS/Android), using the phone’s “USB tethering”, while it connects to a Bluetooth earbud in the classic Bluetooth PAN NAP role. The Dongle translates traffic from the earbud with lwIP NAT (IPv4 forwarding + network address translation) and forwards it out through the USB RNDIS uplink, giving the earbud access to the Internet.

This solution bridges at the IP layer: the Dongle itself has two IP interfaces (b0 on the BT side, u2 on the USB side) and NATs the earbud’s private subnet 192.168.43.0/24 to the phone’s USB network (e.g. 10.196.255.x).

Supported Boards

Verified on the following development boards:

sf32lb52 LCD series

Hardware Required

A SiFli development board with USB Host support
USB Type-C cable
A HarmonyOS/Android phone (with “USB tethering” support)
A Bluetooth PAN (PANU) earbud

Technical Principle

System Topology

Internet (WiFi/4G/5G)
        │
   ┌────┴───────┐
   │ Phone (RNDIS Device) │  USB tethering (DHCP Server)
   └─────┬──────┘
         │ USB Type-C
   ┌─────┴──────┐
   │ Dongle (this example) │
   │  USB RNDIS Host       │  u2: 10.196.255.x (DHCP Client)
   │  BT PAN NAP           │  b0: 192.168.43.1 (DHCP Server)
   │  lwIP NAT             │  NAT: 192.168.43.0/24 → u2
   └─────┬──────┘
         │ BT PAN / BNEP
   ┌─────┴──────┐
   │ Earbud (PANU)          │  192.168.43.x (DHCP Client)
   └────────────┘

Data Flow

Uplink (earbud → Internet):

Earbud 192.168.43.x
  → b0 interface (received over PAN BNEP)
  → lwIP ip4_forward
  → ip_nat_out: source 192.168.43.0/24 hits the NAT entry → source rewritten to u2's IP
  → u2 interface (sent over USB RNDIS)
  → phone USB network → Internet

Downlink (Internet → earbud): ip_nat_input performs reverse NAT (rewrites the destination back to 192.168.43.x) on received replies and sends them back to the earbud over b0.

Key Implementation Points

USB side (u2): after enumerating the phone, the CherryUSB RNDIS Host registers the lwIP netif u2 through usbh_lwip.c; eth_device_init() automatically starts the DHCP client to obtain IP/gateway (e.g. 10.196.255.3 / gw 10.196.255.57).
BT side (b0): when the PAN connection is established, the bt_pan stack registers the lwIP netif b0 (NAP mode) and starts the DHCP server (LWIP_USING_DHCPD), assigning 192.168.43.2 ~ .254 to the earbud with gateway 192.168.43.1 (b0’s address).
NAT: the rndis_monitor_thread in main.c waits for u2 DHCP to bind, then calls update_nat() to install the NAT rule source=192.168.43.0/24 → out_if=u2 (LWIP_USING_NAT). NAT only applies to the forwarding path (ip4_forward) and does not affect traffic originating from the Dongle itself.
Dongle’s own connectivity: once the RNDIS uplink gets an address, the application sets it as the default netdev (netdev_set_default(u2)), so the Dongle’s own ping/DNS/sockets go out the phone uplink.

Using the Example

Compile and Flash

Change to the example project directory and run scons to compile:

scons --board=sf32lb52-lcd_a128r16 -j8

Run the flash command:

build_sf32lb52-lcd_a128r16_hcpu\uart_download.bat

Select the serial port as prompted to download:

please input the serial port num:6

Earbud Side

You can use another development board as the earbud side. For the earbud firmware, use the example\bt\pan example and flash it into the second board using the same compile/flash procedure above for testing.

Usage Steps

Stage 1: Verify USB tethering

Flash the firmware and open the serial terminal
Connect the phone via USB and enable “USB tethering” on the phone
Observe the [USB] DHCP bound ... log
Verify Dongle connectivity: ping 8.8.8.8

Stage 2: Connect the Bluetooth PAN earbud

Flash the earbud-side firmware and power it on
On the Dongle, start the inquiry: pan_net inquiry start
On the Dongle, connect: pan_net conn <earbud MAC> (e.g. pan_net conn 12345678abcd)
On the Dongle, observe [BT] PAN connected ... and [NAT] Enabled 192.168.43.0/24 b0 -> u2

Stage 3: Verify earbud connectivity

The earbud should obtain a 192.168.43.x address from the b0 DHCP server and be able to access the Internet; verify with ping 8.8.8.8.

Example Output

TX:ping 8.8.8.8
   D/BT.lwip bts: rt_bt_lwip_protocol_recv netif 0x20037094
   D/NO_TAG bts: Active mode st: 0, inv: 0.00
   D/btapp_pan bts:  BTS2MU_PAN_STS_IND
   D/BT.lwip bts: rt_bt_lwip_protocol_recv netif 0x20037094
   60 bytes from 8.8.8.8 icmp_seq=0 ttl=106 time=901 ms
   D/BT.lwip bts: rt_bt_lwip_protocol_recv netif 0x20037094
   60 bytes from 8.8.8.8 icmp_seq=1 ttl=106 time=283 ms
   D/BT.lwip bts: rt_bt_lwip_protocol_recv netif 0x20037094
   60 bytes from 8.8.8.8 icmp_seq=2 ttl=106 time=286 ms
   D/BT.lwip bts: rt_bt_lwip_protocol_recv netif 0x20037094
   60 bytes from 8.8.8.8 icmp_seq=3 ttl=106 time=284 ms
   msh />msh />

Troubleshooting

Dongle does not respond when connected to the phone: make sure the USB cable supports data transfer.
CherryUSB