Realtek AmebaD (RTL8720DN) family port — working on BW16, questions before a PR

[Bl00d-B0b]

What this is

A working port of the realtek-ambd family (RTL8720DN / AmebaD), running on an Ai-Thinker BW16. The family is already declared in families.json without a package; this fills it in.

Current state on hardware: boots, connects to WiFi, gets a DHCP lease, and runs ESPHome — adopted in Home Assistant over the native API with OTA available.

Evidence

LibreTiny boot, WiFi association and DHCP on the BW16 (LOG UART, PA7/PA8):

#calibration_ok:[2:19:11]
I [      0.000] LibreTiny v1.13.0 on bw16, compiled at Aug  7 2026, GCC 10.3.1 (-Os)
I [      0.000] Reset reason: Unknown
efuse mac  24:42:E3:16:0E:F2
ota slot   1 (valid 1/0)
I [      0.011] WIFI: Initializing LwIP
interface 0 is initialized
interface 1 is initialized
Initializing WIFI ... WIFI initialized
[2] sta mac 24:42:E3:16:0E:F2
[3] begin HOME-IOT
RTL8721D[Driver]: set pairwise key to hw: alg:4(AES)
Interface 0 IP address : 192.168.11.150
[4] CONNECTED ip 192.168.11.150 gw 192.168.11.254 rssi -23
[6] scan done: 10 networks
alive 0..6  wifi 3  ip 192.168.11.150  heap 260000 (flat)

ESPHome 2026.8.0-dev on the same device, over the encrypted native API:

Successfully connected to test @ 192.168.11.150 in 0.005s
Successful handshake with test @ 192.168.11.150 in 2.057s
[C][api:269]:   Address: 192.168.11.150:6053
[C][api:276]:   Noise encryption: YES
[C][esphome.ota:098]:   Address: 192.168.11.150:8892
[C][wifi:1268]:   IP Address: 192.168.11.150

Build: RAM 7.8% (41,152 / 524,288), flash 29.4% (609,160 / 2,072,576).

What the port contains

• builder/family/realtek-ambd.py — KM4 (Cortex-M33) build: SDK fwlib, FreeRTOS 10.2.0 with the vendor RTL8721D_HP port, lwIP 2.2.0, wlan/mbedTLS glue, generated rlx8721d_img2_all linker script, and OTA image packing
• cores/realtek-ambd/ — LibreTiny API (lt_cpu, lt_mem, lt_ota, lt_device incl. the eFuse MAC at 0x11A), LOGUART printf port, newlib syscalls, UART driver, and fixups
• boards/bw16.json + variant — flash map measured from a factory dump: boot 0x0000, KM0 boot 0x4000, system 0x5000, OTA1 0x6000, littlefs 0x200000, OTA2 0x206000
• Image format: [KM0 XIP][KM0 RAM][KM4 XIP][KM4 RAM][terminator], 32-byte section headers, slot selected by the 81958711 signature (matching OTA_Change() in rtl8721dhp_ota_ram.c)

Open questions before a PR

1. Framework package. The SDK currently comes from a personal fork (framework-realtek-ambd). Would you host it under libretiny-eu, as with the other Realtek families?
2. KM0 image. The KM4 half is compiled, but the KM0 half is a prebuilt blob. Mine is currently extracted from a device dump, which is not shippable. Is building it from the SDK's project_lp the expected route, or would a prebuilt ship in the framework package?
3. ltchiptool flashing. Flashing today is done with an external tool. I have the ROM protocol partly decoded (CMD_USB 0x05 baud negotiation, CMD_XMD 0x07 to enter XMODEM, 1K-checksum packets with a 4-byte LE address prefix, RAM loader at 0x3000A020, confirmed on hardware up to the first ACK). Is working ltchiptool flash support a prerequisite for merging the family, or can it land after?

Also opened #401, which fixes three defects in the shared realtek-amb Arduino WiFi event path that this work uncovered — those affect AmebaZ/AmebaZ2 too.

[Bl00d-B0b]

Port is now working on hardware; opened as drafts: #403 (family) and libretiny-eu/ltchiptool#98 (flasher). The questions above moved into the #403 description where they are easier to answer in context.