BT Home Hub 4.0 Type A
Next generation of home router provided for BT (British Telecom) broadband subscription in UK.
OpenWrt support
Not supported. This platform is not a prime target due as current linux does not support the ARX300 chipset and (currently) unsupported Lantiq WAVE 300 Wifi chip (See https://forum.openwrt.org/t/support-for-wave-300-wi-fi-chip/24690/161 for latest details) 
Check forum for latest and update here when supported
All members of the BT Home Hub OpenWrt community are encouraged to join in ongoing development efforts on the Home Hub 4A, at http://openwrt.ebilan.co.uk/viewforum.php?f=6
Hardware
Info
| Architecture | MIPS |
|---|---|
| Vendor | Lantiq |
| bootloader | U-Boot |
| System-On-Chip | Lantiq Xway ARX368 PSB 50368 EL (MIPS 34Kc) |
| CPU/Speed | 500 MHz |
| NOR Flash | n/a |
| NAND Flash | 128MiB Samsung K9F1G08U0D |
| RAM Chip | Samsung k4t1g164qf-bcf7 / ESMT M14D1G1664A 2.5B |
| RAM Specs | DDR2 128MiB @ 250 MHz |
| Wireless | Lantiq PSB 8301 XWAY WAVE 300 Lantiq PSB 8231 XWAY WAVE 300 |
| Ethernet | 1x external 10/100/1000 XWAY PEF7071 via RGMII for WAN 1x internal 10/100/1000 PHY for LAN4 3x internal 10/100 PHY for LAN1-3) |
| Switch | |
| xDSL | Lantiq XWAY WRX312 Fully integrated 11n 2×2 RF/PA chip Is this the DSL chip? |
| USB | 1x USB 2.0 |
| DECT | no |
| Serial | Yes |
| GPIO Buttons | WPS, Restart and Reset buttons |
| Power | External 12V 1.5A |
Case Opening
The outer case is secured just with plastic clips. Levering apart from the top needs confidence only. Once apart 2 small self-tapping screws secure the PCB. The BT Home Hub 5 is the same casing and there are YouTube videos for Opening the case and Dismantling the router and PCB board which are very good reference.
Photos
Second unit:
Serial
Removing the heatspreader is not required for accessing the RX/TX/boot_sel pins.
However, soldering to SMD solder points is required.
| Resistor number | Function | Info |
|---|---|---|
| unmarked | Serial TX | above R322 (left circle in the image) |
| unmarked | Serial RX | below the C196 and C194 markings (right circle in the image) |
| R158 | boot_sel2 | Solder to the top pad |
Serial uses Baud rate 115200.
To boot via UART you need to pull boot_sel2 (results in CFG 0x4) to GND.