[Android Head-Units] bazanovv:
Disclaimer. Each flasing is a risk, and if you will brick your HU - if something goes wrong, non-compatible formware will be used and so on - nobody, except you, will be responsible. Flashing MCU is a double risk - there are no recovery procedure for the failed MCU flashing, HU will turn into an unrecoverable brick forever. Dont start/stop the engine; don't switch gear to reverse, if camera connected; don't open the doors if CANbus adapter connected; don't even breathe, then flashing the MCU.

Steps to recover depend on the HU model. There are three common types of Head Units made on Allwinner T3 CPU/SOC, with completely different and incompatible hardware and firmware:

"Default one", or "Standard", discussed in this topic. About phone menu is with white letters on the black. Firmware version is like BSP 1.9.4.0.9 - IVT - 170718 (same for all - BT model - date in YYMMDD format). Engineering settings menu is called by clicking Kernel version - OS version 3 times, no passwords in any menus. Firmware update is done by inserting USB flash with os_update.zip or mcu.bin in the root folder, update menu will pop up. HU reboot then cranking / starting the car is fixed in MCU 15.07.2017 or 24.07.2017 AND Android firmware 18.07.2017 (both MCU and Android should be updated), set Sleep mode to at least 30 seconds in the Settings - Settings - General - sroll down. MCU is different for different radio module - most MCUs are for the TDA7786, and few for TEF6686 (written in description). Radio will not work, and Android sound will be with interruptions with the wrong MCU. If you have a 1 DIN HU with moving display, don't flash it with the general firmware - they are not compatible, you will loose display and have to recover with the external display/TV;

"New one", with firmware like was on Allwinner R16 HU. About device menu is with black letters on the white. Firmware version is like V8.2.1_20170419.173320_JYZC1 (version_date.time in YYYYMMDD_maker code). Engineering settings menu is called by Settings -> General (Car Settings) -> Extra settings (password 123456). Other passwords here. Firmware update is done in Settings -> General (Car Settings) -> System update or MCU update. Firmware files are 3 main e24b0eb4-f9b4-11e6-813c-fb743717e00b.0, .1 .2 and e1223aee-f9b4-11e6-912c-3f22a2ba34eb and, may be, others. One of the Android partitions holds the set of the MCU firmwares for different cars with CANbus, different AudioDSP and radio. If you flash it with the wrong set (file bd07a5ee-fbb0-11e4-ae78-000c29ba27c0, better don't flash it) - you will loose choose car model menu. Tool for backup Android firmware here, root required. MCU is flashed each time you choose different car model in settings, don't touch anything then HU is flashing itself.

Owners of this HU frequently report "strange" sound, fixed by moving balance slider slightly to the left or right. If so, check and change the speakers phasing - change the + and - wires on one or some of the loudspeakers, check this sound.

Updated and fixed by the independent programmer firmware, called RedMod, is available for this model , however, it cost additional $35;

"K2001", with menu on the left side and K2001 in firmware version like K2001_YZG_S212104.20170519 (K2001_maker code_time.date YYYYMMDD). Engineering settings menu is called in application Car Setting, Car Settings -> Factory Settings, password 1617 or 16176699 to enable skin selection menu. Sometimes passwords are 66668888 or 3368 or others. Firmware update is done by inserting USB flash with with update.zip and update.script, update will autostart, or in CarSettings -> Factory Settings -> System Update -> Start Update menu, sometimes only one USB connector works.

This HU frequently do have a fake Android version - real 4.4 may be patched to look like 5.1 or 6.0. Check the API Version in any software like CPU-Z, Aida, and so on. API 19 = Android 4.4 and API 23 = Android 6.0. Details here. Firmware with 6.0 really exists, and HU may be updated from 4.4 to it, but the procedure is quite risky and complicated - you have to flash from bootable uSD card with no displlay, or force HU into computer flashing mode by soldering wire, connect it to PC and flash with PhoenixTool. Details here, here and here.

This HU may use a 1024x600 or 800x480 display. Firmware for different resolution is NOT compatible, if you flash the wrong one - part of display will be unused, or image does not fit on display. If so, connect the external monitor (TV), USB mouse, and flash the right one.

More details here, use Google Translate on it.

For all three models, and for other Android devices too, you may get into recovery by:

Connecing external USB keyboard;

Hold Alt + PrintScreen and press "i" repeatedly many many times then HU is working,

... or then HU booting then auto logo appears, before Android logo.

"i" says Android to stop all processes, so getting into recovery depend on luck and time of pressing.

Recovery menu is very limited. There are no custom recovery available. Use keyboard to move in this menu, or right swipe for the Default HU model.

Default and K2001 models do have a firmware image in a .img format to flash it from bootable microSD card, made with PhoenixTool. This will allow to recover Android from ANY state, if hardware and MCU is functional. You should download firmware in .img + phoenix tool (in same archive), prepare uSD card, format it and write firmware on it with Phoenix tool. It creates a special hidden boot partition with firmware. Detailed manual is in the same archive. Insert uSD card in switched off DVD, switch it on - it should start flashing immediately. For the Default type HU progress is displayed, for K2001 screen remains off - just wait for 10-15 minutes and power it off. If nothing started - try another uSD slot. After succesfull flashing remove uSD and clear it with the Phoenix tool, or DVD will start firmware update at every boot with this uSD.

New one model, unfortunately, does not have such recovery firmware now. But you may try to do wipe data / factory reset from recovery, also it have an emergency programming mode, called by:

a) Connect Key1, Key2, or both imput wires to steering wheel keys to ground before HU is powered on, or press resistive (analogue, not CANbus connected) steering wheel keys, and hold them until you see Start update... OR

b) Switch HU on - press Reset - immediately switch HU off (turn off the engine) - after 5 seconds switch HU on (turn on the ACC, better don't crank the car).

USB flash, formatted in FAT32 or FAT with 4 firmware files like e24b0eb4-f9b4-11e6-813c-fb743717e00b.0 .1 .2 and e1223aee-f9b4-11e6-912c-3f22a2ba34eb should be already installed.

All firmwares available here. I will not store it forever, so if you need it - make a backup for yourself.

Default HU type:

Discussed here.

New one HU type:

Discussed here.

K2001 HU type:

Discussed here.

Bonus - new Allwinner T8 HU with 8 cores.

P.S. For all head units, if sleep mode does not work then should, time and, sometimes, radio stations is reset after switching off, and radio starts instead of last sound source used, it may be because of the different pinout standards of the ISO connector. Sometimes you need to change the B+ (battary, always on power) yellow thick wire and ACC (on then car is started) red thick wire on the ISO connector to match the car pinout. Details here.

For all Allwinner T3 head units CPU/SOC overheat is a known problem. CPU by PDF do have an auto (-40 +85C) thermal grade, however, it may be as hot as 105..110 C (220+ F), and start freezing/rebooting at this temperature. To lower the temperature, and prolong CPU life, install additional CPU heatsink or fan, details in the older post.