The most important thing to do before attempting to change / upgrade / modify the default firmware for a tablet (we speak about the WM8505 / WM8650 models), is to create a backup of the internal SPI flash memory. The internal SPI flash memory is the hearth of a tablet. If you can rewrite it with original contents, you can always restore your tablet to the original state.
I highly recommend, for anyone wants to play this “game” with his own tablet, to build a serial interface, that is a basic, but powerful, instrument to “look” inside a tablet. There are many interfaces available to convert the 3.3V signals, present in the tablet, to RS232 levels, and other cheap circuits that directly connect 3.3V true logic from the tablet to USB port, that is seen as COM port from a PC. I suggest to use my own design circuit proposed on my page at this link, ‘cause is powered by the tablet itself, so it doesn’t introduce any voltage in the device input lines when the device itself isn’t powered on. I have had problems, with direct USB to TTL interfaces, due to the presence of positive voltages on transmitting side of the interface even if the tablet was off. So, please, build my circuit (it works !), or use one similar…
In the oldest tablet I own, the Evodroid Devo, based on WM8505 processor, there isn’t any “dongle” for the USB / LAN / Serial. There is just a mini plug for USB, but the serial port is always present inside the tablet. So, opening the case, you can solder the 4 wires needed for a serial terminal connection on the 4 pads as in the picture below :
The TXD is intended as tablet TXD, so is the output from tablet, while RXD is obviously, the data input to tablet.
Using a terminal emulator and the RS232 interface, we can make huge amount of tests and experiments. I recently updated my PC serial terminal program for some problems encountered on WM8505 tablets. In the new version, available for download at this link , I added the sequence “+++” ; if you quickly type this sequence on your PC keyboard, the terminal sends a Ctrl-C plus a CR to the tablet, stopping the autoboot (as default, U-Boot automatically starts the Linux Kernel and Android OS that are stored on tablet’s mass storage NAND flash).
Note that, for storing a copy of the internal SPI flash memory, you must insert a FAT32 freshly formatted microSD card in the tablet’s slot !
Well, if you have the serial interface connected to your PC, launch the terminal program, then turn on the tablet. You will see the W-load and the U-Boot messages ; immediatly type on the PC the +++ sequence (or type Ctrl-C and Enter if using another terminal program)… After a while the U-Boot will stop writing the command prompt WMT # ; the autoboot has been stopped and you can execute commands from the U-Boot provided set. Note that the U-Boot versions are different from one tablet model to another, but many commands are always available, even with small syntax differences. Here, I will describe the backups done from a relatively new 9.7″ tablet based on WM8650 processor, and from old 7″ Evodroid tablet (the one shown in the picture).
The first command to enter is the MMC initialization (MMC is the default name for the SD card), so we must type :
(0 selects the first SD card in wm8650 tablet, that has 2 SD cards capability)
or simply :
(in wm8505 that has only one SD card available)
We receive some messages from the tablet ; if all is OK, the MMC (SD) has been initialized and is ready to operate. Remember to use a FAT32 formatted one !
Now, we type the command :
cp.b FFF80000 0 80000
that means : copy bytes from address FFF80000 (the start of SPI flash memory) to address 0 (the start of RAM) for a length of 80000 bytes (that is hexadecimal, as all the data supplied), so 80000 is 8 * 64K = 512K bytes, the size of the whole SPI flash.
This operation will take a couple of seconds… then we have a copy of the whole SPI flash stored in system RAM. Now, we can use the powerful command “fatstore” to save that area to a binary file on the SD card :
fatstore mmc 0:1 0 backup.bin 80000
This command means : store a file on FAT system device mmc (our SD card), on partition 0:1 (the card is freshly formatted, so has just one partition : card 0, partition 1), reading data from RAM address 0, to a file named backup.bin, for a length of 80000 (hexadecimal) bytes. Note that in the wm8650 tablet I havent had any problem to execute this command, but in WM8505 there was something wrong while writing the SD card. The solution, was (simply) to substitute the original 4GB SD card previously used, with another one, a little bit older, sized just 128 MBytes ! That card was VERY OLD, it was in my LG cellphone by many years. So, if you have errors, try with another SD card, freshly formatted, of different type, brand or size.
When this operation terminates with success, you have a copy of your SPI flash stored on the SD card. This 512KBytes file has the U-Boot.bin, the Env_uboot.bin and the W-Load.bin files all in one. You can expand in 3 different files using my software tool that can be downloaded from this link. The zip file contains just one executable written in FreeBasic ; it does not require installation ; just deflate it in a new directory and copy in the same folder the spi.bin file you want to split ; at end of the process you will have u-boot.bin, w-load.bin and env_uboot files in the same folder.
Obviously, you can simply use the full file spi.bin file with an SPI programmer in case of tablet “hard bricked”.
To turn off the tablet, type the command :
or, if it isn’t recognized, simply press and hold the power button. Extract the SD card and read the contents with your PC. If you want, send the BIN file to my blog, using the file transfer form ; I will collect the different roms for future help requests… 🙂