You can display the serial console from the TXT controler with a USB to TTL adapter. In the following example, a USB to TTL adapter with the chip CP2102 was used. Alternatively you could use an adapter with FT232 chip or similar. It is important that the adapters work on a 3.3 volt basis. First, you should plug the adapter into the desktop PC and check whether there are only 3.3 V between GND <-> RX or GND <-> TX. With 5 volts, the txt controller would be damaged.
The EXT interface from the controller is occupied as follows:
RX
| reserved
| | I2C-Data
| | | Extension
| | | | GND
| | | | |
| +–––+ |
+–––+ +–––+
| O O O O O |
| O O O O O |
+–––––––––––+
| | | | |
| | | | Extension
| | | Extension
| | I2C-Clock
| reserved
TX
When wiring, make sure that RX and TX are cross-connected:
- TX <-> RX
- RX <-> TX
- GND <-> GND
Then the console can be opened with the OSX as follows:
screen /dev/tty.SLAB_USBtoUART 115200
Access with other operating systems is also possible.
Finally, the TXT can be turned on and you can see the boot process.
U-Boot SPL 2013.10 (Jul 04 2015 - 08:30:17)
>>> I2C0 On
Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
Could not get board ID.
>>> setup_dplls vor do_setup_dpll
>>> setup_dplls nach do_setup_dpll
Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
Could not get board ID.
>>> Setting MUX Start
>>> Setting MUX End
Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
Could not get board ID.
>>>Start TXT-Mode - V2
LCD-Init_4:
Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
Could not get board ID.
---> MPU 600 VDD 46
U-Boot 2013.10 (Jul 04 2015 - 08:30:17)
I2C: ready
DRAM: 256 MiB
NAND: 128 MiB
MMC: OMAP SD/MMC: 0, OMAP SD/MMC: 1
Could not probe the EEPROM; something fundamentally wrong on the I2C bus.
Could not get board ID.
Net: usb_ether
NAND read: device 0 offset 0x700000, size 0x40000
262144 bytes read: OK
CopyToLcd
LCD Command called
Hit ENTER key to stop autoboot: 0