I found a strange power-up problem with the IMX28, it is can be simulated on the imx28 evaluation kit.
- Set the board in 5V wall power mode, no battery or USB connected.
- Windows CE installed on NAND
- Power down (Mains) the power supply
- Turn power –on before the 5Vdc is completed discharged, critical point is about 1Vdc
- The CPU will reboot but will get stuck in the xldr part
- Reset button is not working
- oscillator is still running
- The only way to reset the CPU without turn-off the power is by touching one of the XTAL pins with your finger (hence injecting some 50Hz disturbance).
I did some measuring, in some cases the XTAL will continue to oscillate even if the wall 5V power is very low, but the CPU goes in reset and restarts.Boot sequence fails somewhere in the XLDR code. External reset is not working anymore. You need to restart the XTAL oscillator to proper restart.
Keep in mind it is difficult to get in this situation but one can simulate it by unplugging the mains power of the eval kit and reconnect it before the power is completely discharged, critical point is somewhere around 1Vdc measured on the 5Vdc wall.
At this point the power-up of the IMX28 is not reliable, and a external Watchdog or reset chip can´t solve the problem as the external reset is not working once you get in this situation.