My IMX6ULL board is designed booting from EMMC FLASH which connected to SD2 interface. When power on, i burns my program to EMMC FLASH throught USB OTG successfully. i shut down power and then turn on, uart1 output information normally, cpu and the whole board works well. i try this operation many times, the cpu and the whole board is ok every time. But when power is on, i warm reset(i am sure POR_B signal is pulled low for about 200ms) cpu, uart1 have no output. it seems that cpu does not work or cpu can't boot from EMMC FLASH. i am sure cpu is not in suspend or sleep mode when warm reset happens. my four boards have the same problem.
In my designment, EMMC FALSH reset signal is come from max706 SYS_RST# instead of from SD2_RST#(CPU PIN B4). EMMC FLASH and cpu share the same reset signal SYS_RST# from max706. Power-up sequence: 3.3V(VDD_SNVS_IN, VDD_HIGH_IN, NVCC_xxx) ->1.2V(VDD_SOC_IN) ->1.5V(NVCC_DRAM).
In my another IMX6ULL designment, the cpu boots from a 8-bit nand flash instead of EMMC FLASH. the two different boards have the same external reset circuit and the same power up sequence. when warm reset hanppens, cpu can boot from nand flash and work well every time. why this happens?