Saved FAQ from Freescale.com for information only:
Topic: Boot Loader
Question. Is it possible to use 3.3V NAND or SD devices as boot devices with i.MX51 processor?
Answer. Default value of register IOMUXC_SW_PAD_CTL_PAD_NANDF_** HVE bit is 1. This means that supply voltage for NVCC_NANDF_x needs to be between 1.65-1.95V (UHVIO_L) or 2.5-3.1V(UHVIO_H). iROM does not change this value. Applying 3.3V to NVCC_NANF_x may cause problems. If one would like to apply 3.3V to NVCC_NANDF_x, the following sequence is recommended.
1.When powering up i.MX51, apply 1.65-1.95 or 2.5-3.1V to NVCC_NANDF_x.
2.Bootloader sets HVE as 0.
3.Raise NVCC_NANDF_x voltage up to 3.3V.
i.MX Applications Processors > i.MX51 Processors > i.MX512, i.MX513, i.MX514, i.MX515, i.MX516