Our new product have some memory issue which old product didn't, we did the calibration test and update the MMDC registers(uboot/board/freescale/mx6q_sabresd/flash_header.S). now it can pass DDR stress test for range 413 ~ 585MHz. (but old product can pass 413~672MHz)
According Freescale i.MX6 DRAM Port Application Guide-DDR3 , in page 19, it says:
If the DDR3 cannot even pass the stress test at a low frequency, say 200 MHz, it is more likely to be a
hardware problem. Then, it would be a good starting point to check the DDR3 connections. In addition,
you should check the power supply for the DDR3, i.MX6 MMDC and DDR3 I/O to see if voltage is valid
and ripple is not too large.
Our new product never success on frequency lower than 400Mhz, is this situation just fine or not?