Looking at AN4261 (rev4).
In Table 12, do the min and max DDR memory bus clock speeds apply to the asynchronous mode of operation only?
For example, lets say we want DDR3 memory. In asynchronous operation, we provide an external clock (lets say 100MHz) and according to Table 16 we can scale it 6:1 or 8:1 to provide a DDR data rate from 600MHz to 800 MHz?
But if we run in synchronous mode, our memory bus runs at half the Platform Frequency. Lets say our platform frequency is 512 MHz, out memory bus will be clocked at 256MHz. This is outside the limit of what is specified in Table 12 for DDR3 memory bus clock speed.