VIDC20 Data Sheet
So n must be chosen such that 4n/v > (2Pb + Imax + 0.05).
The maximum value for n is deduced from the following condition to avoid the FIFO overflowing:
In 32 bit bus mode, n may take the maximum value of 7, and the FIFO can never overflow, as there will always be 4 words available in the top of the FIFO, even if the video request is serviced immediately.
In 64 bit bus mode, if n takes the maximum value of 6, then the FIFO will never overflow, as there will always be 8 words available in the top of the FIFO, even if the video request is serviced immediately.
However, in very high video bandwidth modes, the value of 7 may be used. Then the criteria is as follows. There are 4 words free in the FIFO when it requests more data. If the request is serviced as soon as possible, then there must be enough room in the FIFO for 8 words, when the last word enters the FIFO. The FIFO requires another 4/v us to free up 4 extra words (giving a total of 8 free words). The minimum time from the FIFO requesting data to getting the last word into the FIFO is Pb + loin + 3P6.
So, if (Pb + lmin + 3P6) < 4/v, then n may be programmed to value 7.