wsinttst@lso.win.tue.nl (Studenten Kees v Overveld ) (09/08/89)
Does anybody out there in netland know the maximum allowed sync. frequencies in external sync. mode ? I have made a calculation that goes as follows: The ST does 2x10^6 video and refresh accesses per second. The typical refresh cycle for 256K and 1M DRAM's is 4 ms, or 250 refresh cycles/second. There are 256 (256K) or 512 (1M) accesses in a refresh cycle. Thus, there are at most 250x512=1.28x10^5 refresh accesses/second. That leaves 2x10^6-1.28x10^5=1.872x10^6 accesses/second for video. There are 80 video accesses needed for 1 scanline (320 pixels, 4 bit/pixel or 640 pixels, 2 bit/pixel). Thus there can be no more than 1.872x10^6/80=2.34x10^4 scanlines/ second, ie. maximal horizontal sync. frequency is 23.4 kHz. There are 200 lines/frame, plus some blank lines (say 50) in the vertical retrace period. This would mean a maximal vertical sync. frequency of 2.34x10^4/250=93.6 Hz. Does anybody know if these figures are correct ? Are there any other factors which limit the maximum sync. frequenies ?