Ok, here's what we're going to do. We've been able to save some money on the design of the board and manage to get most dotclocks we'd want to get. The board has two crystal oscillators on it. One is at 150MHz. The other is at 155.52MHz. Each one goes into an adjustable 8-bit counter divider that lets you divide by any number from 1 to 256 (although some of the smaller numbers will result in clock rates that are too high), then the output of that goes into a clock manager that multiplies it by 32 (adjustable in design but fixed for use). The same logic is duplicated for each head, so each has its own independent clock at any of the available frequences.
The formula for the dot clock is: freq = 150*32/N or 155.52*32/N The actual clock frequency is 1/4 or 1/2 of this, because we process multiple pixels in parallel. At any time, we can adjust the underlying logic to change fixed ratios in the clock managers, we can change crystals (pick something off-the-shelf!), and we need to consider what this looks like for single-link DVI, where we move 2 pixels at a time instead of 4. Also, note that the TV chip generates its own clock that we can select when we want to be in that mode, potentially giving finer control over dot rate for head 1 even when we're not in TV mode. We can select any dotclock below 82MHz with 1MHz precision or finer. Here's a list of frequencies in the higher portion of our range (we can go higher, but we can't use them): 80.00 80.27 81.36 81.58 82.76 82.94 84.21 84.35 85.71 85.80 87.27 87.31 88.87 88.89 90.48 90.57 92.16 92.31 93.90 94.12 95.70 96.00 97.58 97.96 99.53 100.00 101.56 102.13 103.68 104.35 105.89 106.67 108.19 109.09 110.59 111.63 113.11 114.29 115.74 117.07 118.49 120.00 121.38 123.08 124.42 126.32 127.61 129.73 130.96 133.33 134.50 137.14 138.24 141.18 142.19 145.45 146.37 150.00 150.81 154.84 155.52 160.00 160.54 165.52 165.89 171.43 171.61 177.74 177.78 184.32 184.62 191.41 192.00 199.07 200.00 207.36 208.70 216.38 218.18 226.21 228.57 236.98 240.00 248.83 252.63 261.93 266.67 276.48 282.35 292.74 300.00 311.04 320.00 331.78 Feel free to consider other combinations of off-the-shelf oscillators. Also look into Xilinx DCMs to see what programmable ratios there are, in case you want to fine-tune it there. This doesn't affect the time-scale of ODG1 production, but the oscillator frequencies need to be nailed down before we go to production. Otherwise, we'll stick with the values chosen. -- Timothy Normand Miller http://www.cse.ohio-state.edu/~millerti Favorite book: The Design of Everyday Things, Donald A. Norman, ISBN 0-465-06710-7 _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
