There has been much confusion about exactly what the clock limitations of the Chips and Technologies chipsets are. Hence I hope that this section will clear up the misunderstandings.
In general there are two factors determining the maximum dotclock. There is the limit of the maximum dotclock the video processor can handle, and there is another limitation of the available memory bandwidth. The memory bandwidth is determined by the clock used for the video memory. For chipsets incapable of colour depths greater that 8bpp like the 65535, the dotclock limit is solely determined by the highest dotclock the video processor is capable of handling. So this limit will be either 56MHz or 68MHz for the 655xx chipsets, depending on what voltage they are driven with, or 80MHz for the 64200 WinGine machines.
The 6554x and 64300 WinGine chipsets are capable of colour depths of 16 or 24bpp. However there is no reliable way of probing the memory clock used in these chipsets, and so a conservative limit must be taken for the dotclock limit. In this case the driver divides the video processors dotclock limitation by the number of bytes per pixel, so that the limitations for the various colour depths are
8bpp 16bpp 24bpp
64300 85 42.5 28.33
65540/65545 3.3v 56 28 18.67
65540/65545 5v 68 34 22.67
65546/65548 80 40 26.67
For a CRT or TFT screen these limitations are conservative and the user
might safely override them with the "DacSpeed" option to some
extent. However these numbers take no account of the extra bandwidth
needed for DSTN screens.
For the HiQV series of chips, the memory clock can be successfully probed. Hence you will see a line like
(--) CHIPS(0): Probed memory clock of 40.090 MHz
in your startx log file. Note that many chips are capable of higher
memory clocks than actually set by BIOS. You can use the "SetMClk"
option in your XF86Config file to get a higher MClk. However some
video ram, particularly EDO, might not be fast enough to handle this,
resulting in drawing errors on the screen. The formula to determine the
maximum usable dotclock on the HiQV series of chips is
Max dotclock = min(MaxDClk, 0.70 * 4 * MemoryClk / (BytesPerPixel +
(isDSTN == TRUE ? 1 : 0)))
which says that there are two limits on the dotclock. One the overall maximum, and another due to the available memory bandwidth of the chip. For the memory bandwidth 4 bytes are transfered every clock cycle (Hence the 4), but after accounting for the RAS/CAS signaling only about 70% of the bandwidth is available. The whole thing is divided by the bytes per pixel, plus an extra byte if you are using a DSTN. The extra byte with DSTN screens is used for the frame buffering/acceleration in these screens. So for the various Chips and Technologies chips the maximum specifications are
Max DClk MHz Max Mem Clk MHz
65550 rev A 3.3v 80 38
65550 rev A 5v 110 38
65550 rev B 95 50
65554 94.5 55
65555 110 55
68554 110 55
69000 135 83
69030 170 100
Note that all of the chips except the 65550 rev A are 3.3v only. Which is the reason for the drop in the dot clock. Now the maximum memory clock is just the maximum supported by the video processor, not the maximum supported by the video memory. So the value actually used for the memory clock might be significantly less than this maximum value. But assuming your memory clock is programmed to these maximum values the various maximum dot clocks for the chips are
------CRT/TFT------- --------DSTN--------
8bpp 16bpp 24bpp 8bpp 16bpp 24bpp
65550 rev A 3.3v 80 53.2 35.47 53.2 35.47 26.6
65550 rev A 5v 106.2 53.2 35.47 53.2 35.47 26.6
65550 rev B 95 70 46.67 70 46.67 35.0
65554 94.5 77 51.33 77 51.33 38.5
65555 110 77 51.33 77 51.33 38.5
68554 110 77 51.33 77 51.33 38.5
69000 135 116.2 77.47 116.2 77.47 58.1
69030 170 140 93.33 140 93.33 70
If you exceed the maximum set by the memory clock, you'll get corruption on the screen during graphics operations, as you will be starving the HW BitBlt engine of clock cycles. If you are driving the video memory too fast (too high a MemClk) you'll get pixel corruption as the data actually written to the video memory is corrupted by driving the memory too fast. You can probably get away with exceeding the Max DClk at 8bpp on TFT's or CRT's by up to 10% or so without problems, it will just generate more heat, since the 8bpp clocks aren't limited by the available memory bandwidth.
If you find you truly can't achieve the mode you are after with the default
clock limitations, look at the options "DacSpeed" and
"SetMClk". Using these should give you all the capabilities
you'll need in the server to get a particular mode to work. However use
caution with these options, because there is no guarantee that driving the
video processor beyond it capabilities won't cause damage.