Information for Chips and Technologies Users : The Full Story on Clock Limitations
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5. The Full Story on Clock Limitations

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

(--) SVGA: CHIPS: probed memory clock of 40090 KHz

in your startx log file. Note that many chips are capable of higher memory clocks than actually set by BIOS. You can use the Set_MClk 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                    220               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                   220    140      93.33  140      93.33   70.0

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 "Set_MemClk". 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.

Information for Chips and Technologies Users : The Full Story on Clock Limitations
Previous: Modelines
Next: Troubleshooting