The table below summarizes timing capabilities of Matrox controllers in standard VGA modes. All frequencies are in kHz unless noted otherwise.

Clk1 (640 pixels) Clk2 (720 pixels)

Standard CLK freq. 25175 28322

Max CLK freq. 63500 63500

Std HSYNC freq. 31.5 31.5

Std VSYNC freq. 60 Hz 70 Hz

Max HSYNC freq. 90 70.6

Max VSYNC freq 152 Hz 156 Hz

Formula for CLK freq

based on HSYNC freq fc = fh * 800 fc = fh * 900

Formula forVSYNC freq fv = fh / 525 fv = fh / 450

Assume we have a monitor with min. HSYNC frequency of 48 kHz. To use it in standard VGA graphics modes, CLK1 field should be programmed to get pixel clock of 38400 kHz. Rounded to the nearest multiple of 250 kHz this gives 38500, and dividing this number by 250 we get PINS field value of 154. This in turn yields VSYNC frequency of 91 Hz (which must be checked against monitor's limit) . For text modes, the values are: pixel clock - 43200 kHz, rounded to 43250, CLK2 field - 173, HSYNC freq - 96 Hz.

Note that this must comply with monitor limits, otherwise the monitor may be damaged.

While using nonstandard VGA clock values, the picture on fixed frq./ high freq. monitor may be slightly distorted (geometry problems). Anyway it's better to get slightly rounded text screen on bootup than no screen at all until Windows is loaded...