Greenbird

Figure 1 illustrates the basic timing requirements for each row (horizontal) that is displayed on a VGA monitor. An active-low pulse of specific duration (time a in the figure) is applied to the horizontal synchronization (hsync) input of the monitor, which signifies the end of one row of data and the start of the next. The data (RGB) input on the monitor must be driven low for a time period called the back porch (b) after the hsync pulse occurs, which is followed by the display interval (c).During the data display interval the RGB data drives each pixel in turn across the row being displayed. Finally, there is a time period called the front porch (d) where the data (RGB) inputs must again be off before the next hsync pulse can occur. The timing of the vertical synchronization (vsync) is the same as shown in Figure 3.9, except that a vsync pulse signifies the end of one frame and the start of the next, and the display area refers to the set of rows in the frame. Tables 1 and 2 show, for different resolutions, the durations of time periods a, b, c and d for both horizontal and vertical timing.


_{Figure 1 – VGA timing specification}

VGA Mode		Horizontal Timing Spec.
Configuration	Resolution	a (pixels)	b (pixels)	c (pixels)	d (pixels)	Pixel clock
VGA (60Hz)	640×480	96	48	640	15	25 MHz
VGA (85Hz)	640×480	58	79	640	58	36 MHz
SVGA (60Hz)	800×600	128	88	800	40	40 MHz
SVGA (75Hz)	800×600	79	158	800	15	49 MHz
SVGA (85Hz)	800×600	62	152	800	34	56 MHz
XGA (60Hz)	1024×768	136	162	1024	26	65 MHz
XGA (70Hz)	1024×768	135	142	1024	23	75 MHz
XGA (85Hz)	1024×768	95	209	1024	47	95 MHz

_{Table 1 – VGA horizontal timing specification}

VGA Mode		Vertical Timing Spec.
Configuration	Resolution	a (lines)	b (lines)	c (lines)	d (lines)
VGA (60Hz)	640×480	2	33	480	10
VGA (85Hz)	640×480	3	25	480	1
SVGA (60Hz)	800×600	4	23	600	1
SVGA (75Hz)	800×600	3	21	600	1
SVGA (85Hz)	800×600	3	27	600	1
XGA (60Hz)	1024×768	6	29	768	3
XGA (70Hz)	1024×768	6	29	768	3
XGA (85Hz)	1024×768	3	36	768	1

_{Table 2 – VGA vertical timing specification}