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Some VGA questions for Andre' — Parallax Forums

Some VGA questions for Andre'

Jim ColemanJim Coleman Posts: 52
edited 2007-03-30 12:38 in Propeller 1

Andre’

I am trying to correlate your text with reality, please clarify.

Page 121 of HYDRA, a pixel time of 39.721946 X 640 pixels = 25.422045 us

Figure 8:4, Page 121, Parameter D, Active video time = 25.17 us

Why don’t these numbers equal each other?



Page 122 of HYDRA, “However, unlike NSTC, VGA is very unforgiving and you must follow the spec very closely.”

Page 121 of HYDRA, “The main clock used in a VGA generation system is 25.175 MHz;”

VGATERM_013.spin has a vga_rate of 16 MHz.

VGA_DEMO.spin has a vga_rate of 20 Mhz.

How can this be.



Page 121 of HYDRA, “Number of dot clocks for Hsync pulse = 94.90 clocks.” with vga_rate 25.175MHz

VGATERM_013.spin has a Hsync pulse = 32 clocks with vga_rate of 16 MHz

VGA_DEMO.spin has a Hsync pulse = 96 clocks with vga_rate 20 Mhz

Based on the diagram on page 121 of Hydra, Parameters A,B,C,D,E Please help me make sense of these numbers.



Thank you, Jim

Comments

  • AndreLAndreL Posts: 1,004
    edited 2007-03-29 18:19
    It simply doesn't add up exactly, use the values in the timing diagram images. Its an inconsistency with the vga specs. The regions a,b,c... don't all add up. The bottom line is make sure to copy the spec in the diagrams and things will work out.

    Andre'
    ·
  • Jim ColemanJim Coleman Posts: 52
    edited 2007-03-29 19:54
    Andre’

    Thank you for responding, but I guess I’m just too dumb to understand this, so I will try again with only my first point, here goes, please be patient with me:

    On the very top of page 121 of the HYDRA, “25.175 MHz, or inverting this to get the period time we get
    39.72194638 ns. This is how long a pixel takes, …….”.

    Also at the top of Figure 8:4 on Page 121 “ A)VGA 640X480 Layout “

    I assume this means that there are 640 pixels horizontally by 480 vertically. If I multiply

    Pixels X Pixel Time, 640 X 39.72194638 ns it should equal parameter D of the Horizontal Timing Diagram.

    Parameter D is called Active video time and it is marked as 25.17 us.

    But 640 X 39.721946 = 25.422045 us.

    Obviously, these numbers are different and I don’t understand this.

    Jim
  • AndreLAndreL Posts: 1,004
    edited 2007-03-30 07:49
    That's it, they don't equal, its due to numerical inaccuraces the inital calculations and how many places you have the 24.175xxxxxx mhz signal, that's all.

    Andre'
  • Jim ColemanJim Coleman Posts: 52
    edited 2007-03-30 12:38
    Andre’

    Thank you for resolving this, the solution is nothing more than numerical inaccuracy. Now I can move on. I am very stubborn about my dumbness. As Freud would say, I must be anal retentive, but I will never admit it.

    I want to thank you for the HYDRA book, it is like a Bible to me and I reference it constantly.

    My next question concerns the parameter list for VGATERM_013.spin, I believe Chip wrote this routine.

    vgaparams long 0 'status
    long 1 'enable
    long %00_111 'pins
    long %011 'mode
    long 0 'videobase
    long 0 'colorbase
    long cols 'hc
    long rows 'vc
    long 1 'hx
    long 1 'vx
    long 0 'ho
    long 0 'vo
    long 256 'hd
    long 8 'hf
    long 32 'hs
    long 96 'hb
    long 768>>1 'vd
    long 2 'vf
    long 8 'vs
    long 48 'vb
    long 16_000_000 'rate

    My first question concerns the vga_rate or pixel rate (Hz) which is 16_000_000 or 16 MHz. How is this possible when

    Page 122 of HYDRA, “However, unlike NSTC, VGA is very unforgiving and you must follow the spec very closely.”

    Page 121 of HYDRA, “The main clock used in a VGA generation system is 25.175 MHz;”

    Thanks

    Jim
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