Some VGA questions for Andre'
Jim Coleman
Posts: 52
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
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
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