Chameleon-PIC Nurve Networks, Chameleon-PIC Datasheet - Page 43

• Manufacturer Part Number: Chameleon-PIC
• Description: MCU, MPU & DSP Development Tools PIC24 & PROPELLER DEV SYSTEM (SBC)
• Manufacturer: Nurve Networks

Specifications of Chameleon-PIC

• Processor To Be Evaluated: PIC24
• Data Bus Width: 16 bit
• Interface Type: USB, VGA, PS/2, I2C, ISP, SPI
• Operating Supply Voltage: 3.3 V, 5 V
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

© 2009 NURVE NETWORKS LLC “Exploring the Chameleon PIC 16-Bit”
Figure 10.3 – The VGA timing specifications.
To begin with the VGA standard as implemented is 640x480 pixels across as shown in Figure 10.3(a). The image is
rendered left to right, top to bottom, similarly to the NTSC/PAL signals, and thus a similar syncing scheme is used that is
composed of both a horizontal sync pulse each line and a vertical sync pulse each frame. However, there are no color
burst signals, serrations, pre-equalizations pulses, etc. the interface is nearly digital as noted.
The dot clock used in a basic VGA generation system is 25.175 MHz, all timing can be derived from this base frequency.
Typically, designers like to run the system dot clock at this frequency and compute all events as a number of clocks, for
example, getting ahead of myself, take a look at Figure 10.3(b), the Hsync pulse which is labeled “B” in the table (and a
negative polarity), its 3.77us, therefore at a dot clock of 25.175 MHz, or inverting this to get the period time we get
39.72194638 ns. This is how long a pixel takes, anyway, dividing this into our Hsync time we get:
Number of dot clocks for Hsync pulse = 3.77 μs / 39.72194638 ns = 94.90 clocks.
Call it 95 dot clocks, thus you can simply use a counter and count 95 clocks, drive Hsync LOW and that it. The entire VGA
signal can be generated like this. Of course, the tough part is when you get to video, here you only have roughly 39
nanoseconds to do whatever you are going to do, this amounts to more or less doing nothing, but accessing a video
buffer as fast as you can and getting the next data word ready to build the pixel.
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