Chameleon-AVR Nurve Networks, Chameleon-AVR Datasheet - Page 39

• Manufacturer Part Number: Chameleon-AVR
• Description: MCU, MPU & DSP Development Tools AVR8 & PROPELLER DEV SYSTEM (SBC)
• Manufacturer: Nurve Networks

Specifications of Chameleon-AVR

• Processor To Be Evaluated: AVR 328P
• Data Bus Width: 8 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

Therefore, from the D/A’s perspective with both bits on, the circuit looks like that shown in Figure 10.2 (for any particular
channel; R, G, or B). Noting, that the Propeller is a 3.3V device with both channel bits HIGH, that means that we have a
270 Ohm in parallel with a 560 Ohm and this combination in series with the intrinsic impedance of 75 Ohms of the VGA,
thus, we can do a simple computation to compute the voltage at the 75 Ohm VGA input realizing we have a voltage
divider configuration:
Which is what we desire, performing similar math for all 4 combinations of inputs we get the results shown in Table 10.2
for codes.
Considering there are 4 shades of each channel; R, G, and B that means that there are a total of 4*4*4 = 64 colors
available in VGA mode (without any tricks). Not bad! For reference, the VGA connector has the following pin out shown in
Table 10.3. The VGA port on the Chameleon AVR is a female HD15 (High Density) which is standard on the back of video
card hardware. It will connect to any VGA monitor, but be careful when making connections, it’s a special “low profile”
connector and not that sturdy, so hold it when you plug the VGA cable into it.
VGA_IN_RED = 3.3V * 75 Ohm / ( (270 Ohm || 560 Ohm) + 75 Ohm) = 0.95V
Figure 10.2 – Electrical model of VGA inputs and 2-bit D/A channels.
Table 10.2 – VGA input voltages for all 2-bit inputs.
B1
0
0
1
1
B0
0
1
0
1
Code
0
1
2
3
VGA Voltage
0.0V
0.30V
0.64V
0.95V
© 2009 NURVE NETWORKS LLC “Exploring the Chameleon AVR 8-Bit”
Description
Black
Dark Gray
Gray
White
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