#_Toc310936112 Arcade MegaWing
%apply=img width=192 height=144 hspace=12%Papilio_Arcade_Kit_User_Guide_files/image006.jpg The Arcade MegaWing provides all of the arcade hardware in one convenient and easy to connect circuit board. It snaps into the Papilio One and gives it the necessary hardware resources to communicate with the outside world. #_Toc272247104 Continue reading for more information about each hardware section of the Arcade MegaWing.
#_Toc310936113 MegaWing VGA
%apply=img width=145 height=144 hspace=12%Papilio_Arcade_Kit_User_Guide_files/image007.jpg The VGA section of the Arcade MegaWing uses 12 resistors to implement 4K color depth. VGA video is analog in nature so there needs to be some way to vary the RGB (Red, Green, and Blue) signals between 0V and .7V. For each RGB signal the shade, or intensity, of the color is controlled by varying the voltage of the pin between 0 and .7V. The finer control you have over the voltage the more colors you can create. For the Arcade Wing we are able to control 4 different bits per color which allows us to generate 16 different voltage levels between 0 and .7V. This means we can generate 16 shades for each color. If we add all three colors together we have 12 bit video which gives us (2^12=4096) the possibility of 4096 colors.
The usual options for controlling voltage on a pin are to use a DAC, PWM, Delta-Sigma, or a resistor ladder. For the Arcade Wing we wanted to use the lowest cost and easiest method for a hobbyist. PWM and Delta-Sigma were ruled out because we did not think we would be able to vary the voltage with these methods fast enough to keep up with the VGA timing requirements. A DAC would be a good solution but was more than we needed for our modest requirements. A resistor ladder fit the bill perfectly because it is very low cost and easy to implement. The resistor ladder is made up of 4 resistors per RGB color that all connect, in common, to the VGA connector on one side and individually to a digital I/O pin on the other side. The lowest resistor starts at ~500 ohms and each successive resistor doubles in size with the final resistor ending with ~4K ohms. Each digital I/O pin is controlled by the VGA controller inside the FPGA. Each pin can be set to either 0 or 1 with a 1 causing the resistor to contribute its voltage to the final voltage level. The voltage can be stepped up from 0 to the desired output voltage which in this case is .7V. The way the desired output voltage is accomplished is by carefully managing the parallel voltage that the 4 resistors add up to. Using a [[http://www.1728.com/resistrs.htm|Parallel Resistance Calculator]] we see that 500 ohm, 1000 ohm, 2000 ohm, and 4000 ohm give a total resistance of 266 ohm. The final key bit of information is that there is a 75 ohm resistance built into VGA cables, so that means that the point where our 4 resistors come together on our VGA connector forms a Voltage divider circuit. Using a [[http://www.raltron.com/cust/tools/voltage_divider.asp(approve sites)|Voltage Divider calculator]] with 3.3V as the Input Voltage, 266 ohm as R1, and 75 ohm as R2 we end up with an Output Voltage of .73V. The end result is that if all 4 resistors are set to ‘1’ then we will see .73V at the VGA connector. If all 4 resistors are set to ‘0’ then we will see 0V at the VGA connector, any combinations in between will give us evenly stepped voltages between 0 and .73V. The voltages will be evenly stepped because we picked resistors that double in size.
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Note 1: Use these bits for 8 color video.