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TRANSFORMING DIGITAL I/O

The M16C/62P microcontroller has 5-V digital outputs. To transform these into an RGB video signal, you need some additional circuitry. However, you can limit it to a couple of simple resistors and transistors.

First, you need to transform the M16C/62P microcontroller’s 0- to 5-V output to the 0.7-VPP video level. This is easily achieved with a two-resistor voltage divider. R8 and R10 form this voltage divider for the red output signal (see Figure 5, p. 27). R9 adds a small DC offset to it. Transistor T2 is connected as an emitter follower; with R11, it provides the signal with the 75-W output impedance that’s required for video. The green and blue outputs are created with identical circuits. A television’s input impedance is also 75 W so the actual video signal level is half of the level present at the transistor’s emitter. If you calculate the high and low values for the three color components, you’ll find that the difference is 0.6 V. That’s about 85% of the maximum level (0.7 V), and it produces nice bright colors.

A similar circuit is used for the sync output. To generate a pure RGB signal, it’s sufficient to transform only the 0- to 5-V sync output from the M16C/62P microcontroller into a 0.3-VPP signal like with the R, G, and B outputs. But I added something extra. The RGB output from a DVD player often contains a complete CVBS signal on the sync output. The synchronization pulses along with the R, G, and B signals are used when connected to an RGB input, and the complete CVBS signal from the sync output is used when connected to a CVBS input. I did something similar.

I can’t create a complete color CVBS signal (it’s too hard to encode the R, G, and B components into the 3.58- or 4.43-MHz carrier wave), but I can create an old-fashioned black-and-white video signal from it. That involves simply adding up the luminance of the R, G, and B components. In the analog world, adding voltages involves just tying them together with a few resistors.

Take another look at the circuit diagram in Figure 5. R1, R2, R3, R4, and R6 mix the color and sync signals. R5 creates small DC offset. Just like with the color signals, T1 and R7 create a 75-W video output. The three color components are not mixed in the same ratio. When converting to black and white, green is brighter than red, which in turn is brighter than blue. I mixed the green, red, and blue component in a ratio of 4:2:1. This results in eight evenly distributed shades of gray in the black-and-white signal. 

So, if your television set does not have an RGB input, connecting the sync signal alone to the composite (CVBS) input provides a black-and-white image. Alternatively, by tying R1 and R3 to G instead of to R and B, the Sync output becomes a sync-on-green output that can be used along with the R and B outputs to feed into a television or monitor that doesn’t have a separate sync input on its RGB input.