Teletext Experimenter’s Board – The Circuit

Here is the circuit diagram:

The diagram is laid-out so it makes sense from a functionality point of view – obviously the pin numbers on the SAA bear no relation to where they are on the physical device!

The connectors are highlighted in yellow.

Connecting to your microcontroller

On the lower-left is the four-way connector, which should connect to your microcontroller of choice. On the PCB itself, this is in the lower-right corner.. It requires a 5v supply, and an I2C bus. NOT drawn on this diagram are the optional 4.7K pullups on the I2C bus, but I’ve made provision for them on the PCB.

The idea is that if you have more than one device on the I2C bus, you’ll want to add the pullups (there should be one 4.7K resistor from each of the two connections, up to the 5v line). But you should only do this ONCE for the bus – not for each device on it! Also, you don’t seem to need the pullups if there’s only one device on the bus.

For this reason, I make space on the board for them: but they are optional! Whether you need them rather depends on your project. I’d advise that you don’t fit them until you are sure that you do.

Composite Video Input

On the upper-left is the composite-video input. This is the carrier for the Teletext signal you wish to decode. If you’re not doing any decoding (you’re using this project as a display) then you don’t need to connect it to anything.

RGB Output

On the right is the RGB output. This is suitable for feeding into a 15kHz monitor (the old standard “TV res” monitor of the 8-bit and 16-bit eras) or an old TV with an RGB input. Or a RGB-to-composite encoder, or a HDMI upscaler with an RGB input. See my next blog post for wiring details in these cases.

Click here to visit the Teletext Experimenter’s Board main project page.

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