Retromaster’s Electronics Projects

…related to old computers and other assorted stuff…

TFE+ Video WIP

Posted by retromaster on February 19, 2010

For the new TFE+, I’ve chosen the ATmega644 as the main MCU, as opposed to the ATmega32 that was TFE’s MCU. The reasons are twofold: The mega644 has interrupt-on-change capability on all input pins, and twice the RAM of the mega32. Both features will come in handy.

For the TFE+ Amiga version, I need to make two main changes to the circuitry. The first concerns video generation. It will need to be modified so that it will use the sync generated by the host, and that it will overlay its output on top of the analog RGB generated by the host. The second is for user input. The keyboard-to-host connection will pass through the TFE+ board. There will be an additional MCU (an ATtiny24 in this case) continuously monitoring the keyboard lines and looking for a special key combination that activates TFE+. Once the key combination is detected, the keyboard MCU will divert the keypresses to TFE+, until the user is done interacting with TFE+.

So, I have ordered the parts for the new TFE+, and while I am waiting for them to arrive, I’ve decided to experiment with some of the circuitry changes on the breadboard. The gallery shows some of the work I’ve been doing on video overlay. I’ve tapped into the digital CSYNC output and the analog red output on the Amiga video connector. The output stage is based on a few transistors as shown in the schematic. The CSYNC line goes into the ATmega. The PIX line carries the pixels, and comes from the output of the 74hc166 shift register (controlled by the ATmega). The VBL (Video Blank) line becomes low when there is video output from the PIX line. When VBL is high, the analog output basically becomes isolated from the host output. The function of resistor R3 is to dim the video from the host when VBL is low.

As the screenshot shows, the video overlay works, but not very well yet. The sync input somehow works, but is not correct vertically. Furthermore, it seems it is not enough simply to use just one of the analog lines, because it looks very ugly. At least, I can say that there is no visible loss of quality in the host video output (the photo may be misleading, due to the interaction of the plasma screen with the camera). Anyway, it would require too many transistors to control all three analog lines, so I’ll probably switch to an IC here for the output stage. It will probably result in sharper video output, too.


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