Re: Ortonview PCB
I've just thought of another refinement which would be onerous to include on a Veroboard build, but trivial if provided for on a PCB.
One of the few acknowledged drawbacks of OrtonView is that, due to the way the UART is used to generate the video bitstream, there is a thin bright vertical line at the left hand side of the image.
I would therefore propose the addition of a monostable which is triggered from the falling edge of the line sync pulse - the time period to extend to just after the vertical bright line, and the output (via a transistor?) used to hold the video signal at black level until just after the bright line.
If there were any spare pins available on the PIC then one extra port pin + transistor could be used to hold the video output at black level just during the period where the UART output is initially switched on and then released for the rest of the video line.
One way to make this happen would be to use the 44-pin PLCC or flat-pack versions of the 877, as they have several more port pins available. However this would be counter to Karen's stated preference for using DIP parts to make the project easy to hand build.
For some reason Tim never really experienced or was bothered by the left hand vertical bright line, but it is definitely there. It's the leftmost spike on the single video line capture in the first image below. The second image is OrtonView in graphics mode with the screen memory filled with a binary pattern. The bright line is very visible at the left hand side.
On a CRT TV or monitor you can just muck about with the width and picture offset so that the bright line is just off screen out of sight on the left, but on an LCD TV the auto-adjust / auto size feature will no doubt be very pleased with itself for managing to get everything, including the unwanted bright line, on the screen no matter what you do to stop it.
|