c64DTV power on reset circuit

I wanted to get this project all documented at once, but that’s a pretty tall order with how many moving parts it has. I also have issues with being able to track down all the information I want to include in a blog entry months after I did the project. I’m going to start with the latest aspect of this project I’ve finished and work backward through parts that I can document.

This is the issue I have. With my ps/2 keyboard when I turn on the c64DTV I get a “V” on the screen. As I type I get some characters, but not all, and a bunch of “V”s included until some point where it snaps out of it and is ok. The easiest option for this is to press the reset button after the device is powered on and then the keyboard operates properly. I recessed my reset button a little too far to be comfortable so now I want to have an automatic option. The standard option to get around this slight flaw in the c64DTV implementation is a keyboard twister. The problem I have is that even though it fixed the initialization function the keypresses were much slower, there was lag, it got confused if you type too fast. I don’t need the german translation functions or macros offered by that device so I designed my own solution.

I started by looking up “power on reset circuit” and came across the one for the apple 2. The problem is in order to simulate pushing the reset button I wanted to wait some amount of time, then press the button for a different amount of time, then release it for the duration of the power being applied. This circuit simply generates a high pulse for a given amount of time. In order to generate a pulse after some time I need something more complicated.

This circuit has two of those pulse generators that have slightly different periods. I combined them with a 2n7000 to get an open drain output and only have the reset pulse enabled when the output of side A is low and B is high. I did that by grounding the 2n7000 through the A half of the 556 so the pulse on the B side is not causing a reset until the pulse on the A side is done.

prototype circuit installed in the c64dtv
compact module, removable if needed
Fixed!

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