AVR Freaks

Ahh, those haunting '4066s (U18, U21, etc.)!

That smells like messing with scanning matrices and, in my opinion, it has nothing to do with MOS (Metal-Oxide Semiconductor) logic. Actually, all these years we are mostly dealing with Complementary MOS logic, also known as C-MOS, where NMOS and PMOS devices are used in complementary pairs instead of open-drain NMOS transistors with pullup resistors, resulting in circuits that consume power only when they are switching state.

As I can understand it, when U1 decides to scan the Control Ports, sets the analog switches through PA6 and PA7, which, in turn, select the "POT X" and "POT Y" signals of U9 to be used with the PA0..PA4 and PB0..PB4 keyboard lines. So, when you pull these keyboard lines, even when PA6 and PA7 are inactive, you are actually interfering with the keyboard scanning procedure, resulting in confusing U1. All you will have to do is to wait for the Control Port signals to select the Control Ports and only then to pull the PA0..PA4 and PB0..PB4 keyboard lines to the desired state; when the Control Port signals deselect the Control Ports you should immediately tristate the AVR drive lines that control PA0..PA4 and PB0..PB4. But, since I am not sure what kind of signals the "POT X" and "POT Y" are, because as I can see U9 is the SID chip, I would suggest using an AVR to control another set the '4066 type of analogue switches; unless, of course, you have the time to figure this out and use a bare AVR to do the job.

A very similar discussion about driving scanning matrices has been started here, where most of the members praise the use of the '4066 type of analogue switches. There is, of course, an heretical opinion, coming from a couple of members that suggest skipping the use of any external switches, transistors or relays(!) and manipulating the matrix scanning scheme directly, in the manner I have just described, above.

-George

Hi, Svo! I am glad you solved it!

By the way, thanks for sending me out there for research, since it seems I had to do some homework before answering your message in the first place; you see, I think I have not even seen a C64 joystick, ever... :oops:

Anyway, after a brief search I could see what is going on at the C64 Control Ports: The potentiometer inputs have nothing in common with the keyboard connections so they can work independently; but the digital inputs share all their signal lines with the keyboard. This means that the keyboard loses 1..5 of the 8 scan-lines it has in total while any of the two joysticks are in use; but it resumes its full functionality when the joysticks are inactive or disconnected. "This causes the machine to 'type' characters when the joystick is operated," as it has been exactly reported, definitively confirming the "known feature" you were talking about! :-)

-George