RE: Commodore joystick ports

From: Scott McDonnell (
Date: 2007-04-11 15:58:57

Hmm... played with a simulation a bit more on the o'scope and
it doesn't work out quite right. john/lori called it with the 
voltage changing as the cap charges. When I had the scope on 
there yesterday, I could see varying charge rates, but I hadn't 
bothered to measure them. Thanks for pointing that out.

So, let's get back to the basics here.

Here's the raw numbers, compiled here:

Cap is 1nF (C64) to 2.2nF (SX-64)
CD4066 causes approx. 250 ohms in series with the cap
Cap is *grounded* to discharge for 256 cycles.
SID counts how long it takes for the cap to charge to threshold voltage
2V is 40% of VCC (5V)
1 count is 1uS, 256 counts is 256uS
Current needed to charge the cap to 40% in 1uS is 2.5mA, equiv.
resistance is ~1.9K
Current needed to charge the cap to 40% in 256uS is 10uA, equiv.
resistance is ~501K

This is what needs to be emulated.

We have 5V, Ground, and POT lines to work with from the control port.

Back to the drawing board.

Scott M.

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