Re: Commodore joystick ports

From: Marko Mäkelä (
Date: 2007-05-09 08:19:07

On Wed, May 09, 2007 at 02:33:11PM +1000, Daniel O'Shea wrote:
> I was wondering, in both Jim and Levente's microcontroller to joystick 
> port projects - is interfacing a microcontroller's digital output 
> (high/low) to the joystick input likely to cause the same issues with 
> the CIA chip as noted here:

The reported issue was that Sega gamepads pull lines high or low, while
joysticks are open-collector outputs that pull lines low or leave them
floating.  Yes, you will have to be careful with this.

In Atmel's AVR microcontollers, you can get an open-collector active-low
output by initializing PORT=0 (which it is by default after a reset) and
controlling DDR.  When DDR=0 (the default after a reset), the output driver
is inactive (or the line is an input).  When DDR=1, the output driver is
active, and it would be pulling the line low.


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