[stella] PIA/(RIOT) "undocumented' feature

Subject: [stella] PIA/(RIOT) "undocumented' feature
From: Mark De Smet <de-smet@xxxxxxxxxxxxxxx>
Date: Fri, 5 May 2000 22:59:48 -0500 (CDT)
In doing some reasearch into the VCS memory map, I've discovered something
interesting about the PIA.  I was going through the datasheet for the
chip(you can find it at www.6502.org), and noticed that it has interupt
generation capabilities.  

Schematics show that the interupt output pin is left disconnected, but
there is an interupt flag register.  It can generate interrupts for timer
overflow and from edges on PA7 (which is joystick 0 right or paddle 0
button). Reading a flag to tell if the timer overflowed is kinda useless
because simply loading the value of the timer, and checking the zero bit
is much easier.(technically they are different in that the overflow
interupt bit is set once the overflow has occured(ie wrapped to FF), but I
can't imagine how this would be helpful) 

However, you can use this as an edge (positive or negative) detection
circuit on this joystick/paddle line.  This could be used with some
external hardware, or if you are using P0 paddle button, and are concerned
about catching the fastest presses.(or want to know about the button
release, but don't care about the press)  This can be used in the same way
as the latches on the joystick trigger buttons.(but in this case for
paddle 0) 

I mention this, not because I have a use, but because I'm sure that the
creative people on this list can come up with some good use for this. 

Ideas?


Mark


--
Archives (includes files) at http://www.biglist.com/lists/stella/archives/
Unsub & more at http://www.biglist.com/lists/stella/

Current Thread