Xref: utzoo comp.sys.atari.8bit:2174 comp.sys.cbm:1989
Path: utzoo!utgpu!watmath!clyde!att!ucbvax!hplabs!pyramid!cbmvax!fred
From: fred@cbmvax.UUCP (Fred Bowen)
Newsgroups: comp.sys.atari.8bit,comp.sys.cbm
Subject: Re: commodore mouse
Message-ID: <5644@cbmvax.UUCP>
Date: 9 Jan 89 15:21:33 GMT
References: <33889@bbn.COM>
Reply-To: fred@cbmvax.UUCP (Fred Bowen)
Organization: Commodore Technology, West Chester, PA
Lines: 34

In article <33889@bbn.COM> slackey@BBN.COM (Stan Lackey) writes:
>I am trying to use the Commodore mouse on my Atari. [...]
>The mouse docs say that the X/Y coords are sent on the pot lines.
>Now - It seems that the way Commodore reads the pot lines is different from
>the way Atari does.  Simply reading the pot registers as one would on the
>Commodore does not work on the Atari.  Does the mouse send an analog voltage
>proportional to the distance the mouse has moved?  Does anyone know?

I doubt you will be able to use the 1351 mouse's proportional mode- it takes
advantage of special features built into the C64/C128 computers.

The mouse uses the SID (the audio chip) POT registers to pass digital position
information from the mouse directly into the C64/C128 memory map without
processor intervention.

The SID A/D conversion cycle is 512 CPU cycles long.  During the first 256
clocks, SID sinks both POT lines, discharging some small caps in the computer.
During the next 256 cycles, SID starts an 8-bit counter and releases the POT
lines, allowing them to charge at a rate determined by the mouse.  When the
voltage at the POT input crosses a threshold the counter value is latched into
the appropriate SID POT register.  The conversions are automatic, and cannot
be affected by the processor.  Tricky, eh?

It is only necessary to poll the mouse at a 60Hz rate to accurately determine
its position.  If you buy a 1351 mouse, a diskette is included with 6502 source
code and demo programs.

--
-- 
Fred Bowen			uucp:	{uunet|rutgers|pyramid}!cbmvax!fred
				arpa:	cbmvax!fred@uunet.uu.net
				tele:	215 431-9100

Commodore Electronics, Ltd.,  1200 Wilson Drive,  West Chester,  PA,  19380