Xref: utzoo comp.sys.atari.8bit:2187 comp.sys.cbm:2003
Path: utzoo!attcan!uunet!cbmvax!fred
From: fred@cbmvax.UUCP (Fred Bowen)
Newsgroups: comp.sys.atari.8bit,comp.sys.cbm
Subject: Re: commodore mouse
Message-ID: <5690@cbmvax.UUCP>
Date: 12 Jan 89 22:40:15 GMT
References: <33889@bbn.COM> <5644@cbmvax.UUCP> <13398@cup.portal.com>
Reply-To: fred@cbmvax.UUCP (Fred Bowen)
Organization: Commodore Technology, West Chester, PA
Lines: 42

In article <13398@cup.portal.com> Bob_BobR_Retelle@cup.portal.com writes:
> 
>Fred, if you're still tuned in here, is this A/D process similar to the way
>the C-64 reads the Paddle pots..?
>The 8-bit Atari uses a somewhat similar way of reading the A/D ports through
>hardware, although the scaling seems to be a little different.   [...]
>As far as the proportional mode of the 1351 mouse, does the resultant data
>get stored in the same locations as the paddle values..?   [...]
>I guess I'm just a little bit confused as to the actual input method from
>the mouse... if it's a variable resistance, as in a paddle, does the computer
>translate the x-y values into an absolute position, rather than relative
>direction and speed values, as an Amiga or Atari ST mouse..?

The POT lines are read on the C64/128 via the SID POTx,y registers, just as
the 1351 mouse movements are.  The difference between an actual paddle and
the mouse inputs is how they are driven- the mouse manipulates the resistance
(normally variable as in the paddle) such that the value one would read from
the SID POT register is equivalent to the digital data corresponding to the
mouse's relative movement (bits 6-1).  The mouse's speed can only be inferred,
taking into account the polling frequency and the relative change in position.
The left button appears as a joystick "fire" signal, and the right button as a
joystick "up" signal.

The specifics pertaining to the SID and the values of the caps and resistors
(accounts for the difference between CBM and Atari scaling factors) can be
found in the C64 or C128 Programmer's Ref Guide.  The specifics pertaining to
the value read from the SID POT register can be found in the 1351 User's Guide.

One important note- on the C64/128, a 4066 analog switch is used to toggle the
POT lines between the two joy/paddle ports.  The selection is made via one of
the lines shared with the keyboard scan lines.  This means a keyscan will
temporarily clobber anything in the POT registers.  Since the POT register
requires some time to "charge" before its data is valid, at least 1.6ms must
transpire after this switch has been twiddled.

--
-- 
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