[stella] Paddle controller docs?

Subject: [stella] Paddle controller docs?
From: "C. Bond" <cbond@xxxxxxxxxxxxx>
Date: Tue, 17 May 2005 11:59:01 -0400
I haven't been able to locate any comprehensive docs about
the VCS paddles online, although there are bits of
information here and there, including the info in the Stella

Programmer's Guide.

Here is what I'm looking for:

1) Is there a schematic of the electrical circuit for the
paddles, including component values? (I know that a
capacitor is charged at a rate depending on the position of
the variable resistor controlled by the knob, and that a
Schmitt trigger is activated when the capacitor voltage
exceeds its threshhold. But have not been able to find the
time constants or component values anywhere.)

2) What is the approximate minimum and maximum delay times
from the start of charge to the activation of INPT0? (or
INPT1, INPT2, etc.)

3) What is the best way to interrogate the paddles?
Apparently it is expected that the capacitors will be
discharged and clamped to ground by writing 1xxx xxx
(binary) to VBLANK, and that the charging process can begin
when the capacitor is unclamped by writing 0xxx xxx to
VBLANK. But should the timer be used to measure the delay?
Is it sufficient to count scan lines? That is,

VBLANK <- 1xxx xxx ;    discharge capacitors
VBLANK <- 0xxx xxx;    unclamp capacitors
???    ; how do I determine the elapsed time?

I tried to figure out the method used in BREAKOUT by
examining the code, but got lost!

4) What is a good way to use the keyboard to emulate a
paddle? Clearly the LEFT and RIGHT cursor keys can be used
to increment or decrement a number which could represent the

paddle position, but what are the normal paddle limits and
how do I guarantee that the software will accept the

Thanks for any pointers.

C. Bond

Democracy: The triumph of popularity over principle.

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