*** Series Feature (Part 2) ***                         
*** Xav (Mark Crutch) ***                            
*** Programming/Games/Hardware/DIY! ***                      
*** STe, Jaguar or Falcon ***


*** Heading & Standfirst ***
Just A Bit Of Padding

In the second article on uses of the Enhanced Joystick Ports, Xav 
takes a more detailed look at the humble joypad.

*** Main Text ***

In the unlikely event that there is anyone reading this who is 
unfamiliar with exactly what I mean by a "joypad" let me start with a 
brief description. The joypad is an input device, designed primarily 
for games playing, and is the standard controller for a Jaguar. You've 
probably seen pictures of them, festooned with buttons, back in the 
"old" days when Atari were actually making some noises about the 
Jaguar. Standard joypads have three fire buttons, a twelve button 
numeric keypad, pause and option buttons, and a directional pad 
(sometimes referred to as a "D-pad").

This article is intended to explain the inner workings of the joypad, 
as the principles used here are fundamental to many of the EJP 
projects that can be made. A brief description of the joypad hardware 
was given in issue 1, and it may be useful to refer back to that if 
you have a copy. That article included a very simple circuit diagram 
of part of the hardware, but the full thing can be found just over 
there, in the corner.

*** IMAGE ***
pad_1.gem

No, please don't turn over just yet - it's not as intimidating as it 
looks and is quite simple when viewed at as a number of building 
blocks. Honest.

*** Subtitle ***
Trigger Happy

Taking the circuit diagram vaguely from left to right, the numbered 
circles (and those along the top) represent the pins of the joypad 
connector. Those on the left carry the input to the computer as simple 
high or low voltages - which are provided by that big rectanglular 
object. This is a "Schmitt triggered octal buffer."

Impressive sounding stuff, maybe, but all it does is take signals in 
from one side (the wide end of the triangles), clean them up a bit so 
they don't scare the computer too much, then squirt them out the other 
side (the pointy end of the triangles). At the top is a connection for 
the positive power supply (from pin 7), and at the bottom is the 
connection to ground (pin 9). There are also a couple of other inputs
at the bottom - the "enable" connections. These effectively switch the 
whole chip on and off, and are activated when they are pulled low 
(that's what the bar over the top means). For our purposes we always 
want the chip active, so they are tied permanently to ground.

Next along we have a number of resistors which connect the Schmitt 
triggers to the positive supply. Their sole purpose here is to ensure 
that the computer normally receives "high" values unless a button is 
being pressed and read from. All of which brings us neatly onto...

*** Subtitle ***
The buttons

Each button, as can be seen, is connected at one end to the Schmitt 
trigger, and at the other end to one of the pins along the top. 
Associated with most of the buttons is a diode. This can be thought of 
as a one-way valve for electricity, and prevents the Schmitt trigger 
from reading the buttons of adjacent columns when it's not supposed 
to. Since the Pause button doesn't share its Schmitt trigger with 
anything else, it doesn't need a diode.

When a button is pressed it effectively transfers a high or low 
voltage from the pin to which it's connected, onto the diode. If the 
voltage is high, the diode blocks it. If the voltage is low, the diode 
"pulls" the input to the Schmitt trigger low as well. This signal is 
then cleaned up and passed to the computer.

In order to read any one switch, therefore, it is necessary to make 
the appropriate column low, and all others high. This is done by using 
some of the pins on the EJP as outputs and setting them accordingly. 
Then it is a simple case to read the values returned on the input 
pins.

*** Subtitle ***
Next Issue

It's worth reading through this page a couple of times to make sure 
you understand what's going on, as a good grounding in the hardware of 
the joypad will make it much more understandable to program - which is 
exactly what we'll be starting on next time.

Meanwhile you'll find a couple of goodies on the reader disk (you do 
get the reader disk, don't you?). First of all, there is a GEM file of 
the joypad circuit diagram, for those hardware hackers amongst you 
that would like it printed a little larger. Secondly there is a joypad 
test program - nothing too exciting at the moment, but I hope to 
develop it in line with this series as an aid to the testing of some 
of the projects.