JOY_STIK.ASM Theory of Operation        2001

Overview:
The program is designed to convert the movements of most analog
PC joysticks into serial data formatted to operate a Scott Edwards
SSC II servo controller.
The hardware is a PIC 16F84, 5v regulator, 3 caps and
a few resistors for current limiting.

Data aquisition:
The joystick's two variable resistors representing X and Y axis
positions are read sequential with an RC based analog to digital
conversion.  Some of the routines are borrowed from Microchip AN512.
Measured values are then stored for later calculations.

Joystick calibration:
When a joystick is first used with the converter, a calibration
must be done by powering up with the fire button pressed.
When the button is released, the routine begins measuring the
position in both directions.  The joystick is then moved in a
sweep covering the full range of motion.  At this point the fire
button is pressed again which stores the maximum and minimum
resistance in EEPROM memory.  Afterwards, the LED begins blinking,
indicating the calibration is complete.
Because the calibration is stored in non-volatile memory, the data
is saved and will be present the next time the interface is
turned on.  The calibration routine must be done when changing
joysticks and can be performed anytime desired thereafter.

Program operation:
The unit is powered up w/o any buttons pressed for normal use.
The calibration data is retrieved from PIC EEPROM memory.
If the data is corrupt or not found, the program stops and
the LED begins blinking.  If good data is read back, the
PIC starts measuring both axis with 16 bit resolution.
The measurements for each axis are then scaled to 8 bits by the
math routines prior to storage.
At this point the fire button which controls a third servo is
read.  This servo shifts from minimum to maximum when the
fire button is pressed, returning to minimum when released.

Delta Compression:
Data from all three inputs are compared to the previous readings.
Only the data which has changed is sent serially.
The LED will blink once for each update sent and the
LED on a connected SSCII should blink in unison.

Serial data format:
Data out is jumper selectable 2400/9600,8,N,1 serial at TTL levels.
The jumper control mimics the SSCII. On = 9k6, Off = 2k4.
The present code is designed to operate with the SSC II connected
directly.  If the data is to be sent via a modem or other RS232
device, optional code written for use with an inverting TTL to
serial converter (i.e. MAX232) will soon be availble.  The data
out line from the MAX232 would be tied to DB9 pin #3 instead of
pin #2 for this type of connection.

Other applications:
Because the resistance measurements are self powered, this
unit will work with any pair of potentiometers with values
from 5k to 400k and a SPST switch replacing the fire button.
Would make a great pan / tilt control for a camera.
This version of the code requires all three standard joystick
inputs (X,Y,fire) be present to operate properly.  If you
have problems, check the resistance at the joystick connector
between pins #1 / #3 for X axis and #1 / #7 for Y axis, note
change matching movement of the joystick.  If none is seen,
check the other pins.  I wired mine to work with my F-15E
Hawk joystick and am assuming other analog sticks are the same.
You may also want to check which of your fire buttons pulls
pin #6 low.  This will be the one used for calibration and
third channel control.

Pinout:
DB15 joystick - pin #1 is x,y axis common.  Pin #3 is x axis out.
Pin #7 is y axis out.  Pin #6 is the fire button out.  
Pin #4 is fire button common (ground).

DB9 serial - pin #2 is serial out.  Pin #5 is ground.
Obviously, this assumes you've wired the SSCII the same.

This software is copywritten 2001 by Mike Berg N0QBH.
It's free to use in a non commercial manner by experimentors.

Mike Berg       N0QBH