Hello everyone,

Feels good to get back to my favorite pastime: designing hardware/software

projects for the ZX81. This time we mix a little hardware and software and

squeeze more performance out of two simple chips than you might think

possible, all of which echoes the simplicity of the original ZX81 design.

A PROPORTIONAL JOYSTICK INTERFACE

1996 - wilf rigter - Revised 2005

WHAT IT IS

This 2 chip (74HC138 and 74HC245) joystick interface provides 8 analog

inputs with 5 bit accuracy and is suitable for connecting 2 proportional

(IBM/APPLE) joysticks complete with left/right fire button inputs.

It can also be used for making 8 analog measurements which use resistive

transducers such as thermistors, CdS photocells, pressure sensitive __

resistors, servo position encoders, potentiometers, etc.

In this project, the 245 bi-directional buffer is used as a threshold

detector to measure the time constant of a simple resistor and capacitor

network. This CMOS digital logic chip is not an analog device but its

accuracy is sufficient for a joystick interface and similar applications.

I used a SN74HC245 chip made by TI. Other chips may require different

RC values.

HOW THE HARDWARE WORKS

The 74HC138 is used as a conventional I/O decoder and I used Q6 for

address F5 in this application but any other decoder output except Q0

can be used.

A 3300 pf capacitor is connected between a HC245 input and ground and a

joystick potentiometer (100K) or switch is connected from the input and

the joystick common line to +5V. A 4.7K series resistor is sets the

minimum value for R. Most joysticks also include a ZERO adjustment pot.

The joystick resistance is measured by first discharging the capacitor

by writing a zero to the output port and then sampling the inputs 32

times with a fast software loop and recording the samples in memory.

The RC values are approximate and should be adjusted to match the

joystick pot values and 245 chip. For example a 40K pot required a

0.01 uF cap and a minimum 2K series resistor.

If a switch is used instead of a pot you will of course only be able to

read a two state output. Light Dependent Resistors and thermistors will

need calibration and linearization. Some experimentation may be required

to give the desired results.

HOW THE SOFTWARE WORKS

The hardware and sampling software are tightly coupled in this simple

hardware circuit. The interface is designed to be used in the SLOW mode

and a special software routine is required to sample during a time when

it cannot be interrupted by the Non Maskable Interrupt (NMI). This is

accomplished by hooking the I/O sampling routine into the ZX video service

routines.

The annotated listing 1 shows how SAMPLE is embedded between the video

service routines right after DPILE processing and before NMI is enabled.

The VIDEO1 routine can only be started indirectly by calling START to

change the video routine vector in the IX register to the start of the

VIDEO1 routine.

Note the use of the INIR instruction to transfer 32 samples from the I/O

port to the system variable Printer Buffer (PRBUFF). The VIDEO1 routine

has a constant execution time.

The DECODE software executes in a variable time and must be called from

the application program. DECODE converts the stored samples to analog values

0 to 31 which are then stored into a variable array of Character values

representing the time constant of each input. The 32 samples in the PRBUPF

are scanned S times and bits are tested with a bit mask in C until the bit

is 1 or the loop counter in B is zero. In either case the value in B is

stored in the corresponding variable byte. The decoded analog values can

be used by BASIC or ML programs for example:

10 DIM A$(8)

20 RAND USR 16518

30 RAND USR 16516

40 PRINT AT 0,0; A$

50 GOTO 30

If you would like to experiment, you can store the analog values directly

to the screen by replacing the instruction LD HL, (4010) at the start of

DECODE with LD DE, (400C) and using the following BASIC code.

10 DIM A$(8)

20 RAND USR 16518

30 RAND USR 16516

40 GOTO 30

And finally an example of positioning software which assumes that the

joystick X and Y pots are connected to input 0 and 1:

10 DIM AS (8)

20 RARD USR 16518

30 RAND USR 16516

40 PLOT (CODE A$(l)),(CODE A$(2))

50 GOTO 40

LISTING 1

I/O SAMPLER ROUTINE 1996 WILF RIGTER

CONVERT JR DECODE ;CALL WITH RAND USR 16516 TO DECODE SAMPLES

START LD IX,VIDEO1 ;CALL WITH RAND USR 16518 TO START SAMPLE

RET ;BY INTERCEPTING THE ZX VIDEO ROUTINE

VIDEO1 LD A,R ;THE VIDEO / SAMPLING ROUTINE

LD BC,l901 ;24 LINES OF VIDEO

LD A,F5 ;DELAY

CALL 2B5 ;JP TO DFILE

SAMPLE XOR A ;NOW WE DO THE ACTUAL SAMPLING

OUT 5F,A ;DISCHARGE THE CAPS

LD HL,40C3 ;USE THE PRINTER BUFFER @ 403C

LD BC,205F ;TO STORE 32 BYTES

INIR ;AT 150,000 SAMPLES/S

VIDEO2 CALL 292 ;NOW THE APPLICATION CODE

CALL 220 ;AND VSYNC

LD IX,SAMPLE ;RESTORE SAMPLE VECTOR IN IX

JP 2A4 ;RETURN TO APPLICATION

;SCAN AND CONVERT THE SAMPLE BYTES

DECODE ;INTO 8 ANALOG VALUES BETWEEN 0-31

LD HL,(4010) ;START OP BASIC VARIABLES

LD DE,0006 ;SKIP THE HEADER

ADD HL,DE ;FIRST ELEMENT OF AS ARRAY

LD C,01 ;SET UP BIT MASK BIT 0

LOOP1 LD DE,403C ;START OF PRINTER BUFFER

LD B,1F ;VALUE COUNTER

LOOP2 LD A, (DE) ;GET SAMPLE

AND C ;COMPARE BIT

JR Z LOOP3 ;RECORD VALUE IF BIT IS 1

INC DE ;NEXT SAMPLE BYTE

DJNZ LOOP2 ;LAST SAMPLE BYTE?

LOOP3 LD A,B ;RECORD VALUE

LD (HL),A; STORE TO VARIABLE

INC HL ;NEXT VARIABLE

RLC C ;NEXT BIT

JR NC LOOP1 ;ANY MORE BITS

RET ;RETURN IF LAST BIT DONE

THE CREDITS

This project is simplified version of Fred Nachbaur’s Proportional

Joystick Interface (PSI) in one of his excellent ZX BREADBOARDER

articles written up in 1989 in ZXAPPEAL, the Vancouver Sinclair

User Group newsletter

Fred’s design uses 3 chips including a 556 type timer to measure

2 analog voltages and two switch inputs.