Another little project on the old ZX81 breadboard is finished and

presented here to be shared by one and all!

 

                     A ZX81 BREADBOARD PROJECT

                       8 CHANNEL / 8 BIT ADC

                         1996 wilf rigter

 

WHAT IT IS

 

Here is the next installment in a series of simple hardware projects

for the ZX81 this time an 8 channel 8 bit Analog to Digital Converter.

The simplest version is shown in FIG 1 using the widely available

ADC808 and a 74HC4002 dual 4 input NOR gate. The hardware is pretty

useless without some software and to make things easy a have written

an interrupt routine, shown in listing 1, which automatically converts

each analog channel to binary data and stores the converted data in a

BASIC string variable. The unit uses the ZX81 +5V supply for reference

to provide a 0 to +5V analog input voltage range. This is fine for

ratiometric measurements like sensing the position of a potentiometer

type joystick. If absolute measurements are required, a software

calibration factor can be used or better accuracy can be obtained by

using a seperate LM317 voltage regulator trimmed to +5V.

 

THE HARDWARE

 

The circuit in FIG 1 decodes the I/O address 5F (plus echos) with IORQ,

A7 and A5 together with RD and WR to generate the required active high

control signals. The WR signal is used to load the analog channel

address and start the ADC808 A/D conversion and the RD signal is used

to read the ADC808 data. In order to keep the circuit simple, the MREQ

line is used as the clock source. With the MREQ line used for the clock

source, the actual conversion time is uncertain but can be determined

by sensing the End Of Conversion (EOF) line. The EOF signal is normally

tied to the INT line to tell the CPU when conversion is completed.

However the ZX81 INT and NMI lines are not available. Rather than

convert and poll the EOC line to determine when data is ready, I put a

small hook into the video routines to convert and transfer the ADC data

in the "background". All 8 channels are converted about 8 times per

second and automatically stored in A$. The typical low pass filter

shown on ch7 will clean up noisy input lines. VCCand GND of the ADC808

should be bypassed with a 1 uF tantalum cap in parallel with a 0.1 uF

ceramic. The +5V line is also connected to pin 14 of the 74HC4002 and

0V goes to pin 7.

 

          FIG 1 - 8 ch / 8 bit ADC for the ZX81 (and SPECTRUM)

 

                                 ADC808

            _____                _________

   RD --- 2)     \              |     VCC |11-------- +5V

   A5 --- 3) 74HC  \ 1________ 9| OE +REF |12-------- +5V

   A7 --- 4) 4002  /            |    -REF |16--------  0V

   IORQ - 5)_____/              |     GND |13--------  0V

            _____          ___ 6| START   |

   WR -- 12)     \        |     |     EOC |7 -------- not used

   A5 -- 11) 74HC  \ 13___|__ 22| ALE IN0 |26 ------- CH1  see IN7

   A7 ---10) 4002  /            |         |

   IORQ - 9)_____/              |     IN1 |27 ------- CH2  see IN7

   MREQ --------------------- 10| CLK     |

   D0 ----------------------- 25| A0  IN2 |28 ------- CH3  see IN7

   D1 ----------------------- 24| A1      |

   D2 ----------------------- 23| A2  IN3 |1 -------- CH4  see IN7

   D0 ----------------------- 17| D0      |

   D1 ----------------------- 14| D1  IN4 |2 -------- CH5  see IN7

   D2 ----------------------- 15| D2      |

   D3 ------------------------ 8| D3  IN5 |3 -------- CH6  see IN7

   D4 ----------------------- 18| D4      |

   D5 ----------------------- 19| D5  IN6 |4 -------- CH7  see IN7

   D6 ----------------------- 20| D6      |

   D7 ----------------------- 21| D7  IN7 |5 ---+----[1k]--->  0-5V

                                |_________|     |             ANALOG

                                               === 0.01       INPUT

                                                |

                                               0V

 

THE SOFTWARE

 

The software is for the ZX81 only. The conversion is started by first

writing the channel number to I/O address 5F and the converted data can

be read sometime later at the same I/O address. The assembly language

routine in listing 1 ties the conversion to the video routines, a

channel conversion is started and the data for that channel is read once

every video frame. The start occurs right after vertical sync and the

data is read right after the DFILE display which guarantees that the

data conversion is complete when the data is read. Each channel is

converted by using the lower 3 bits from FRAMES to select a channel and

data is automatically loaded into an element of an 8 byte A$ string

variable array. The process is repeated for each channel as the FRAMES

counter is incremented each 1/60 second. A DIM A$(8) BASIC line should

be the first line after the 1 REM line. The machine code test for A$ as

the  first variable and skips data storage if not found. If you read all

zero data check that A$(8) is the first defined variable. The routine is

started with RAND USR 16516 and any BASIC program can easily use the

data from the a$ variable. For example this little demo shows how the

calibration factor (FACTOR) is calculated. In this case it assumes the

ZX81 Vcc is exactly 5V but you can substitute the actual value for

greater accuracy). It also illustrates how easily the raw data in the A$

array is accessed and converted to volts and converted to a equivalent

string in B$. Next the voltage of each channel is displayed but first

the old voltage reading is erased by writing 6 spaces to the appropriate

screen location.

 

enjoy wilf

 

 

BASIC LISTING

 

1  REM ;this line contains the machine code for conversion

10 DIM A$(8)

20 RAND USR 16516

30 LET FACTOR = 5/256

30 PRINT "CHANNEL 1 - 8 INPUT LEVEL (VOLTS) "

40 FOR N=1 TO 8

50 LET B$=STR$ (CODE A$(N)*FACTOR)

60 PRINT AT N+2,0;"      ":AT N+2,0;B$

70 NEXT N

80 GOTO 40

 

ASSEMBLY CODE LISTING 1

 

START   LD IX,CONVERT   ; ALWAYS START CONVERT ROUTINE HERE

        RET

 

CONVERT LD A,R          ;SET UP THE NORMAL SINCLAIR SCREEN

        LD BC,1901

        LD A,F5

        CALL 02B5       ;AND NOW DISPLAY DFILE

        HL,(4010)       ;START OF BASIC VARIABLES

        LD A,(HL)       ;GET VARIABLE NAME

        CP C6           ;IS IT A$?

        JR NZ CONV1     ;MUST BE FIRST VARIABLE OR ELSE NO DATA

        LD A,(4034)     ;GET FRAMES

        AND 07          ;LOWER 3 BITS

        ADD 06          ;SKIP THE VARIABLE HEADER

        LD E,A          ;PUT IN OFFSET

        LD D,0

        ADD HL,DE       ;POINTER TO ELEMENT

        IN A,5F         ;GET DATA

        LD (HL),A       ;STORE IN VARIABLE

CONV1   CALL 0292       ;RETURN TO APPLICATION

        CALL 0220       ;DO VSYNC

        LD A,(4034)     ;GET FRAMES

        AND 07          ;LOWER 3 BITS

        OUT 5F,A        ;LOAD CHANNEL AND START CONVERSION

        LD IX,CONVERT   ;SAVE THE NEW VIDEO VECTOR IN IX

        JP 02A4         ;RETURN TO APPLICATION

 

END

 

 

 

 

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