PIC 16f877 A/D conversion Allegro ACS712

[thronborg]

Is this the way to get negative reading? i am new to this, so sorry if i mixed it up.

I use Allegro ACS714 but with a PIF16F690 with A/D set to 8bit, should not be any difference.

It works and can read +30Amp to -30Amp.

The problem is that i use 0V and 5V as reference. I cant figure out to get 0.52V to read 0 and 4.48V to read 255? Is it a math problem or must i add an external ref?

The ACS gives 66mV/A:
Amp Vin Bit
+30A 4.480 255
+10A 3.160
+1A 2.566
0A 2.500 127
-1A 2,434
-10A 1.840
-30A 0.520 0

See CODE below and attached DRAWING

'Define OSC and ADC
DEFINE OSC 4             ' Set internal Oschillator to 4Mhz
DEFINE    ADC_BITS 8       ' Set number of bits in result
DEFINE    ADC_CLOCK 2      ' Set clock source (3=rc)
DEFINE    ADC_SAMPLEUS 50  ' Set sampling time in uS
' Define LCD pins
Define LCD_DREG  PORTC   'LCD data port
Define LCD_DBIT  0       'LCD data starting bit 0 or 4
Define LCD_RSREG PORTC   'LCD register select port
Define LCD_RSBIT 4       'LCD register select bit
Define LCD_EREG  PORTC   'LCD enable port
Define LCD_EBIT  5       'LCD enable bit
 
 
TRISA = %00001001                         ' RA0 = A/D input
ADCON1.7 = 0                                  ' RA.1 = +Vref, Set PORTA analog and left justify result
PORTb.6  =0                                     ' Prepare RB0 for high-going pulseout

ANSEL = %00000101                         ' Set PORTA.2 analog, rest digital
ANSELH = %00000000

' Variables
outpuls VAR WORD                           ' Variable for the calculated puls out in mS
POT_POS VAR BYTE                          ' Potentiometer position CC=0, CCW=255
amp var word                                   ' Ampere 30A-0-30A (0-127-255)
                                 
Pause 500                                 ' Wait for LCD to start

MainLoop:                                 ' The Loop start here!
ADCIN 0,POT_POS                           ' Read A/D channel 0 to variable SVO_POS

'Check if negative or positive Ampere
IF POT_POS=>127 then
amp=(235 * (POT_POS-127))
else
amp=(235 * (128-POT_POS))
endif

Lcdout $fe, 1, "POT_POS= ", #POT_POS      ' Display POT Valu between 0-255 on line 1

' Check if negtive or positive Ampere, only for - minus sign on LCD display
IF POT_POS=>127 then
LCDOut $fe,$C0, "AMP= ",DEC (amp/1000),".", DEC1 amp' Display +Amp on line 2
else
LCDOut $fe,$C0, "AMP= -",DEC (amp/1000),".", DEC1 amp' Display -Amp on line 2
endif 

PAUSE 20                                  ' Constant 20mS pulses(low) between outpuls
GOTO MainLoop                             ' Forever
End