Scaling ADC Result to a Set Range

[RossWaddell]

I found the math behind the Arduino map function and have replicated it in a subroutine in PBP:

PR2     = 62                ; For 16Mhz OSC the desired output freq of 15,873Hz is
                            ; achieved with this PR2 value (8-bit resolution
                            ; with 1:4 prescaler)
                            
                            ; PWM freq must be ~ 16-20kHz to reduce noise
MinDuty     CON 100         ; Minimum speed to rotate motor for this application
MaxDuty     VAR WORD        ; According to Darrel:
                            ;   MaxDuty = (PR2 + 1) * 4

MaxDuty = (PR2 + 1) * 4     ; 252 but with prescaler resolution it's actually 250
MotorDuty   VAR WORD        ; Actual duty cycle for motor
MaxADCVal   CON 255         ; 255 for 8-bit; 1023 for 10-bit
ADCInVal    VAR BYTE        ; stores ADCIN result read from trim pot
compVal     VAR BYTE        ; stores last-changed ADC value

Main:
    gosub Do_ADC
    pause 100

    If ADCInVal <> compVal Then
        #IFDEF USE_LCD_FOR_DEBUG
            HSEROUT [LCD_INST, LCD_CLR]
            pause 5
            HSEROUT ["new ADCInVal=", DEC ADCInVal, "       ", 13, 10] ; Send text followed by carriage return and linefeed
        #ENDIF
        
        GOSUB Map_ADC_Val_to_PWM_Duty
        gosub ChngMotorHPWM
        compVal = ADCInVal
    endif

 
GOTO Main

Do_ADC:
    PAUSEUS 50              ' Wait for A/D channel acquisition time
    ADCON0.1 = 1            ' Start conversion
    
    WHILE ADCON0.1 = 1      ' Wait for it to complete
    WEND

    ADCInVal = ADRESH

    return

Map_ADC_Val_to_PWM_Duty:
'   Arduino Map function to emulate:
'   ===============================
'   map(value, fromLow, fromHigh, toLow, toHigh)

'   long map(long x, long in_min, long in_max, long out_min, long out_max)
'   {
'     return (x - in_min) * (out_max - out_min) / (in_max - in_min) + out_min;
'   }

    MotorDuty = (ADCInVal - 0) * (MaxDuty - MinDuty)/(MaxADCVal - 0) + MinDuty

    #IFDEF USE_LCD_FOR_DEBUG
        HSEROUT [LCD_INST, LCD_CLR]
        pause 5
        HSEROUT ["MotorDuty=", DEC MotorDuty, "       ", 13, 10] ; Send text followed by carriage return and linefeed
        pause 1500
        HSEROUT [LCD_INST, LCD_CLR]
    #ENDIF

    RETURN

It doesn't look like this is working as expected - any ideas?

[HenrikOlsson]

Hi,
I don't know exactly how you expect it to work but if Iäm not mistaken your formula, when re-written looks like:

MotorDuty = ADCInVAL * 252 / 255 + 100

When ADCInVal = 1 Motor Duty will be 100
When ADCInVAL = 255 MotorDuty will be 352

Is that what you're seeing?

/Henrik.

[richard]

(MaxDuty - MinDuty) would be 250(2)-100= 150

this would make more sense if your pwm value is a byte
it now becomes

MotorDuty = ADCInVAL */ 150(2) + 100

not knowing whats in the sub ChngMotorHPWM

snippets lead to speculation

[RossWaddell]

I'm using 8-bit resolution for PWM so it should be a byte but I have the variable defined as WORD in case the math takes it over 255:

MotorDuty   VAR WORD        ; Actual duty cycle for motor

ChngMotorHPWM:

    CCP3CON.4 = MotorDuty.0
    CCP3CON.5 = MotorDuty.1
    CCPR3L    = MotorDuty >> 2

    CCP4CON.4 = MotorDuty.0
    CCP4CON.5 = MotorDuty.1
    CCPR4L    = MotorDuty >> 2
    
    RETURN

Do you think the issue would be resolved if I change the datatype of MotorDuty to BYTE? I was also thinking I might switch to 10-bit resolution for both ADC and PWM to get the finest grain control of the motor speed, but that may be overkill.

[HenrikOlsson]

> Do you think the issue would be resolved....

What issue? What exactly is it that doesn't work the way you expect? What do you expect and what does it do?

75% dutycycle (8bit resolution) +/-25% from an 8bit ADC readin:

PWMDuty = 191 + (64 - ADCValue >> 1)

191 is 75% of 255, your initial PWM duty cycle. When ADCvalue (BYTE) is 0 PWMDuty (BYTE) will be 255=100%. When ADCValue is 255 PWMDuty will be 127=50%.

/Henrik.

/Henrik.

[RossWaddell]

The goal is to scale the ADC values of 0-255 to a range of duty cycles appropriate for my project (100-252 with the chosen prescaler). If I manually calculate the results are correct:

• ADCInVal =0 yields 100
• ADCInVal=127 yields 175
• ADCinVal=255 yields 252

But perhaps I don't have the order right in the expression because I don't see those results with the above code.

[amgen]

from excel chart, multiply adc val by .6 and add 100........ but can't mult decimal so multiply by 6 then divide by 10 then add 100