Don_Kirby
Joined: 15 Oct 2006
Posts: 329
Location: Long Island, New York
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Posted: 27 May 2007, 13:42 PM Post subject: Scaling PWM Duty Cycles |
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Here's a bit of code that might be useful to new users, or those who's math skills are as poor as mine.
I had the need to generate a PWM output that followed the value read from an ADC input. I also needed to define minimum and maximum limits for the duty cycle of the PWM output. The code below is what I came up with.
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Public Sub PWM_OUT()
Const PWM_PIN as Byte = D.5 'PWM Output Pin
Const ADC_PIN as Byte = A.6 'ADC Input Pin
Const UPDATE_RATE as Single = 0.5 'Seconds between updates
Const MIN_DUTY_CYCLE as Single = 25.0 '%
Const MAX_DUTY_CYCLE as Single = 95.0 '%
Dim DUTY_CYCLE as Single 'PWM Duty Cycle
Dim I as Single 'ADC Input
'Open the pin as an input.
Call PutPin(ADC_PIN, zxInputTriState)
'Initialize the PWM output
Call OpenPWM(1, 1000.0, zxCorrectPWM)
Do
'Read the ADC
Call GetADC(ADC_PIN, I)
'And Scale the ADC value to the min/max settings defined above
DUTY_CYCLE = ((((MAX_DUTY_CYCLE - MIN_DUTY_CYCLE) / 100.0) * I) * 100.0 + MIN_DUTY_CYCLE)
Call PWM(1, DUTY_CYCLE) 'update the PWM output...
Call Sleep(UPDATE_RATE) 'But not real often
Loop
End Sub
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Given an ADC value of 0.0 (tied to ground), and a MIN_DUTY_CYCLE of 25.0 (25%), the DUTY_CYCLE is 25%. Conversly, an ADC value of 1.0 (tied to Vcc) and a MAX_DUTY_CYCLE of 90.0 (90%) the DUTY_CYCLE is 90.0 (90%). ADC values in between are scaled appropriately.
Of course, the specifics of the OpenPWM call will depend on your particular application.
Apologies if this is common knowledge, but I didn't find anything similar while searching the forum.
-Don |
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