Hey guys, will this work?

[jmgelba]

Well the little guy is sick and I got about 4 hours sleep last night.

Havent tried this yet but I came up with this alternative and included Darrels changes.

START:
ADCIN 6, CSET 'READ VALUE OF CURRENT SET POT
ADCIN 4, CSEN   'READ VALUE OF CURRENT SENCE
IF CSEN > CSET Then LET DUTY = DUTY - 1
IF CSEN < CSET THEN LET DUTY = DUTY + 1
GOSUB NewDuty                '
PAUSE 1                         
GOTO Start

NewDuty:
        TMR2IF = 0
        while !TMR2IF : wend       ; let current cycle complete
        CCP1CON = 0  
        CCPR1L = Duty >>2   
        CCP1CON.5 = Duty.1
        CCP1CON.4 = Duty.0
        CCPCON.7 = 0
        CCPCON.6 = 0
        CCPCON.3 = 1
        CCPCON.2 = 1
        CCPCON.1 = 0
        CCPCON.0 = 0              
        RETURN

[Darrel Taylor]

Poor little guy. :sad:

As for the proposed additions ...
You don't want to do that.

The idea is to minimize the glitches, not create more.
Leave the module in PWM mode and just change the dutycycle.
<br>

[jmgelba]

Hmm, well it dosent work. It's all over the place and then goes high and stays high. I have to power down to reset because adjusting the CSET pot does nothing to gain a recovery. Also, adc value for CSET does very little to alter the duty cycle.

Is my thinking right here? Will this try to find the right duty cycle so CSEN = CSET?

[mister_e]

Refresh your duty cycle only when it need to, unless you'll have problem.

try something like

START:
        ADCIN 6, CSET 'READ VALUE OF CURRENT SET POT
        ADCIN 4, CSEN   'READ VALUE OF CURRENT SENCE
        IF CSEN > CSET Then LET DUTY = DUTY - 1
        IF CSEN < CSET THEN LET DUTY = DUTY + 1
        if DUTY != OldDuty then
            OldDuty = Duty
            GOSUB NewDuty 
            ENDIF               
        GOTO Start

NewDuty:
        TMR2IF = 0
        while !TMR2IF : wend       ; let current cycle complete
        CCP1CON = 0  
        CCPR1L = Duty >>2   
        CCP1CON.5 = Duty.1
        CCP1CON.4 = Duty.0
        RETURN

You'll probably need to allow an error margin in your IF-THEN, remember you have a 7 bits or so PWM resolution but 10Bit ADC results. Reduce your ADC resolution to 8 bit may help.

You also want to make sure you don't overflow the Duty value...

Seeing your schematic would be nice.

[Bruce]

Does this work OK?

OSCCON = $70
DEFINE OSC 8

CSET   VAR WORD 'CURRENT SET
CSEN   VAR WORD 'CURRENT SENCE  
DUTY   VAR WORD 'PWM DUTY CYCLE
O_Duty VAR WORD 'previous Duty-cycle
MinDuty CON 2   'minimum duty cycle
MaxDuty CON 40  'maximum duty cycle

ADCON0 = %00011001
ADCON1 = %00001111
ADCON2 = %10111111
TRISA = %00000000
TRISB = %00110011
Define  ADC_BITS        10     	' Set number of bits in result
Define  ADC_CLOCK       3     	' Set clock source (5=fosc/16)
Define  ADC_SAMPLEUS    10    	' Set sampling time in uS

CSET = 0
CSEN = 0

DUTY = 20           ' duty value for 50% duty cycle
PR2 = 9             '
T2CON = %00000100   ' timer2 on, prescale 1:1
CCPR1L = DUTY>>2    ' MSB of duty cycle value
CCP1CON.4=DUTY.0    ' set PWM mode and store the 
CCP1CON.5=DUTY.1    ' 2 LSB of duty
CCP1CON = %00001100

LOOP:
ADCIN 6, CSET   'READ VALUE OF CURRENT SET POT
ADCIN 4, CSEN   'READ VALUE OF CURRENT SENSE

' test for CSEN > CSET*2
IF CSEN > (CSET*2) THEN  ' minimize short-cycling (increase as needed)
   IF DUTY > MinDuty THEN ' don't allow below minimum duty
      DUTY = DUTY - 1
   ENDIF
ENDIF

IF CSEN < (CSET/2) THEN  
   IF DUTY < MaxDuty THEN ' don't allow greater than max duty
      DUTY = DUTY + 1
   ENDIF
ENDIF

IF O_Duty = DUTY THEN LOOP ' does duty-cycle need updating?
O_Duty = DUTY    ' yes. copy last duty value for next comparison

CCP1CON.4=DUTY.0 ' set PWM mode and store the 
CCP1CON.5=DUTY.1 ' 2 LSB of duty
CCPR1L = DUTY>>2 ' MSB of duty cycle value
GOTO LOOP

A low-pass filter on each A/D input would also help.

[Darrel Taylor]

At 200khz, you only get a Duty range of 0-40.
Not going to be a very precise regulator with that.

Your previous examples were at 32.767khz, which PicMultiCalc shows 0-244 duty range (better). But like mister_e said, you'll need to limit the range.

With a 20mhz crystal you can get 0-610 @32khz, or 0-100 @ 200khz.
<br>

[jmgelba]

Guys thanks for the ideas and snippets. I have not been around today to work on this. I need to up the frequency to 300KHz now to reduce inductor size and value. I was going to use a 20mhz resonator but just wanted to get something up quick and dirty so I used the internal osc at 8mhz.

My schematic is a scribble on a piece of paper at the moment, but its essentially a classic buck converter design that could be found anywhere on the web. I think some of the problem is ripple voltge on the 5v line. This changes the internal vref of the adc.

I'll mill or get the boys in the shop to etch me a pcb in the morning so I can load up on the capacitance and cut down on the noise.

Oh, last night, I killed 12 Osram Dragon infra red led's that are not mine. Oops. I think they are something like $15 a piece