ProblemwithPWMfrequency

[sangaboy]

Hi HenrikOlsson
I have manage to combine the code,compile it successfully and get the correct PWM frequency(5khz) thanks for all.
(a) As you told me early the DutyVal of PR2*4=100% that means that my value of PR2=199 the DutyVal can vary within (0-796).One problem I have is in my lookup table the there is value greater than 796.What happen to this values greater than 796?

(b) Also I have read PIC Midrange manual(http://ww1.microchip.com/downloads/e...doc/33023a.pdf) page 211 and 212 it says
(1) " If the PWM duty cycle value is longer than the PWM period, the CCPx pin will not be cleared. This allows a duty cycle of 100%".
(2) " Any value greater than 255 will result in a 100% duty cycle".
is this two reason cause some part of my wave form not to have a PWM frequency of 5khz? having a long off time
here is code and two phase PWM output

STEPCOUNT var byte
STEPCOUNT1 var byte

LED1  VAR  PORTB.1

STEPCOUNT = 0   'pointer for phase one in sinearray
STEPCOUNT1 = 12 'pointer for phase two  in sine array
ADCON0 = %00000000
ADCON1 = %00000000

TRISB = %11111101
TRISC = %11111001  'PMW output for CCP1 AND CCP2
TRISA = %11111111
TMR2 = 0
PR2 = 199 'generate 5khz  'set for 18Khz HPWM
CCP1CON = %00001100 'set CCP1 for PWM OPERATION
CCP2CON = %00001100 'set CCP2 for PWM OPERATION
T2CON = %00000100  'TIMER2ON and prescale of 1:1 

sineval var byte[36]
sineval[0] = 512    
sineval[1] = 583   
sineval[2] = 652
sineval[3] = 717
sineval[4] = 775
sineval[5] = 825
sineval[6] = 866
sineval[7] = 896
sineval[8] = 915
sineval[9] = 921   
sineval[10] =915
sineval[11] =896
sineval[12] =866   
sineval[13] =825
sineval[14] =775
sineval[15] =716
sineval[16] =652
sineval[17] =583
sineval[18] =512   
sineval[19] = 441
sineval[20] = 372
sineval[21] = 308 
sineval[22] = 249
sineval[23] = 199
sineval[24] = 158
sineval[25] = 128
sineval[26] = 109
sineval[27] = 103
sineval[28] = 109 
sineval[29] = 128
sineval[30] = 158
sineval[31] = 199
sineval[32] = 249
sineval[33] = 308
sineval[34] = 441
sineval[35] = 512

timerone var word 
Temp var byte
Temp1 var byte
INCLUDE "DT_INTS-14.bas"     ; Base Interrupt System          emp
INCLUDE "ReEnterPBP.bas" 
 
ASM
 
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler   TMR1_INT,   _sine,   PBP,  yes
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
 
T1CON = 000001                 ; Prescaler = 1;1, TMR1 ON
@   INT_ENABLE  TMR1_INT     ; Enable Timer 1 Interrupts 
 
timerone =  65080' gives1818hz interruptfrequency or about 50 htz sine 
Main:
       PAUSE 5
 
GOTO Main
 
'---[TMR1_INT - interrupt handler]------------------------------------------

sine:
     T1CON = 000000 'stop the timer

    TMR1L = timerone.byte0  'reload the timer
    TMR1H = timerone.byte1
    T1CON = 000001          ' start the timer
    
    
    TeMP = sineval[STEPCOUNT]
    CCP1CON.4 = Temp.0 ' bit 0
    CCP1CON.5 = Temp.1 ' bit 1       
    CCPR1L = Temp >>2  'Bit 2-7 
    
    TeMP1 = sineval[STEPCOUNT1]
    CCP2CON.4 = Temp1.0 ' bit 0
    CCP2CON.5 = Temp1.1 ' bit 1       
    CCPR2L = Temp1 >>2  'Bit 2-7
    TOGGLE LED1 
    stepcount =  stepcount +1
    stepcount1 =  stepcount1 +1
    
    if stepcount =36 then stepcount =0
    if stepcount1 =36 then stepcount1 =12
     
 
@    INT_RETURN

Help to help me to solve this problems
thanks

[HenrikOlsson]

Hi,

One problem I have is in my lookup table the there is value greater than 796.What happen to this values greater than 796?

You answered your own question in (1) from the Midrange manual:

(1) " If the PWM duty cycle value is longer than the PWM period, the CCPx pin will not be cleared. This allows a duty cycle of 100%".

Any dutycycle value above 796 will result in 100% dutycycle. In other words your SIN-output will be distorted/clipped.

is this two reason cause some part of my wave form not to have a PWM frequency of 5khz? having a long off time

Don't think so, I'd expect it to stay at 100% but I may be wrong. However this isn't correct:

    if stepcount =36 then stepcount =0
    if stepcount1 =36 then stepcount1 =12

You can't restart at the 12th byte, that will make the second output miss 1/3 of the cycle. You start at the 12th byte to get the correct phase angle offset but you must restart at 0 once it reaches 36.

/Henrik.

[sangaboy]

Thanks HenrikOlsson for your information.But this look like is not clear to me. From what I now when stepcount1 =36 for 2nd phase then stepcount1=12 so as it start to the 12thbyte again because initially is declared as stepcount1=12.What confusing me is that initially in my declaration of variable stepcount1=12 why after one cylecle stepcount1 should declared as stepcount1=0.How can I archieve phase difference of 120 if stepcount=0 after one cylecle.

[HenrikOlsson]

Hi,
You have 36 values in you SIN table. To get a correct output you need each phase to cycle thru these 36 values. You get the phase difference by starting each phase 120° apart in the lookup table but from that point they will keep that phase difference if you reset the index pointer to 0 when they reach 36:


If you don't reset it to 0 but instead keep restarting the second phase at the 12th value (like you're currently doing) the output will look something like this:



Why? Because the second phase never "gets" the first 12 values of the lookup table so the output gets A) distorted and B) of another frequency since it doesn't contain the same number of "samples" as the the first phase.

Thinking even further and adding the third phase, starting at the 24th value keep restarting at 24 the output would look something like:


I hope you see that the above is not correct and that you understand WHY it's not correct.



Using your lookup table, starting the pointers at 0, 12 and 24 and restarting them all at 0 when they reach 36 the output looks like:

Which I think is more inline with what you're after, isn't it?

Now, the waveform is distorted because you have the value 512 as the first AND as the last the value in the table which means that you'll get a dutycycle of 512 for two "samples" in a row. That is where th flat spot in the wave form is comming from.

/Henrik.

[sangaboy]

Hi HenrikOlsson

Now Inderstand why stepcount1 should restart to zero.
(1) But I have been working on the code and and modify my look up table as you told me about the first value and the last value, but the problem of flat spot still remaing I didn't understand why is this?
(2)If I want to make acertain value of dutycylecle maximum (specific range of variation) let say(0- 80)% or (0-75%) should I do in the look up table or modify the formular?
formular : ((sin(degree*pi/180)+1) /2)*1023 for 10 bit resolution.


help me to solve this problems

Thanks

[HenrikOlsson]

Hi,
1) Can't help without seeing the values.
2) Not sure I understand what the problem is.... If you want a certain maximum dutycycle then simply rescale the values before storing them in the lookuptable (change the formula to produce a table with lower values). If you want to rescale at runtime then you need to perform the math in the PIC. Retrieve the "original" value, rescale, and output.

/Henrik.

[sangaboy]

Hi HenrikOlsson
1) Can't help without seeing the values.
Here is the values

Wsave var byte   $70system
Wsave1 var byte  $A0system
Wsave2 var byte  $120system
Wsave3 var byte  $1A0system

STEPCOUNT var byte
STEPCOUNT1 var byte

LED1   VAR  PORTB.1

STEPCOUNT = 0   'pointer for phase one in sinearray
STEPCOUNT1 = 12 'pointer for phase two  in sine array
ADCON0 = %00000000
ADCON1 = %00000000

TRISB = %11111101
TRISC = %11111001  'PMW output for CCP1 AND CCP2
TRISA = %11111111
TMR2 = 0
PR2 = 199 'generate 5khz  'set for 18Khz HPWM
CCP1CON = %00001100 'set CCP1 for PWM OPERATION
CCP2CON = %00001100 'set CCP2 for PWM OPERATION
T2CON = %00000100  'TIMER2ON and prescale of 1:1 

sineval var byte[36]
sineval[0] = 512    
sineval[1] = 583   
sineval[2] = 652
sineval[3] = 717
sineval[4] = 775
sineval[5] = 825
sineval[6] = 866
sineval[7] = 896
sineval[8] = 915
sineval[9] = 921   
sineval[10] =915
sineval[11] =896
sineval[12] =866   
sineval[13] =825
sineval[14] =775
sineval[15] =716
sineval[16] =652
sineval[17] =583
sineval[18] =512   
sineval[19] = 441
sineval[20] = 372
sineval[21] = 308 
sineval[22] = 249
sineval[23] = 199
sineval[24] = 158
sineval[25] = 128
sineval[26] = 109
sineval[27] = 103
sineval[28] = 109 
sineval[29] = 128
sineval[30] = 158
sineval[31] = 199
sineval[32] = 249
sineval[33] = 308
sineval[34] = 372
sineval[35] = 441

timerone var word 
Temp var byte
Temp1 var byte
INCLUDE "DT_INTS-14.bas"     ; Base Interrupt System          emp
'INCLUDE "ReEnterPBP.bas" 
 
ASM
 
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler   TMR1_INT,   _sine,   ASM,  yes
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM
 
T1CON = 000001                 ; Prescaler = 1;1, TMR1 ON
@   INT_ENABLE  TMR1_INT     ; Enable Timer 1 Interrupts 
 
timerone =  65080' gives1818hz nterruptfrequency or about 50 htz sine 
Main:
       PAUSE 5
 
GOTO Main
 
'---[TMR1_INT - interrupt handler]------------------------------------------

sine:
     T1CON = 000000 'stop the timer

    TMR1L = timerone.byte0  'reload the timer
    TMR1H = timerone.byte1
    T1CON = 000001          ' start the timer
    
    
    TeMP = sineval[STEPCOUNT]
    CCP1CON.4 = Temp.0 ' bit 0
    CCP1CON.5 = Temp.1 ' bit 1       
    CCPR1L = Temp >>2  'Bit 2-7 
    
    TeMP1 = sineval[STEPCOUNT1]
    CCP2CON.4 = Temp1.0 ' bit 0
    CCP2CON.5 = Temp1.1 ' bit 1       
    CCPR2L = Temp1 >>2  'Bit 2-7
    TOGGLE LED1 
    stepcount =  stepcount +1
    stepcount1 =  stepcount1 +1
    
    if stepcount =36 then stepcount =0
    if stepcount1 =36 then stepcount1 =0
     
 
@    INT_RETURN