sinusoidal PWM

[HenrikOlsson]

Previously you started at the "end" of table and decremented the "pointer" but now you've apparently changed that so it starts at 0 and increments the pointer instead.

If you want it phase shifted 180° ("inverted") then simply "reverse" your sin-table. 128, 108, 89, 71.... instead of 148, 167, 185....

[bwaxing]

hellow,

i've managed to generate SPWM signal using 4MHz xtal ,thanx to Henrik for the tip of modifying the lookup table values,this is the screen shoot taken from the actual scope.....

however,there are things i'm not happy with this signal...
1)i expected to see at least one complete cycle on the screen for a single screen shoot,but this is not the case,it is either positive or negative cycle!what should i do?

2)in theory,SPWM signal is obtained by comparing the triangular wave having a certain switching frequency with a sine wave of certain modulating frequency (50Hz for my case),now the HPWM frequency is 18KHz,is this the switching frequency?if not how do i know the switching frequency for this kind of generation?i want a switching frequency around 2KHz. i want to use this signal to control a single phase inverter.

here is my code:

define OSC 4

;*****************VARIABLE DECLARATION******************************************
wsave   VAR BYTE    $70     SYSTEM      ' alternate save location for W 
wsave1  VAR BYTE    $A0     SYSTEM      ' location for W if in bank1
wsave2  VAR BYTE    $120    SYSTEM      ' location for W if in bank2
wsave3  VAR BYTE    $1A0    SYSTEM      ' location for W if in bank3
STEPCOUNT var byte                      'Define stepcount as byte type variable
stepcount = 36

;**************SETTING THE REGISTERS WITH APPROPIATE BIT DEFINITION************* 
ADCON0 = %00000000
ADCON1 = %00000000  ;all anolog output
trisb = %11111111   ;define porta as input
trisc = %11111011   ;make ccp1/portc.2 an output pin
trisa = %11111111   ;define porta as input
TMR2 = 117
PR2 = 55          ;set for 18Khz HPWM(=36 steps*10 times*50hz)        
CCP1CON = %000001100       ;set to pwm mode
T2CON=%00000100           ;enable timer2 and set timer2 prescaler value of 1:1

;****************A sine lookup table in an array********************************
sineval var byte[36]
sineval[0] = 115
sineval[1] = 102
sineval[2] = 90
sineval[3] = 79
sineval[4] = 70
sineval[5] = 62
sineval[6] = 57
sineval[7] = 53
sineval[8] = 52
sineval[9] = 53
sineval[10] = 57
sineval[11] = 62
sineval[12] = 70
sineval[13] = 74
sineval[14] = 90
sineval[15] = 102
sineval[16] = 115
sineval[17] = 128
sineval[18] = 141
sineval[19] = 154
sineval[20] = 166
sineval[21] = 177
sineval[22] = 186
sineval[23] = 194
sineval[24] = 199
sineval[25] = 203
sineval[26] = 204
sineval[27] = 203
sineval[28] = 199
sineval[29] = 194
sineval[30] = 186
sineval[31] = 177
sineval[32] = 166
sineval[33] = 154
sineval[34] = 141
sineval[35] = 128
 
timerone var word 
Temp VAR byte 
INCLUDE "DT_INTS-14.bas"     ; Base Interrupt System

;********Define INT_Handler as ISR********************************************** 
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, TMR1ON          
TMR1L = 255    
TMR1H = 254
@   INT_ENABLE  TMR1_INT     ; Enable Timer 1 Interrupts  
timerone = 64980          ;gives about 50 hz sine 
Main:
        pause 5
        
GOTO Main
 
'---[TMR1_INT - interrupt handler]------------------------------------------


sine:
    TMR1L = timerone.byte0
    TMR1H = timerone.byte1     
    Temp = SineVal[StepCount]   
    CCP1CON.4 = Temp.0   'Bit 0
    CCP1CON.5 = Temp.1   'Bit 1
    CCPR1L = Temp >> 2     'Bit 2-7
    if stepcount = 0 then stepcount = 36 
    stepcount = stepcount -1
   

@    INT_RETURN

[HenrikOlsson]

Hi,

1)i expected to see at least one complete cycle on the screen for a single screen shoot,but this is not the case,it is either positive or negative cycle!what should i do?

I haven't checked your code but going by the comments you expect a 50Hz "sinewave", a full period at 50Hz is 20ms but your scope is set to 200us/div, there's 10 divisions so you're only displaying 2ms on the screen. I see a 10:1 note there but does that apply to the horizontal sweep?

I don't understand the part with the triangular wave being compared with a sine etc but the switching frewuency IS the PWM frequency, or 18kHz in this case. If you want 2kHz you need to reconfigure the PWM module to output 2kHz which might also require adjusting the sine-table depending on how the numbers play out.