Andre,

Bill Cornut's advice is excellent.

Have you considered putting the code to generate the output waveform in
the interrupt handler, but leave the A to D code in the main loop?

A while back, I designed some PIC code for a circuit that went into a
ruggedized portable PC that had internal heating elements. This PC was for
data collection in test aircraft, the PC had to function at -30 deg C. The
PIC had to sample four temp sensors, calculate the required PWM duty
cycles for the five heating elements, and generate the five PWM outputs
that controlled FETs to switch power to the heaters.

I put the A to D code and the calculation code in the main program loop -
real easy stuff there.

The difficult part was the PWM outputs. The heaters consume a huge amount
of power, so all five could not be on at the same time. I had to
synchronize and interleave the PWM outputs to not exceed the power budget
(and burn up the power supply section!) In the interrupt code, I had a
counter that would increment each interrupt. The main loop calculated the
start and end count number for each of the five PWM outputs. The interrupt
turned on/off the appropriate PWM outputs when the count equalled the
start/stop values. Worked great, so I submitted the design.

In testing, the client wanted the PWM frequency increased. But I already
had the TMR0 prescaler at 0, and the PIC was running at 20Mhz. The
interrupt code could not be made smaller, so I decided to modify the TMR0
value from inside the interrupt routine. Since TMR0 caused an interrupt
when it rolls from 0xFF to 0x00, TMR0 has a value of 0 when the interrupt
routine begins. The first instructions in my interrupt routine (except
saving STATUS and W) were to load TMR0 with a new value. This forced the
interrupt frequency to increase.

To get the maximum PWM frequency, the interrupts happened so often that
the main loop code would execute about five instuctions between
interrupts. Any faster and the main loop would never get a chance to run!

So, try putting your output code in the interrupt handler, and if the top
frequency is still inadequate, modify TMR0 in the interrupt handler to
force interrupt frequency to increase. Carefully calculate worse-case
timing and make sure the main loop gets a chance to execute.

Dave

Andre Abelian wrote:

> Hi to all engineers.
>
> I am trying to make analog control A/D generator. I connected
> 20 Mhz crystal with 16c71. on output I am getting only 20 Khz.
> I know 20/4 = 5 Mhz do you think A/D conversion slowing it down.
> Is there any thing I am doing wrong?
>
> here is my code.
>
> TMR0        EQU     0x01              ; COUNTER
> PC          EQU     0x02              ; PROGRAM COUNTER
> STATUS      EQU     0x03              ; STATUS REGISTER
> FSR         EQU     0x04              ; FILE SELECT REGISTER
> PORTA       EQU     0x05              ; OUTPUTS
> PORTB       EQU     0x06
> RP0         EQU     0x05
> TRISA       EQU     0x05             ;
> TRISB       EQU     0x06             ;
> OPTION_REG  EQU     0x81             ;
> ADCON0      EQU     0x08
> ADCON1      EQU     0x08             ;
> ADRES       EQU     0x09
> PCLATH      EQU     0x0A
> INTCON      EQU     0x0B
> CARRY       EQU     0x00              ; CARRY BIT
> DCARRY      EQU     0x01              ; DIGIT CARRY BIT
> PDOWN       EQU     0x03              ; POWER DOWN BIT
> WATDOG      EQU     0x04              ; WATCHDOG TIMEOUT BIT
> F           EQU     0x01
> Z           EQU     0x02
> PORTA       EQU     0x05               ; PIN
> PORTB       EQU     0x06               ; INPUTS FROM DIP SWITCH
> CH_1_RESOLT EQU     0x0C               ; TEMP REGISTER
> CH_2_RESOLT EQU     0x0D
> CH_3_RESOLT EQU     0x0E
> CH_4_RESOLT EQU     0x0F
> COUNT       EQU     0x1D
> COUNT1      EQU     0x1C
> TEMP        EQU     0x1E
>
>  LIST P=16C71
>  ERRORLEVEL  -302
>
> #DEFINE  BANK1 BSF STATUS,RP0
> #DEFINE  BANK0 BCF STATUS,RP0
>
>     ORG     00H
>     GOTO INIT
>
> ; ****************** SUBROUTINES **********************
> DELAY
>         MOVF  CH_1_RESOLT,W     ; LOAD DECIMAL .25 TO COUNT
>         MOVWF COUNT            ; SAVE IT THERE
>
>         DECFSZ COUNT,F         ; DECRIMENT COUNT2
>         GOTO   $ - 1           ; IF NOT JUMP
>         RETURN
>
> ; ****************** INITALISATION ********************
>
> INIT    CLRF    PORTB
>         BANK1                   ; SET UP PAGE 1
>         MOVLW   B'00000000'
>         MOVWF   TRISB           ; B0 AND B1 INPUTS THE REST OUTPUTS
>         MOVLW   B'00011111'
>         MOVWF   TRISA           ; SET AS INPUTS A4 OUTPUT
>         MOVLW   B'00000000'
>         MOVWF   ADCON1          ; SET A/D INPUTS ON A0/1 REST DIGITAL
>         BANK0                   ; RETURN TO PAGE 0
>         MOVLW   B'11000001'     ; SET A/D INTERNAL RC CLOCK AND 11 FOR
> CH 4
>         MOVWF   ADCON0          ; DO IT
>
> ; ****************** THE START ************************
>
> MAIN
>                 BSF PORTB,2
>                 BSF     ADCON0,2        ; START CONVERSION
>                 MOVLW   10              ; THIS LINE IS 10
>                 MOVWF   TEMP
>                 DECFSZ  TEMP,F          ; WAIT A WHILE
>                 GOTO    $-1
>                 BTFSC   ADCON0,2        ; TEST FOR END OF CONVERSION
>                 GOTO    $-1
>                 MOVF    ADRES,W         ; LOAD A/D RESULT INTO W
>                 MOVWF   CH_1_RESOLT     ; STORE IT
>
>                 BSF PORTB,0
>                 CALL DELAY
>                 BCF PORTB,0
>                 CALL DELAY
>                 GOTO MAIN
>
>                 END