18F4431 Quaduature Encoder

[BigWumpus]

I'm using the 18F2431 too.
And I begun with the PICDEm MC LV board...

It's a great uC ! (UART, I2C, fast AD, 3-times PWM...)
I use it as a PI(D)-Controller for a BLDC-Motor.
And I use it for his fast AD-Converters in a CR-Tuning-Box.

The Quadrature-Encoder isn't very strange ... the whole business with this chip is to use the right timers for the right tasks and use the right prescalers !!

;-)

[Bruce]

Yeah this one is a nifty series PIC for sure. Sampling 4 x A/D inputs at once is
a very handy feature.

See this post http://www.picbasic.co.uk/forum/showthread.php?t=6768

I just received a few samples of the new USB 18F2553. 12-bit A/D on this
one is nice, but the 18F2431 is still my favorite. Now, if I can just find more
time to play with it..;o}

[hell_pk]

Thanks a lot Bruce.
I just want to get started with PPWM and then build my own code for brushless DC motor control.
Thanks for your code although its in assembly.i will figure it out.

[Bruce]

Here's the BASIC equivalent commands to the right of the assembly version.

FOR Duty = 50 TO $500 STEP 4
        ASM
        bsf   PWMCON1,UDIS    ; PWMCON1.1 = 1
        MOVFF _Duty,PDC0L     ; PDCOL = Duty.LowByte
        MOVFF _Duty+1,PDC0H   ; PDC0H = Duty.HighByte
        MOVFF _Duty,PDC1L     ; PDC1L = Duty.LowByte
        MOVFF _Duty+1,PDC1H   ; PDC1H = Duty.HighByte
        MOVFF _Duty,PDC2L     ; PDC2L = Duty.LowByte
        MOVFF _Duty+1,PDC2H   ; PDC2H = Duty.HighByte
        ;MOVFF _Duty,PDC3L    ; PDC3L = Duty.LowByte
        ;MOVFF _Duty+1,PDC3H  ; PDC3H = Duty.HighByte
        bcf   PWMCON1,UDIS    ; PWMCON1.1 = 0
        ENDASM
        PAUSE 50
    NEXT Duty

bsf PWMCON1,UDIS just sets bit 1 in PWMCON1. bcf PWMCON1,UDIS then
clears the UDIS bit.

MOVFF is move file to file.

MOVFF _Duty,PDC0L moves the low byte of Duty into file register PDC0L.

MOVFF _Duty+1, PDC0H moves the high byte of Duty into file register PDC0H.

Placing the underscore before the _Duty variable allows you to access the
variable declared in BASIC with assembler.

So the BASIC version would look like this;

  FOR Duty = 50 TO $500 STEP 4
    PWMCON1.1 = 1
    PDC0L = Duty.LowByte
    PDC0H = Duty.HighByte
    PDC1L = Duty.LowByte
    PDC1H = Duty.HighByte
    PDC2L = Duty.LowByte
    PDC2L = Duty.HighByte
    PDC3L = Duty.LowByte
    PDC3H = Duty.HighByte
    PWMCON1.1 = 0
    PAUSE 50
  NEXT Duty

  FOR Duty = $500 TO 50 STEP-4
    PWMCON1.1 = 1
    PDC0L = Duty.LowByte
    PDC0H = Duty.HighByte
    PDC1L = Duty.LowByte
    PDC1H = Duty.HighByte
    PDC2L = Duty.LowByte
    PDC2L = Duty.HighByte
    PDC3L = Duty.LowByte
    PDC3H = Duty.HighByte
    PWMCON1.1 = 0
    PAUSE 50
  NEXT Duty

[NatureTech]

Bruce,

That's a super start to a beginner like me.

But,i gotta some problem with the PWM Period value.I mean how to find the values for .. say...a PIC18F4431 running at OSC 20 , wish to set base period 19.5KHZ.How to find this using equation 17-1 ( i use this equ since it's free running mode here).Apart from that , how to calculate the min & max value of dutycycle?.Dohh...it's really bothering me no matter how many times i try it.

Could you guide me how to do this.I am a bit confused.The rest seems yeeha as it's taught in "863 MTR" from Microchip.


Regards.

[NatureTech]

Anyone..know how to do it?.

[Bruce]

       (PTPER+1) x PTMRPS
TPWM = ------------------
            Fosc/4

Equation 17-1 assumes you know the value in PTPER.

TPWM is the period of the PWM frequency.
PTPER is the value loaded into the PWM period register.
PTMRPS is the PWM timer prescaler. 1, 4, 16 or 64.

1/19,500 = 0.000051282. This is the period for a PWM frequency (TPWM) of 19.5kHz.

At 20MHz the instruction cycle time = 1/5MHz = 200nS.

To find the value to load into PTPER, divide the PWM period by the instruction cycle time.

0.000051282/0.0000002 = 256.410. Subtract 1 for 255.

If you load 255 into the period register, it will generate a PWM frequency of around 19.5kHz.

PTPERL=$FF ' low byte = $FF
PTPERH=$00 ' high byte = $00

Assuming a 20MHz osc, 1:1 prescaler, Fosc/4 = 5Mhz, so plug this into equation 17-1.

TPWM = (255+1) * prescaler
       -------------------
             Fosc/4

TPWM = 256/5MHz = 0.0000512.
Frequency = 1/TPWM so 1/0.0000512 = 19.531kHz.

The period register is 12-bits wide, so the minimum period for a given oscillator speed = the
instruction cycle time * ($0FFF+1) * the prescaler.

I.E. at 40MHz the instruction cycle time = 1/10Mhz = 100nS.

100ns * $1000 = 0.0004096. 1/0.0004096 = 2.441kHz.

At 10MHz 1/2.5Mhz = 400nS. 400nS * $1000 = 0.0016384.

1/0.0016384 = 610Hz, etc, etc..

You can see from Table 17-1 how the PWM timer prescaler affects the PWM frequency.
I.E. 2.441kHz/64 = 38Hz, etc,..

Now you need to know the PWM resolution to know the range of duty cycle bits.

Note: Resolution is the number of bits you have to control the duty cycle. Not the PWM
frequency.

For PWM resolution use the equation below. Equation 17-3 'as shown in the data sheet'
doesn't produce the same figures for PWM resolution shown in Table 17-2.

               log(Fosc/Fpwm)
Resolution = ------------------
                   log(2)

At 20MHz, with a PWM frequency of 19.5kHz;

             log(20MHz/19.5kHz)   3.010995384
Resolution = ------------------ = ----------- = 10.003
                   .301               .301

So we 10-bit resolution. The value loaded into duty cycle registers would range from 0 to
1023 or $00 to $03FF.

512 loaded into the duty registers should give ~50% duty at 19.5kHz.

There's a lot more to the PCPWM module than this, but it should help you get
started. I haven't played much with this particular feature.

[NatureTech]

Hi Bruce,

Sorry if i pester you a lot.Well,it drive me crazy to know about it.This should be more than enough for me to get started.Yes.You are right.This is part of it and not complete yet.But anyhow,it's clear enough for me to understand.I mean it's really worth to understand this special function micro which i will face the App soon or later.

Thanks a lot.