8-bit MicrocontrollersのトピックRe: Lookup table to Generate PWM tone

Lookup table to Generate PWM tone

mylim — Mon, 19 Nov 2007 11:22:35 GMT

Hi guys, do you have any example on Lookup Table for PWM tone generation?

Thanks.

Re: Lookup table to Generate PWM tone

tonyp — Mon, 19 Nov 2007 19:16:47 GMT

Check this thread on a similar topic.

Re: Lookup table to Generate PWM tone

mylim — Thu, 29 Oct 2020 09:12:29 GMT

Hi Tony,

Basing on the code from the thread;

Code:

lb00                ldhx      #TabSinlb01                clr       TPMC1VH                    mov       x+,TPMC1VL                    mov       #$28,TPMSC                    brclr     TOF,TPMSC,*                    cphx      #EndTabSin                    blo       lb01                    bra       lb00TabSin              dc.b      $A0,$A0,$da,$da,$A0,$A0,$da,$da                    dc.b      $A0,$A0,$da,$da,$A0,$A0,$da,$daEndTabSin

Where/how does the pointer increment in order to load the next TabSin value to x?

Thanks.

Regards,

Re: Lookup table to Generate PWM tone

tonyp — Thu, 29 Oct 2020 09:12:31 GMT

mov x+,TPMC1VL
(the + is for auto-increment after the move)

The TabSin table value isn't loaded to X, it's copied ('moved') from memory to memory using HX as pointer

Message Edited by tonyp on 2007-11-22 10:30 AM

Re: Lookup table to Generate PWM tone

mylim — Thu, 22 Nov 2007 16:49:39 GMT

Hi Tony,

does that mean once this routine is called, I do not require to have any external pre-calculated pointer for #TabSin?

It will increment itself in the routine until it matched #EndTabSin?

To generate tone, it's preferable to init my pin to PWM or Output Compare?

Any difference between these 2 as PWM will also either set/clr output on compare?

Does the channel compare value = TPMxCxV?

Thanks

Regards,MY

Re: Lookup table to Generate PWM tone

Ake — Thu, 22 Nov 2007 16:52:29 GMT

Hi,

The pointer gets incremented at

mov x+,TPMC1VL

and when it hits EndTabSin, it gets reset to TabSin.

Note that the program assumes that the conversion table starts at address $xx00.

So put an

org $xx00

before the TabSin label.

Regards,

Ake

Re: Lookup table to Generate PWM tone

tonyp — Thu, 22 Nov 2007 16:58:45 GMT

Ake wrote:
Note that the program assumes that the conversion table starts at address $xx00.
So put an
org $xx00
before the TabSin label.

No such assumption!

Re: Lookup table to Generate PWM tone

tonyp — Thu, 22 Nov 2007 17:13:51 GMT

mingyee wrote:
does that mean once this routine is called, I do not require to have any external pre-calculated pointer for #TabSin?

Read the code carefully. First instruction initializes HX to the beginning of the table.

To generate tone, it's preferable to init my pin to PWM or Output Compare?
Any difference between these 2 as PWM will also either set/clr output on compare?

PWM uses an Output Compare. Normally, varying the pulse-width changes the volume, while varying the frequency changes the tone. (How you do it depends on your actual hardware design.)

Re: Lookup table to Generate PWM tone

mylim — Thu, 22 Nov 2007 17:44:52 GMT

tonyp wrote:
PWM uses an Output Compare. Normally, varying the pulse-width changes the volume, while varying the frequency changes the tone. (How you do it depends on your actual hardware design.)

Hi Tony,

As I'm aware, the frequency can be manipulated thru TPMC1VL.

But how do I come about the Pulse Width?

I'm a little confused here.

Thanks.

Regards,

Re: Lookup table to Generate PWM tone

tonyp — Thu, 22 Nov 2007 17:59:14 GMT

You'll have to read the various references for detailed explanations. In short, (quoting from one such reference):

The period of this PWM signal is determined by the setting in the modulus register
(TPMMODH:TPMMODL). The duty cycle is determined by the setting in the timer channel value register
(TPMCnVH:TPMCnVL). The polarity of this PWM signal is determined by the setting in the ELSnA
control bit.

Re: Lookup table to Generate PWM tone

mylim — Thu, 22 Nov 2007 19:36:58 GMT

Hi Tony,

I've tried the code with my TPM modulo ctr $FF00.

I've obtained the graph below basing on circuit attached.

I'm not able to get a 'nice' sine wave is it due to my hardware filtering problem?

Please advise.

Thanks.

Regards,

Re: Lookup table to Generate PWM tone

tonyp — Thu, 22 Nov 2007 20:26:33 GMT

Others can help you much better with this. What frequency are you running the PWM at? Don't you need a resistor from between R14, R15 to ground?

Re: Lookup table to Generate PWM tone

mylim — Fri, 23 Nov 2007 00:20:36 GMT

Hi Tony,

My controller is running at internal osc. Which my bus freq is 2MHz.

My TPM Modulo is FF00 at 261.124ms ~ 3.83Hz.

I'm not targeting at any specific freq because I'm curretly testing out whether I'm able to obtain a Sine Wave using PWM.

Thanks.

Regards,

tpm_config.jpg

Message Edited by t.dowe on 2009-10-27 01:17 PM

Re: Lookup table to Generate PWM tone

bigmac — Fri, 23 Nov 2007 01:10:21 GMT

Hello MY,

It would appear the primary reason your code doesn't generate a sine wave is that the table data you are using does not represent a sequence of amplitude values within a sine wave.

Firstly, you will need to decide how many PWM output periods should represent one sine wave cycle. More periods per cycle should give simpler filtering, but there will be a trade off between amplitude resolution and sine wave frequency.

For example, let's assume you require 1000Hz sine wave using 10MHz TPM clock rate, i.e. 10,000 bus cycles per output cycle. One possible solution would be to have 100 PWM periods, with each PWM period consisting of 100 TPM clock cycles ( 1% amplitude resolution). and this would seem a reasonable compromise. Using these figures, the size of the lookup table would need to be 100 bytes.

Since the PWM generates an output amplitude between 0 percent and 100 percent, the sine waveform will be offset so that the negative minimum corresponds with 0 percent, and the positive maximum with 100 percent amplitude. The lookup table might have the following structure -

Table index: Entry value:

0 50

25 100

50 50

75 0

The intermediate values, in increments of 3.6 degrees, would need to be calculated and scaled to follow the sine waveform.

Since the PWM frequency for the above example would be 100 kHz, the cut-off frequency for the output filter would be non-critical, perhaps a value of 2 - 3 kHz. A two-pole filter will probably be adequate, however, I would probably use an active filter type for better control of the filter characteristic, and to provide output buffering.

Regards,

Mac

Re: Lookup table to Generate PWM tone

PeterHouse — Fri, 23 Nov 2007 12:23:44 GMT

You could have the lookup table only contain the data for a single quadrant of the sine since all four quadrants will have identical values and will differ only in output order and magnitude. If you have amplitude resolution of 8 bits your table would contain only seven bits and half the time would be subtracted from 128 and the other half added to 128. This could reduce your table size to 25% or you could inccrease the frequency resolution - your call.

I am sure Mac knows this and left it off for simplicity!

Good Luck,

Peter House

Re: Lookup table to Generate PWM tone

mylim — Thu, 29 Oct 2020 09:12:33 GMT

Hi, I've tried to implement a table like below which i think it looks more like a sine table according to your comment.

Code:

tone        ldhx   #tonetab 
tone1       clr    TPM2C0VH
            mov    X+,TPM2C0VL
            mov    #$28,TPM2SC
            brclr  TPM2SC_TOF,TPM2SC,*
            cphx   #tonetabend
            blo    tone1
            rts

tonetab     fcb    $00,$7F,$94,$A7,$B9,$C8,$D5,$DE,$E4,$E6
            fcb    $E4,$DE,$D5,$C8,$B9,$A7,$94,$7F,$6C,$59
            fcb    $47,$38,$2B,$22,$1C,$22,$2B,$28,$47,$59,$6C
tonetabend

I'm having another problem which after executing

mov X+,TPM2C0VL

I do not see the X value written into TPM2C0VL register.

Please advise.

Thanks.

regards,

7.jpg

Message Edited by t.dowe on 2009-10-27 01:19 PM

Re: Lookup table to Generate PWM tone

tonyp — Fri, 23 Nov 2007 20:53:32 GMT

No, you're not. The first value in your table is $00 indeed.

Re: Lookup table to Generate PWM tone

bigmac — Sat, 24 Nov 2007 03:22:38 GMT

Hello MY,

There are still some problems with your lookup table. Since you requre symmetry between the positive and negative half-cycles of the waveform, the table should have an even number of entries, possibly 32 (not 31), and the first entry should not be zero. There also seems to be some degree of asymmetry between positive and negative peak amplitudes.

For a 32 byte table, the following would seem to be a little more accurate for a sine shape -

tonetab:

fcb 128,148,166,184,199,211,220,226

fcb 228,226,220,211,199,184,166,148

fcb 128,108,90, 72, 57, 45, 36, 30

fcb 28, 30, 36, 45, 57, 72, 90,108

For the range of values you have selected, the TMOD value for the TPM module should be 228 ($00E4) minimum. Higher values will affect both output amplitude and sine frequency. The sine wave period will be 32 times the TPM overflow period.

For a TPM clock of 2.0 MHz, and a TMOD value of, say 255, the sine frequency should be about 244 Hz, and the PWM frequency 7.8 kHz. The low-pass filter cut-off frequency should be about 500 Hz, perhaps somewhat higher if a CR passive filter is used.

Regards,

Mac

Re: Lookup table to Generate PWM tone

mylim — Sat, 24 Nov 2007 14:33:08 GMT

Hi Bigmac,

I've tried using the sine table you gave me & the TPM2MOD value.

The waveform doesnt seems quite right.

Another thing is that after executing mov X+,TPM2C0VL
I still can't see the respective Tonetab value on the memory table in the debug window.

Doesnt this mean the value is not written into the register?

Thanks.

regards,

mingyee

TPM2_config.jpg

8_waveform.jpg

9_debug.jpg

Message Edited by t.dowe on 2009-10-27 01:21 PM

Re: Lookup table to Generate PWM tone

mylim — Thu, 29 Oct 2020 09:12:35 GMT

Hi, i also tried the method proposed by Peg (http://forums.freescale.com/freescale/board/message?board.id=8BITCOMM&thread.id=6656&view=by_date_ascending&page=1) message 10 of 11.

The code goes;

Code:

        mov    #$28,TPMSC    ; TPM: No int, ext source, prescale 1
          clr    PNTR          ; Initialise pointer value
          clrh
LOOP1:    ldx    PNTR
          ldx    TabSin,x      ; Get byte value from table
          sthx   TPMC1V        ; Set PWM value
          inc    PNTR          ; Increment for next table entry
          brclr  TOF,TPMSC,*   ; Wait for timer overflow
          mov    #$28,TPMSC    ; Clear overflow flag
          bra    LOOP1         ; Loop always

I'm still unable to write my lookup table value into the TPM2C0VL.

Is there anything wrong?

I've attached the debug window & waveform captured below.

Btw, I'm using MC9S08AW60CFG.

Please advise. Thanks.

Regards,

C_debug.jpg

B_Waveform.jpg

Message Edited by t.dowe on 2009-10-27 01:23 PM