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Gradual dsPIC33FJ (Microstick II board) DSP tutorial - from blinking LED to real-time DSP

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Tutorial for dsPIC33FJ on Microstick II

Here is step by step tutorial that will be hopefully someday full example of real-time DSP - ADC conversion, processing, and DAC conversion. However it will take some time...

Common required hardware for all projects:

Required software:

  • MPLAB X IDE v6.00 - last version that works.

    • Do NOT use v6.05 - it will freeze forever when Master/Slave project is associated (not used in this case, but anyway...)
    • Do NOT use v6.10 - it has broken support for PKOB (PicKit on Board programmers) and also for PicKit 3 - that IDE version is simply rubbish (if it was not ill intention - to force people to buy new hardware?).

  • XC16 v2.10

List of projects

Important note:

All MPLAB X IDE projects have 2 configurations:

  • default - used for programming and debugging real Microstick II hardware
  • Simulator - running code in software simulator - no hardware required.

Before building and/or Debugging/running please select right configuration in MPLAB X IDE toolbar.

Tutorial 1 - use FRC oscillator, blink LED in main loop using delay()

Tutorial 2 - use FRC + PLL at 79.12 MHz, blink LED in main loop using delay()

  • dsPIC33FJ_tut02.X/
    • Tutorial No. 2
    • use FRC (at 7.37 MHz) + PLL to get fosc= 79.12 MHz (most close value to 80 MHz target)
    • PIN10 CLKO - should be fcy = 79.12/2 MHz = 39.56 MHz (= 39.56 MIPS)
      • I'm unable to verify it on Digilent AD2 (it has maximum sampling frequency 100 Mhz which is not enough to get precise waveform)
    • PIN2 RA0 - LED blinking with rate 200ms (=5Hz), toggle rate 100ms (10 Hz)

PLL notes:

  • used example from: http://ww1.microchip.com/downloads/en/DeviceDoc/DS-70596A.pdf

  • on DS70596A-page 48-27, Example 48-2

  • equation for PLL is (for M=43, N1=2, N2=2): f_osc = f_in * M / N1 / N2 = 7.36 * 43 / 2 / 2 = 79.12 MHz, or:

    $$ f_{osc} = f_{in} \times \frac{ M }{ N1 \times N2 } = f_{in} \times M / N1 / N2 = 7.36 \times 43 / 2 / 2 = 79.12 \,\text{MHz} $$

  • so we are a bit under wanted 80.00 MHz, because using higher M=44 would yield 80.96 MHz, which is overclocked over maximum 80.00 MHz.

Tutorial 3 - Using OC PWM as DAC

Here we will use Output Compare 1 (OC1) module (standard since 8-bit PIC) for "slow" PWM (we will later use MC motor control PWM for much faster results). PWM is set to fixed 25% duty - to verify that it works as expected. PWM Resolution is 8-bit limited by TMR2 period which is set to 256.

Existing features (from previous tutorial):

  • PIN10 CLKO - should de fcy = 79.12/2 MHz = 39.56 MHz
  • PIN2 RA0 - LED blinking with rate 200ms (=5Hz), toggle rate 100ms (10 Hz)
  • PIN4 & 5 = reserved for programming debugging

Additional features:

  • PIN3 Timer2 overflow, Toggle fcy/256 = 39.56/256 = 154.53 kHz, Frequency = 77.27 kHz Timer2 (TMR2) is used as time base for Output Compare 1 (OC1) PWM
  • PIN6 RB2,RP2: OC1 - PWM with frequency 154.53 kHz, and Fixed duty 25%

WARNING! Measured values are slightly lower:

  • expected TMR2 freq 77.27 kHz
  • got 77.024 kHz
  • error: -0.3% Which corresponds to clock error (see previous tutorial)

Here are measurements using Digilent Analog Discovery 2 scope and software WaveForms v3.20.1:

Tutorial 3 - OC1 PWM 25%

And here is workspace file: (dsPIC33FJ_tut03.X/digilent-ad2/dsPIC33FJ-TUT03.dwf3work](dsPIC33FJ_tut03.X/digilent-ad2/dsPIC33FJ-TUT03.dwf3work]) for WaveForms software.

Tutorial 4 - Using OC PWM and RC filter

In this stage we will generate PWM signal chainsaw /|/|/| and we will use simple low-pass RC filter to convert PWM pulses to Voltage levels:

                    4700 Ohm
                  +-------+
PIN6 PWM OC1  >---|   R   |---+---- Analog Output: /|/|/|
                  +-------+   |
                             === C=10 nF
                              |
                             GND

Please see https://ww1.microchip.com/downloads/en/AppNotes/00538c.pdf (However I used R=4k7 instead of R=4k because lack of suitable parts).

RC=1/(2*PI*f)

f*RC = 1/(2*PI)

f [Hz] = 1/(2*PI*R*C)

f [Hz] = 1/(2*3.14*4700* 1.0E-8) = 1/ ( 2 * 3.14 * 4.7 * 1.0E-5 ) =
f [kHz] = 1 / (2 * 3.14 * 4.7 * 1.0E-2) = 1 / 2 * 3.14 * 4.7 * 0.01 ) = 
f_cutoff = 3.388 kHz

Chainsaw frequency is TMR2 frequency / PWM period = 154.53 / 256 = 603.64 Hz

Please note that down slope is not perfect because capacitor takes some time to discharge. However it can't too low otherwise it will not flatten PWM pulses well. It is always compromise.

PWM Chainsaw output

Workspace for Digilent WaveForms software: dsPIC33FJ_tut04.X/digilent-ad2/dsPIC33FJ-TUT04-chainsaw.dwf3work

Below are future tutorials - work in progress

Tutorial 4+ - requisites for ADC and PWM as DAC

For real DSP example we need signal input and output.

For input we need:

For output we need:

In both cases we may need FilterLab program:

Example for both at speech processing sample frequency (fs = 8kHz) is on

Known traps

  • MCC Tool does not support dsPIC33FJ series (yes, that's it). So we have to code manually - however we can sometime reuse MCC generated code for similar CPU.
  • PDIP version of dsPIC33FJ does NOT include 2 channel Audio DAC (please see feature matrix on datasheet DS70291G-page 2 - it is really that). So we will use PWM + RC filter in future as poor-man DAC.

ADC Resources

Most important - MPLAB starter kit for dsPIC (I don't have one, but there are important docs and code examples):

More theory on ADC and anti-aliasing (to prevent high frequency to appear as parasitic low frequency - aliasing, there is needed low-pass filter):

From above User Guide (51700B.pdf) we know that on ADC input there is

This sixth-order Sallen-Key low-pass filter has a cut-off frequency of 3300 Hz. The output of the anti-aliasing filter is connected to input AN0 of the ADC module on the device.

From source https://ww1.microchip.com/downloads/en/DeviceDoc/SASK%20Record%20Play%20Demo%20With%20Intro.zip We can find:

// SASK Record Play Demo With Intro/h/ADCChannelDrv.h
#define ADC_CHANNEL_FCY	40000000
#define ADC_FSAMP	8000	/* Sampling Frequency	*/

#define ADCON1VAL 0x0744  /* 12 bit ADC with signed fractional format
		     	  * Triggered by Timer 3 and auto start 
		   	  * sampling after conversion complete. */
#define ADCON2VAL 0x0000  /* AVdd and AVss voltage reference, 
		   	  * use channel 0 with no scan	*/
#define ADCON3VAL 0x0010  /* Tad is 16 Tcy				*/     
#define ADCHSVAL  0x0000  /* AN0 input on channel 0		*/	  
#define ADPCFGVAL 0xFFFE  /* AN0 input is Analog			*/
#define ADCSSLVAL 0x0000  /* No channel scanning			*/

// src/ADCChannelDrv.c
void ADCChannelInit	(ADCChannelHandle * pHandle,int * pBufferInDMA)
{
// ...
	AD1CON1 	= ADCON1VAL;    /* Load the ADC registers with  value 	*/
	AD1CON2 	= ADCON2VAL;    /* specified in 12bitADCDriver.h	*/
	AD1CON3 	= ADCON3VAL;
	AD1CHS0 	= ADCHSVAL;
	AD1PCFGLbits.PCFG0 = 0;
	AD1CSSL 	= ADCSSLVAL;

	TMR3 		= 0;
	PR3		= (ADC_CHANNEL_FCY/ADC_FSAMP) - 1;
}

So filter cut-off is 3_300 Hz (3 kHz) and ADC sample rate is 8_000 Hz (8 kHz)

PWM DAC Resources

Related application notes:

From above User Guide (51700B.pdf) we know that on PWM output there is connected low-pass filter:

The PWM signal from the Output Compare module on the dsPIC33FJ256GP506 device on the board is demodulated by the PWM low-pass filter (A4). This fourth-order filter uses two Op amps (U8:A and U8:B) on the MCP6022 quad Op amp IC.

But there are no more details (for example one important - cut-off frequency)

From source we can find:

// SASK Record Play Demo With Intro/h/OCPWMDrv.h
#define OCPWM_FCY	40000000
#define OCPWM_FPWM	64000
#define OCPWM_FSAMP	8000	/* Sampling Frequency	*/
#define OCPWM_FPWM_FSAMP_RATIO 	(OCPWM_FPWM/OCPWM_FSAMP)
#define OCPWM_TMRPRESCALE 			1
#define OCPWM_MAX_PWM_PERIOD	(((OCPWM_FCY/OCPWM_FPWM)*OCPWM_TMRPRESCALE) - 1)

// c

void OCPWMStart	(OCPWMHandle * pHandle)
{
//...
PR2	= OCPWM_MAX_PWM_PERIOD;		/* PWM Period			*/
OC1RS	= ((OCPWM_MAX_PWM_PERIOD)/2);	/* Initial Duty Cycle at 50% 	*/
OC1R	= ((OCPWM_MAX_PWM_PERIOD)/2);
OC1CON	= OCPWM_OCCON_WORD;		/* Turn module on		*/
// ...
}

// how is Duty cycle computed
#define OCPWM_HIGHEST_INPUT_VALUE	32767
#define OCPWM_LOWEST_INPUT_VALUE	-32768
#define OCPWM_INPUT_RANGE	(OCPWM_HIGHEST_INPUT_VALUE - (OCPWM_LOWEST_INPUT_VALUE))

unsigned int sample;
unsigned int pwmDutyCycle;

	/* Compute Duty cycle values for every input sample	*/
	sample = data[i] - (OCPWM_LOWEST_INPUT_VALUE);
	pwmDutyCycle = ((sample * OCPWM_MAX_PWM_PERIOD))/OCPWM_INPUT_RANGE;
	// the pwmDutyCycle has enforced boundaries:
	// 	minimum pwmDutyCycle = 1
	// 	maximum pwmDutyCycle =  OCPWM_MAX_PWM_PERIOD - 1

So it seems that PWM period (or freq) is 64 000 Hz (64 kHz). So for each sample (at 8 kHz) there are 8 PWM periods on output.

Resources

Very good study material - how to do audio + DMA and also option so use PWM + RC filter as DAC:

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Gradual dsPIC33FJ (Microstick II board) DSP tutorial - from blinking LED to real-time DSP

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