DDS (generating sine waves) with onboard DAC using latest PIC 16F chips?

[HankMcSpank]

Just wondering if anyone has dabbled with the DAC aspect on the latest range of PICS.

IFor example a PIC16F1824.....I'm figuring that it having a DAC onboard is going to be a significant win over say using a PWM & low pass filter?

My goal is to generate sine waves to a reasonable level of accuracy up to about 3Khz. (ie plus/minus 0.3Hz)

Never having dabbled with PIC DACS previously (or any DAC!), would anyone have any example code or setups?

[amgen]

it looks like that so-called dac is just the 5-bit Vref part.
I don't know why they won't put a 10 bit dac in a pic, there is just about everything else available in the pics.

waiting,
don

[HenrikOlsson]

Hi Hank,
I've never played with them but I'd say it depends on what kind of resolution you need as the DAC in the 16F1824 only has 5 bits of it. This means that you will only be able to get 32 discrete voltage levels which may, or may not, be enough.

/Henrik.

Edit: Don beat me to it, I was reading the datasheet while writing.

[HankMcSpank]

Hi Hank,
I've never played with them but I'd say it depends on what kind of resolution you need as the DAC in the 16F1824 only has 5 bits of it. This means that you will only be able to get 32 discrete voltage levels which may, or may not, be enough.

/Henrik.

Edit: Don beat me to it, I was reading the datasheet while writing.

Actually 5 bits would probably ok for my needs, but I need to be able to resolve the frequency to 0.1% (eg plus or minus 1Hz @1khz & so on)

I can see now that is's a very very basic DAC (just a 32 step pot really)...hey, ho.

Thanks for your input though.

[ronsimpson]

The DAC is very high impedance. You will need a buffer. Normally I use a 2 pole low pass filter to remove the digital noise.

[Mike, K8LH]

Hi Hank,

Not exactly what you asked for, but... have you considered using the PWM module with a low pass filter on the CCP1 output? You may need to amplify the signal as well.

I was toying with this idea a while back for a CTCSS Access Tone Generator but I never got around to building and testing it.

This example (free/lite version of BoostC) uses a 12F683 and an 8.388608-MHz crystal to generate CTCSS sine wave tones spanning 100 to 250-Hz with 0.000625-Hz frequency resolution. If you were to use a 12F1822 with that crystal and 4xPLL, or any other device with 4xPLL, you could bump the upper frequency range to about 1000-Hz with 0.0025-Hz frequency resolution.

Food for thought.

Cheerful regards, Mike McLaren, K8LH


PS: A quick search came up with this interesting old thread; Generating tones with decimals using PWM

/********************************************************************
 *                                                                  *
 *  Project: CTCSS 12F683                                           *
 *   Source: CTCSS_12F683.c                                         *
 *   Author: Mike McLaren, K8LH                                     *
 *     Date: 26-Nov-10                                              *
 *  Revised: 26-Nov-10                                              *
 *                                                                  *
 *  untested 12F683 DDS(PWM) CTCSS Tone Generator with 24-bit       *
 *  Phase Accumulator (Fosc = 8.388608-MHz)                         *
 *                                                                  *
 *      IDE: MPLAB 8.56 (tabs = 4)                                  *
 *     Lang: SourceBoost BoostC v7.01, Lite/Free version            *
 *                                                                  *
 ********************************************************************
 *
 *    CTCSS tone frequencies
 *
 *     67.0 Hz    69.3 Hz    71.9 Hz    74.4 Hz    77.0 Hz
 *     79.7 Hz    82.5 Hz    85.4 Hz    88.5 Hz    91.5 Hz
 *     94.8 Hz    97.4 Hz   100.0 Hz   103.5 Hz   107.2 Hz
 *    110.9 Hz   114.8 Hz   118.8 Hz   123.0 Hz   127.3 Hz
 *    131.8 Hz   136.5 Hz   141.3 Hz   146.2 Hz   151.4 Hz
 *    156.7 Hz   159.8 Hz   162.2 Hz   165.5 Hz   167.9 Hz
 *    171.3 Hz   173.8 Hz   177.3 Hz   179.9 Hz   183.5 Hz
 *    186.2 Hz   189.9 Hz   192.8 Hz   196.6 Hz   199.5 Hz
 *    203.5 Hz   206.5 Hz   210.7 Hz   218.1 Hz   225.7 Hz
 *    229.1 Hz   233.6 Hz   241.8 Hz   250.3 Hz   254.1 Hz
 *
 */

#include system.h

#pragma DATA _CONFIG, _MCLRE_OFF, _WDT_OFF, _HS_OSC

#pragma CLOCK_FREQ 8388608      // using an 8.388608-MHz crystal

//--< function prototypes >------------------------------------------
//--< type definitions >---------------------------------------------

typedef unsigned char u08;
typedef unsigned int u16;
typedef unsigned long u32;

#define r08 rom char*

//--< variables >----------------------------------------------------

u32 accum;                  // phase accumulator
u32 phase;                  // phase offset (DDS tuning word)

r08 sinetbl =   { 100,102,104,107,109,112,114,117,119,121,
                  124,126,129,131,133,135,138,140,142,144,
                  147,149,151,153,155,157,159,161,163,165,
                  167,168,170,172,174,175,177,178,180,181,
                  183,184,185,187,188,189,190,191,192,193,
                  194,194,195,196,197,197,198,198,198,199,
                  199,199,199,199,200,199,199,199,199,199,
                  198,198,198,197,197,196,195,194,194,193,
                  192,191,190,189,188,187,185,184,183,181,
                  180,178,177,175,174,172,170,168,167,165,
                  163,161,159,157,155,153,151,149,147,144,
                  142,140,138,135,133,131,129,126,124,121,
                  119,117,114,112,109,107,104,102, 99, 97,
                   95, 92, 90, 87, 85, 82, 80, 78, 75, 73,
                   70, 68, 66, 64, 61, 59, 57, 55, 52, 50,
                   48, 46, 44, 42, 40, 38, 36, 34, 32, 31,
                   29, 27, 25, 24, 22, 21, 19, 18, 16, 15,
                   14, 12, 11, 10,  9,  8,  7,  6,  5,  5,
                    4,  3,  2,  2,  1,  1,  1,  0,  0,  0,
                    0,  0,  0,  0,  0,  0,  0,  0,  1,  1,
                    1,  2,  2,  3,  4,  5,  5,  6,  7,  8,
                    9, 10, 11, 12, 14, 15, 16, 18, 19, 21,
                   22, 24, 25, 27, 29, 31, 32, 34, 36, 38,
                   40, 42, 44, 46, 48, 50, 52, 55, 57, 59,
                   61, 64, 66, 68, 70, 73, 75, 78, 80, 82,
                   85, 87, 90, 92, 95, 97 };

//--< defines >------------------------------------------------------

//--< isr >----------------------------------------------------------

/*
 *  12F/16F1xxx DDS-PWM CTCSS "Access Tone Generator" Notes
 *  =======================================================
 *
 *  using an 8.388608 MHz crystal
 *
 *    Tcy = 1 / 8388608 * 4 = 476.837158203125 nsecs
 *
 *  using a 200 cycle PWM period provides a DDS frequency of
 *
 *    Fdds = 1 / (200 Tcy) = 10,485.76 Hz
 *
 *  frequency resolution using a 24 bit phase accumulator is
 *
 *    Fres = Fdds / 2^24
 *    Fres = 10485.76 / 16777216 = 0.000625 Hz
 *
 *  dds tuning word (phase offset) can be calculated a couple
 *  different ways.  since the dds frequency resolution (Fres)
 *  is basically 0.01-Hz divided by 16, you can calculate the
 *  phase offset by mulitplying desired Fout output frequency
 *  by 1600.  the phase offset for an Fout of 254.1-Hz is;
 *
 *    phase = (Fout  * 100) * 16
 *          = (254.1 * 100) * 16 = 406560
 *
 *    phase =  25410 << 4 = 406560
 *
 *  or you can also calculate phase offset like this (yuch!);
 *
 *    phase = Fout / Fres
 *          = 254.1 / 0.000625 = 406560
 *
 *    phase = Fout * 2^24 / Fdds
 *          = 254.1 * 16777216 / 10485.76 = 406560
 *
 *  the highest CTCSS frequency (254.1 Hz) will produce the
 *  smallest number of sine wave D/A output points per cycle;
 *
 *    INT(10485.76 / 254.1) = 41 output points per cycle
 *
 *  use the most significant 8 bits of the 24-bit phase
 *  accumulator as the sine table index.
 *
 */

void interrupt()                // 200-cycles (10485.76-Hz)
{ pir1.TMR2IF = 0;              // clear TMR2 interrupt flag
  accum += phase;               // add phase offset to accum
  ccpr1l = sinetbl[accum>>16];  // sine duty cycle value for
}                               // next PWM period

//--< main >---------------------------------------------------------

void main()
{
  cmcon0 = 7;                   // comparator off, digital I/O
  ansel = 0;                    // a2d module off, digital I/O
  trisio = 0b00111011;          // GP2 output, all others input
 /*
  *  setup PWM module for 200 cycle interrupts (10485.76-Hz using
  *  an 8.388608-MHz clock)
  *
  */
  ccp1con = 0b00001100;         // 00001100
                                // --00---- DC1B<1:0>, duty cycle b1:b0
                                // ----1100 CCP1M<3:0>, pwm mode
  t2con = 0b00000000;           // 00000000
                                // -0000--- TOUTPS<3:0>, postscale 1
                                // -----0-- TMR2ON, timer off
                                // ------00 T2CKPS<1:0>, prescale 1
                                // for 476.837158203125 nsec 'ticks'
  pr2 = 200-1;                  // for 95.367431640625 usec interrupts
  ccpr1l = 0;                   // 0% duty cycle
  pie1.TMR2IE = 1;              // enable Timer 2 interrupts
  pir1 = 0;                     // clear all peripheral interrupt flags
  intcon.PEIE = 1;              // enable peripheral interrupts
  intcon.GIE = 1;               // enable global interrupts
  t2con.TMR2ON = 1;             // turn TMR2 on

  phase = 25410 << 4;           // phase offset for 254.1-Hz tone

  while(1);                     // loop forever
}

[HankMcSpank]

Hi Mike,

yes I've done that....I just got a bit over excited at the prospect of an onboard DAC, but I now realise it's pretty poor....looks as if it's meant to generate a ref voltage, but being high impedance, I'd be better off using two resistors and an opamp!

[ScaleRobotics]

Im with you Hank, I want to see what you can make it do!

[HankMcSpank]

Well I did use a $1 MCU to generate that sine wave. So by your logic, that would be the right tool for the job.

Apologies...I misread you...if you paid a $1 (vs $15 for a DDS IC) & all you wanted was a 20Khz sine wave or less, then yes, by my logic that would be the right tool to do the job! (even if you had to dedicate the PIC to nothing else, due to the burden placed on it). They sell Aston Martins here in London...but I wouldn't buy one to go to the corner shop - my bike will do!

After just a short bit of dabbling, I've approximated a sine wave using DDS principles & the 16f1828's meagre inbuilt 5 bit DAC & a modest 16 bit accumulator (& no filtering cap),



Sure, the tops are a little pointy (16 levels per swing isn't much!) & I doubt it'll win any distortion/noise figure awards, but if I can squeeze another 2 bits resolution by getting creative with the FVR, then that'll probably do me. If not, I'll just go the 10 bit PWM method....but that method uses a filtering cap, so the amplitude will change with frequency - I was really after a fixed output amplitude no matter what the output frequency (& without going the R2R ladder way)

[rmteo]

Actually, the CPU load is zero while that $1 microcontroller is generating the 20KHz sine wave. You can just as easily generate any frequency up to about 250KHz with that device. Here is staircase waveform on the same MCU - great for doing things like testing power supplies, etc.

[ScaleRobotics]

Which PIC16F chip are you using rmteo?

[rmteo]

Which PIC16F chip are you using rmteo?

I said it was a a $1 device - didn't say it was a PIC16.

[ScaleRobotics]

I said it was a a $1 device - didn't say it was a PIC16.

Oh, it must be one of those new PIC12F's or maybe the PIC18F's? Because those are the only ones compatible with PBP, on the top bar of the forum.....

[rmteo]

Oh, it must be one of those new PIC12F's or maybe the PIC18F's? Because those are the only ones compatible with PBP, on the top bar of the forum.....

I doubt it.

[ScaleRobotics]

rmteo,

It is interesting that 99.9% of the posters here talk about .... well the topic of the forum ... Pic Basic Pro. Your posts are often misleading, because newer users assume you are talking on-topic, rather than about some 16 or 32 bit chip which can not be programmed in PBP. If you are going to give advice that 100% of our users didn't ask for, and 95% of our users can't use, you should probably mention that your solution can't be used with the tools which this forum is based on.

[cncmachineguy]

even if you had to dedicate the PIC to nothing else, due to the burden placed on it

Hank, this is a great idea. Seems like you should be able to do it with any basic device if this is all you want it to do. The comm's to it will be the only thing to worry with. So lets be clear what you want here -
1Hz-5KHz sine wave
fixed amplitude
A way to get its attention to change the freq.

Do I have this correct?

just remembered one thing, did you say NO passive devices?

[HankMcSpank]

Hi Bert,

Seems like you should be able to do it with any basic device

Agreed...even though I'm using a 16f1828 to practise with, my favourite low chip end of the moment is an 8 pin 12f1822. So a simple DDS program + servicing an odd interrupt (for frequency change)...should be a walk in the park. I also think even a low end 12f chip is going to be much more capable of outputting more than 5Khz...but I mentioned that frequency, only because it's all I need.

1Hz-5KHz sine wave
fixed amplitude
A way to get its attention to change the freq.

Do I have this correct?

Correct, I would add, granularity to 0.01Hz or better (this is perhaps the most important requirement for me). That's where the larger accumulator is needed
(see my other post - http://www.picbasic.co.uk/forum/show...887#post106887 )

Without want to teach granny to suck eggs, the output frequency granularity is established from this formula...

interrupt frequency/ accumulator

So for an interrupt rate of say 20khz & a 16 bit accumulator, it'd be

20,000/65536 = 0.30Hz granularity which is insufficient for my needs, which means either making the accumulator larger, or the sample rate smaller....I don't want to do the latter, hence wanting to make the accumulator larger, but I then start hitting my knowledge boundary dealing in 17 bit+ numbers witin the confines of picbasic.

Also, there's likely to be a fair degree of floating point...which I'll have to work around. For example to set the required freqency, it's

(required frequency/sinterrupt rate)*accumulator , so for 1500Hz

(1500Hz/20,000) * 65536 ........clearly the first part of that equation is going to result in a decimal, so quite a few things impeding me at the moment!

re no passive devices ...well, ideally no reactive passive devices (on account they alter the amplitude as the frequency changes)....using an R2R ladder would be cool, except the take up a lot of space & that's something I always try to avoid (I can't seem to locate anyone that sells prefabbed R2R ladders here in the UK)

[cncmachineguy]

On the passives, I was thinking about the fact we can only output 2 levels, Vcc and Vdd, so the "sine" wave won't be. I have found R2R sips before, but that would also require enough I/O to service it in parallel.

What do we do about the not analog output?

[HankMcSpank]

I think we're at crossed wires a little.

My first (present) plan of attack is to use the 5 bit internal DAC to get an anlogue output... (that's what my screenscrape was earlier - using the PIC's internal DAC to get a rough analogue output) clearly 32 steps is insufficent for sine waves, so I was trying to add an extra 2 bits by creatively using the PIC Fixed voltage reference (the PIC FVR output canfeed the DAC and the voltagae output can be set to 1.024V, 2.048V or 4.096V) - I might not be grasping a fundamental issue here, so I need to ponder this one until I have an "Oops, that was an embarrassing idea" moment!

If the above comes to nothing then I'll relent & use HPWM and a filter (I only had a brief go at this a year or so ago & didn't actually test how much the reactance of the cap in such an arrangement impacts the amplitude...over the range I'm interested (0-5khz) it mght not be that much. I'm of the opinion that I'll end up having to go this way...then all I have to crack is the accumulator problem (need more than 16 bits) & the little bit of floating point (to get the accumulator value needed for the required frequency)

Clearly if I end up going the R2R route, then I'll need to bring a 14 pin PIC into service (likely to be a 16f1824)