DAC support

Yes, for audio applications, if voltage range of the DAC is from (1/6) x VADVREF to (5/6) x VADVREF then you use a differential op-amp with the second input connected to (1/2) x VADVREF - though it would require a symmetric power supply. Or just use a DC blocking capacitor.

It is a surprising limitation for other uses, though.

Hello,
i'am experiencing the same problem as smay4finger described in original post, using this code:

void setup() {
  analogWriteResolution(12);
  analogWrite(DAC0, 4095);
  analogWrite(DAC1, 4095);
}
void loop(){}

however, this code set correct output values:

void setup() {
  analogWriteResolution(12);
  analogWrite(DAC0, 4095);
  analogWrite(DAC1, 4095);
  delay(2);
  analogWrite(DAC0, 2047);
  analogWrite(DAC1, 1023);
}

i suspect problem in this line in wiring_analog.c:

while ((dacc_get_interrupt_status(DACC_INTERFACE) & DACC_ISR_TXRDY) == 0);

but you guys know better.
Regards,Vladimir

@vdorr

Hi,
Are you using the amended code (Reply #2)?

Jim

@jgmdavies

Hi, i'm using Arduino 1.5.1 which allready contains this patch.

V.

@vdorr

OK, I'm using 1.5.1r2.

The following works as expected here, giving independent control of the two DACs:

        while (true)
        {
            analogWrite(DAC0, 240);
            delayMicroseconds(200);

            for (int i = 0; i < 8; i++)
            {
                analogWrite(DAC1, 240);
                delayMicroseconds(20);
                analogWrite(DAC1, 120);
                delayMicroseconds(80);
            }

            analogWrite(DAC0, 0);
            analogWrite(DAC1, 0);
            delay(9);
        }

Several people in this forum seem to have had trouble and have possibly blown their DAC output(s) by following an Audio example with a loudspeaker (which I haven't tried).

For anyone following with an 'early production' Due like me, please note that the constant 'DAC0' (66) corresponds to pin 'DAC2' on the board header, and 'DAC1' (67) corresponds to pin 'DAC1'.

Attached are screenshots from a Rigol scope. N.B. I'm using x10 probes.

HTH
Jim

aaa.bmp (146 KB)

bbb.bmp (146 KB)

@jgmdavies

i guess you are writing DAC in loop() but my (non-working) example was writing DAC in setup(), your code also has delay before writing second DAC channel, just like my second (working) example. In my opionion, there is bug in analogWrite(), related to initialization of DAC.
V.

Hi V,

The following works as expected here, with outputs as per the earlier scope 'screenshots':

void setup()
{
       while (true)
        {
            analogWrite(DAC0, 240);
            delayMicroseconds(200);

            for (int i = 0; i < 8; i++)
            {
                analogWrite(DAC1, 240);
                delayMicroseconds(20);
                analogWrite(DAC1, 120);
                delayMicroseconds(80);
            }

            analogWrite(DAC0, 0);
            analogWrite(DAC1, 0);
            delay(9);
        }
}

void loop()
{
}

What do you get?

Jim

Hi V,

And commenting out the initial delay like so:

void setup()
{
       while (true)
        {
            analogWrite(DAC0, 240);

            for (int i = 0; i < 8; i++)
            {
                analogWrite(DAC1, 240);
                delayMicroseconds(20);
                analogWrite(DAC1, 120);
                delayMicroseconds(80);
            }

            analogWrite(DAC0, 0);
            analogWrite(DAC1, 0);
            delay(9);
        }
}

void loop()
{
}

gives the outputs shown below.

Jim

ccc.bmp (146 KB)

@jgmdavies

I tried your code and from what i can tell (using Velleman HPS5), it works as expected, BUT it is equivalent to example from my first post which works as expected:

void setup()
{
analogWrite(DAC0, 255);
delayMicroseconds(200);
analogWrite(DAC1, 255);
}
void loop(){}

both outputs are at 2.8V, what is NOT working is this piece of code:

void setup()
{
analogWrite(DAC0, 255);
analogWrite(DAC1, 255);
}
void loop(){}

using this code DAC0 is at 0.54V and DAC1 at 2.8V
V.

@jgmdavies

Insertion of some delay between first two writes is perfectly acceptable workaround for me, but IMO this behaviour is bug and should be fixed. Am i crying on wrong place(forum)?

@vdorr

OK, I've caught up with you now!

And I agree, it seems to be a bug. The delay between the writes in start() needs to be at least 30 us on mine (using the default 8 bit resolution).

Jim

@jgmdavies

This modification seem to fix this buggy behaviour:

diff --git a/hardware/arduino/sam/cores/arduino/wiring_analog.c b/hardware/arduino/sam/cores/arduino/wiring_analog.c
index 58bd87a..97f50e8 100644
--- a/hardware/arduino/sam/cores/arduino/wiring_analog.c
+++ b/hardware/arduino/sam/cores/arduino/wiring_analog.c
@@ -243,8 +243,8 @@ void analogWrite(uint32_t ulPin, uint32_t ulValue) {
 
                        // Write user value
                        ulValue = mapResolution(ulValue, _writeResolution, DACC_RESOLUTION);
-                       while ((dacc_get_interrupt_status(DACC_INTERFACE) & DACC_ISR_TXRDY) == 0);
                        dacc_write_conversion_data(DACC_INTERFACE, ulValue);
+                       while ((dacc_get_interrupt_status(DACC_INTERFACE) & DACC_ISR_EOC) == 0);
                        return;
                }
        }

also with this change should desired value be on output pin by the time analogWrite() exits. Opinions?

V.

@vdorr

Looks good to me. I repeated the basic test in start() with no delays and now it's fine, and my other tests are still good.

I think the person to discuss this with is cmaglie in this forum.

Thanks,
Jim

@vdorr,

thank you!
If you can open a pull request it would be perfect, otherwise I'll apply the fix by myself.

@cmaglie

I believe i've just opened pull request for this issue.

V.

vdorr:
I believe i've just opened pull request for this issue.

Yes, this is the preferred way to submit patches, thanks.

Thanks for saving my nerves! :grin:

Hi Guys,

Is it possible to change the DAC resolution using the dacc_write_conversion_data(DACC_INTERFACE, value);?
It's always using 12 bits, even if I change using that command: analogWriteResolution(8);.
Is it seem the analogWriteResolution(8); only works when we write a value using analogWrite(DAC0, value);

Thanks

Hi. I'm trying to use the DAC from my due to make a square wave. After analogWrite(DAC0,value), the output voltage won't reach 0V again. I know that the voltage output will always be between 0.55 and 2.75V, but i really need 0V... There is a way to turn off the DAC0? I've tried digitalWrite(DAC0,LOW) with and without pinmode() function.

Best regards,
Jorge

How to get rid of the 0.55 Vdc from the DAC output---

You will have to waste an amplifier on this to do it right, otherwise a series capacitor will do the trick. I'll show you in text so everyone can see it:

----|(---- use a nice big capacitor; 47uf will pass just about all frequencies, 0.047pf will block all frequencies.

Better to use an amplifier, Freescale has one that works in front of a comparator; here's the link:
http://cache.freescale.com/files/microcontrollers/doc/app_note/AN3552.pdf

Best regards