DAC support

Hey there,

I tried the DACC on the Arduino Due, but it doesn't work as expected. I used the function analogWrite() for setting a new DAC value. The following code will work as expected:

byte value;
void loop() {
  analogWrite(DAC0, value++);
}

Same applies if you replace DAC0 with DAC1. For some unknown reason, I couldn't get both DACs to work simultaneously. The following code does not work:

byte value;
void loop() {
  analogWrite(DAC0, value++);
  analogWrite(DAC1, value * 2);
}

I think it has something to do with the code in hardware/arduino/sam/cores/arduino/wiring_analog.c. In the function analogWrite() there is the following lines:

 // Write user value
 ulValue = mapResolution(ulValue, _writeResolution, DACC_RESOLUTION);
 dacc_write_conversion_data(DACC_INTERFACE, ulValue);

I'm missing the channel selection before writing conversion data. Did someone else use the DACs before with analogWrite()? Is the reported behaviour bug or feature?

mfg, Stefan.

smay4finger,

I fear that you’re right, the analogWrite on DACs didn’t work if you write on both DACs.
I don’t know why, but seems that nobody (including me) ever dreamed to test BOTH channels together.

I’ll put the fix to this in the to-do list, before the next beta release 1.5.1 (that is very close).

C

Fix was trivial. Please find attached the patch for wiring_analog.c. Should I make bug report?

mfg, Stefan.

wiring_analog.c (9.62 KB)

patch (1.65 KB)

smay4finger,

thank for the patch I'm testing it in the next hours, in case may you open a pull request on github so i can merge it?

C

Hi,
I tested your patch and I think it works!

I attach a sketch that generates a sine wave on both the DAC outputs phase shifted of pi. To prove I add the screen capture of the oscilloscope.

+1 Karma is deserved :slight_smile:

DAC_Sine_Due.ino (2.63 KB)

DS0003.BMP (16.6 KB)

I made a bug report here: http://code.google.com/p/arduino/issues/detail?id=1092

Please use the patch attached to the ticket. There is also a bugfix regarding corruption of the data register, when writing too fast.

Done, thank you.

C

“I tried the DACC on the Arduino Due, but it doesn’t work as expected. I used the function analogWrite() for setting a new DAC value. The following code will work as expected:”

byte value;
void loop() {
  analogWrite(DAC0, value++);
}

Hello, when i test the DAC on the DUE i measured an Outputvoltage from 0,55V (value 0) to 2,77V (value 255). I thought the DAC has a range from 0V to 3,3V?! Do anybody knows more or can help me?

greets
parasound

"Hello, when i test the DAC on the DUE i measured an Outputvoltage from 0,55V (value 0) to 2,77V (value 255). I thought the DAC has a range from 0V to 3,3V?! Do anybody knows more or can help me?"

Hi all! I would be interested in this too. Could not figure out yet why DAC range ist approx. 0,54V - 2.74V (both channels). andy_G

Hi parasound and andy_g,

I could find no information about the DAC range in the SAM3X datasheet. But on page 1059 of SAM3*S* datasheet, it is said that the voltage range of the DAC is from (1/6) x VADVREF to (5/6) x VADVREF. So I suspect, the same applies to SAM3U, and range is 1/6 x 3.3V = 0.55V to 5/6 x 3.3V = 2.75V, which is verified by your measures.

wibauxl

Ok. Thank you wibauxl.

wibauxl: Hi parasound and andy_g,

I could find no information about the DAC range in the SAM3X datasheet. But on page 1059 of SAM3*S* datasheet, it is said that the voltage range of the DAC is from (1/6) x VADVREF to (5/6) x VADVREF. So I suspect, the same applies to SAM3U, and range is 1/6 x 3.3V = 0.55V to 5/6 x 3.3V = 2.75V, which is verified by your measures.

wibauxl

That seems pretty limiting. You mean one can't measure output signals between below .55 volt? There must be more to the subject then that?

Lefty

@retrolefty

We're talking about the DAC here. The ADC works at voltages below 0.55v. As far as the DAC is concerned it's a pity to have such limits but I guess for audio applications there are way to circumvent the issue.

m

[quote author=Massimo Banzi link=topic=129765.msg993253#msg993253 date=1352733776] @retrolefty

We're talking about the DAC here. The ADC works at voltages below 0.55v. As far as the DAC is concerned it's a pity to have such limits but I guess for audio applications there are way to circumvent the issue.

m [/quote]

Well I don't yet own a Due and not up to the task just yet of taking on the huge datasheet for this uP, but they must talk about this range restriction if indeed it's a hardware based restriction. I just find it very hard to except at this time that there is a legitimate reason for this. I guess it could be compensated for with external op-amp with a -.55v offset, but still there just must be more to this subject then 'that's just the way it is'? It's really not a problem for audio type applications as one could just capacitance couple the signal, but for precision DC DAC applications, not being able to reach ground potential is a pretty big deal, no?

Lefty

...what would also make sense resolution-wise: LSB seems to be 0.5mV (measured) which is in line with wibauxl's theory ((2.75-0.55)/4096=0.537mV). This DAC behaviour -and te lack of it's documentation- is a little odd to my opinion, but at least there's an explanation that makes sense.

MANY THANKS FOR ALL YOUR INPUTS!!

andy_g

andy_g

The Due is still in beta, there is still work to be done on the documentation.

m

wibauxl: I could find no information about the DAC range in the SAM3X datasheet. But on page 1059 of SAM3*S* datasheet, it is said that the voltage range of the DAC is from (1/6) x VADVREF to (5/6) x VADVREF. So I suspect, the same applies to SAM3U, and range is 1/6 x 3.3V = 0.55V to 5/6 x 3.3V = 2.75V, which is verified by your measures.

I've just checked with Atmel support and they confirm that SAM3S and SAM3X features the same DAC IP, what you have measured is correct.

[quote author=Massimo Banzi link=topic=129765.msg993253#msg993253 date=1352733776] As far as the DAC is concerned it's a pity to have such limits but I guess for audio applications there are way to circumvent the issue. [/quote]

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