16bit DAC parallel bits, AD669

I will be implementing a single channel DAC in my current/upcoming project.

The DAC IC that I have available is the AD669;

ftp://ftp.signalogic.com/data_sheets/AD_DA_Converters/AD669.pdf

It wants to receive a 16-bit parallel input. Fine by me, but what is the best/fastest way to do that? I can hook it up to 16 digital outputs on my Arduino Due, no problem, but maybe there are smarter options.

It would take 19 pins to drive the DAC. I need a lot of pins for the ADC's too, but I think it would be acceptable.

However, if I can use a shift register without adding "loop-time", that would be fine too.

Application_AD669.png

Why not use a serial DAC instead? I'm pretty sure you can get some reasonably fast single channel ones and you might find it easier to implement. If you can't find one in a package you like then there's always the option of a breakout board.

you might use a couple of shift registers - http://arduino.cc/en/Tutorial/ShiftOut - to control it with 3 lines.

You could use a couple of 595 shift registers:

But by the time you have bought those you may as well buy an SPI DAC chip.

if I can use a shift register without adding "loop-time", that would be fine too.

What do you mean by loop time?

You an always use a MCP23S17, this is an SPI 16 bit port expander, that can run quite fast. How fast do you need?

If you use shift registers it will be a bit slower than writing 2 8-bit ports. But what do you need for speed? The difference is a small fraction of a millisecond.

I withdraw my suggestion about the 595 register, that is an output one. But something like the 74HC165 will do input.

Some of these are even bi-directional:

I don't know those people but I have bought from them a few times and never a problem with an order. Obviously though, you don't just buy 1 or 2 50-cent chips on a $4+ shipping charge. But they have component bags and all kinds of goodies. The prices are close enough to eBay for me and these guys do have datasheets so I know what I'm ordering, not just hoping it is what it seems.

(ships from Hong Kong, there's a how-long table)

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Hmmmm, after the USPS charges thread I have to wonder if the site needs updating or they subsidize the difference. And heyyy, ship to Down-Under is the same as to the US!

Ok, I think I got it up and running. I now need to write some code to transform a value of desire to 16 parallel bits. I have enough digital output pin available so I would like to use just the 16 output pins for the data.

Any suggestions?

I have looked at bitRead. That seems to work but puts out a value of 1 or 0 while ideally, I want a LOW or HIGH.

for instance, for bit 0, I would like to output;

I did not expect this to work but it does;

  • digitalWrite(2,bitRead(8,3));
    delay(2000);
    digitalWrite(2,bitRead(8,2));
    delay(2000);

Here's you pin map:

You have Ports A, B, C, D with pins as SAM3X pin names PA##, PB##, etc.
If you can get 16 pins in a row on the same port then you should be able to write 16 bits to that port in one go. Once the bits are written however you do it, the pins should reflect what you wrote. It's a lot quicker than setting 16 pins 1 at a time.

I can hook it up to 16 digital outputs on my Arduino Due, no problem, but maybe there are smarter options

I read the entire thread before I realised you're using a Due, just hook up 16 (or 19) pins and learn how to directly write to a port. Job done, there's no "better" way unless you're desperate to save pins for another purpose.


Rob

That seems to work but puts out a value of 1 or 0 while ideally, I want a LOW or HIGH.

So what is the difference between a 1 and a high?
I will tell you, nothing at all.
See it is ideal!

Grumpy_Mike:

That seems to work but puts out a value of 1 or 0 while ideally, I want a LOW or HIGH.

So what is the difference between a 1 and a high?
I will tell you, nothing at all.
See it is ideal!

Once here I was warned that TRUE/FALSE and HIGH/LOW might some day not be 1/0 so my code should not assume that and won't be compatible. I had to LOL since hey, I should live so long! But then that person advocates using C++ Strings on UNO's and not being hardware-centered.

Ok, I got it working in "slow"-mode.
I indeed put each individual pin either High or Low. It is more of a proof-of concept than it is the end result. The loop now takes as much a 80-100uS with a analogread and normal digitalwrites (16 of them)
Now I know what kind of signals are required to use the DAC, I will try to use one of the ports for a shorter looptime.

I will try to use portC to driving the DAC.

"So what is the difference between a 1 and a high?"
In the digitalWrite help it says explicitly it want either HIGH or LOW so I was under the assumption that those two inputs were the only ones accepted.

More to come.

For the ADC I will be implementing I will use this code, which speeds up the digitalwrites and reads drastically. I will first try to implement the same type of bitsetting on the DAC and if that works nicely, keep it that way.

inline void digitalWriteDirect(int pin, boolean val){
if(val) g_APinDescription[pin].pPort -> PIO_SODR = g_APinDescription[pin].ulPin;
else g_APinDescription[pin].pPort -> PIO_CODR = g_APinDescription[pin].ulPin;
}

inline int digitalReadDirect(int pin){
return !!(g_APinDescription[pin].pPort -> PIO_PDSR & g_APinDescription[pin].ulPin);

Once here I was warned that TRUE/FALSE and HIGH/LOW might some day not be 1/0 so my code should not assume that and won't be compatible. I had to LOL since hey, I should live so long! But then that person advocates using C++ Strings on UNO's and not being hardware-centered.

Yes but two things, one that is rubbish, two he was talking about a chip outputting something, so that means a physical output. Lets see the guardians of C redefine a TTL output.

In the digitalWrite help it says explicitly it want either HIGH or LOW so I was under the assumption that those two inputs were the only ones accepted.

As an engineer once told be assumption is the mother of all **** ups.

For the ADC I will be implementing I will use this code,

Won't that only work with 8 bits?

I have not tried it yet with more than 4 pins. For the ADC I will read 1 byte per ADC. When I put "byte" high, the lowbyte and highbyte flip so per ADC I can read the 16 bits with "just" 8 inputs. Therefore I can use 2 ADC's and a DAC without running out of pins.

Grumpy_Mike:

Once here I was warned that TRUE/FALSE and HIGH/LOW might some day not be 1/0 so my code should not assume that and won't be compatible. I had to LOL since hey, I should live so long! But then that person advocates using C++ Strings on UNO's and not being hardware-centered.

Yes but two things, one that is rubbish, two he was talking about a chip outputting something, so that means a physical output. Lets see the guardians of C redefine a TTL output.

I'm not sure that person is a he but was talking about source code. I called it garbage too. But perhaps that person doesn't recognize the relation of bits to wire states as anything but incidental or thinks that code now should be compatible to some future age where binary logic is outdated and the crass obfuscation of using tricky operations like XOR will die with the ancient ones who lived close to the metal.
I've seen it in new compsci degree holders back since 87 but we're still here and LOW is still 0 and HIGH is still 1 without any sign of a big change, and XOR is still useful and used!

Ok, now the loop takes 12.5 us. So every 12.5us, a new value is put into the dac and outputted. I think I can reduce this period by smarter bitsetting, but I don't know how to do best. Ideally I want to stick to the pins I use now, 30-45, so not an unique port. I expect the for loop to take most of the time.

//Definieer digitalwrite en digitalread-direct

inline void digitalWriteDirect(int pin, boolean val){
  if(val) g_APinDescription[pin].pPort -> PIO_SODR = g_APinDescription[pin].ulPin;
  else    g_APinDescription[pin].pPort -> PIO_CODR = g_APinDescription[pin].ulPin;
}

inline int digitalReadDirect(int pin){
  return !!(g_APinDescription[pin].pPort -> PIO_PDSR & g_APinDescription[pin].ulPin);


}


int update = 8;

//trim DC output to 0, depends on resistor specs and board layout
int offset = 23;

//Halfway ideal value 2^16 / 2 + the offset adjustment
int zero = 32768+offset;

//2^16 / 20 volts is value per volt
float scale = 3276.8;

//DAC reference, what should the output be? In volts.
float input = 0.1; 

//Define the value which is send to the DAC, with scaling and offset.
int value = 0; //Value to DAQ;

void setup() {                
  
  
  //Define DAC PINS
  pinMode(update, OUTPUT);
  digitalWrite(update, LOW);
  pinMode(A0, INPUT);
  pinMode(30, OUTPUT);
  pinMode(31, OUTPUT);
  pinMode(32, OUTPUT);
  pinMode(33, OUTPUT);
  pinMode(34, OUTPUT);
  pinMode(35, OUTPUT);
  pinMode(36, OUTPUT);
  pinMode(37, OUTPUT);
  pinMode(38, OUTPUT);
  pinMode(39, OUTPUT);
  pinMode(40, OUTPUT);
  pinMode(41, OUTPUT);
  pinMode(42, OUTPUT);
  pinMode(43, OUTPUT);
  pinMode(44, OUTPUT);
  pinMode(45, OUTPUT);
                          
  
}

// the loop routine runs over and over again forever:
void loop() {

//Put value out to DAC

//Determine integer to DAC
value = round(zero+input*scale);

//Put DAC pins high  
for(int x = 30; x < 46; x++){digitalWriteDirect(x, bitRead(value,x-30)); }

//Update DAC output value
digitalWriteDirect(update, HIGH);
digitalWriteDirect(update, HIGH);
//Fix outpur value
digitalWriteDirect(update, LOW); 
    
}

Get rid of the floats and it will speed up.

scaling integers ---- scaled_value = value x scale_numerator / scale_denominator

don't forget to use an integer type that will hold intermediate results.