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[on:]

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

The DAC IC that I have available is the AD669;

http://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.

[Reply #1 on:]

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.

[Reply #2 on:]

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

[Reply #3 on:]

You could use a couple of 595 shift registers:

http://www.gammon.com.au/forum/?id=11518

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

[Reply #4 on:]

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?

[Reply #5 on:]

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.

[Reply #6 on:]

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

[Reply #7 on:]

Some of these are even bi-directional:
http://www.futurlec.com/cgi-bin/search/search.cgi?search=shift_register&search_base=0&page_no=1

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!

[Reply #8 on:]

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);

[Reply #9 on:]

Here's you pin map:
http://arduino.cc/en/Hacking/PinMappingSAM3X

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.

[Reply #10 on:]

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

[Reply #11 on:]

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!

[Reply #12 on:]

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.

[Reply #13 on:]

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.

[Reply #14 on:]

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);