HugoW:
That sounds amazing, but I do not understand it at all! The hardware I can just replicate, but the code it beyond me. After I do the code you posted in the void setup part, what would the void loop bit look like? Say I use a simple 0 - 5V (0-1023 bit) input signal over an analog in port (a pot hooked up)?
Cheers,
Hugo
What we are doing is this:
(1) Setup both of the ports we are using as outputs. The "DDR..." lines are equivalent to "pinMode ()" statements. (I'll explain WHY I didn't use pinMode a bit later).
(2) Take the 16 bit number and split it into two 8 pit "pieces".
(3) Send one piece to one port, send the other piece to the other port (which ends up outputting the voltage we want).
As you know, "pinMode" sets one particular BIT as an input or as an output. Since we are using a total of 16 pins to drive the R-2R parts, it would be a pain to do 16 "pinMode" commands. Doing it by using DDR sets all 8 bits of one port at once, so all I have to do is setup the first port and the second port, not 16 bits individually.
OK, now we have a 16 bit value that can be anything between 0 and 65535. The 65535 number is simply 2 "to the" 16'th power, which is the largest number you can make with 16 bits (all of you who are dying to say "it's 65536, not 65535" just zip it... don't want to complicate things now!)
The full range of the 16 bit value represents 0 volts to 5 volts (broken into 65536 steps!). So, for example, a value of "0" outputs 0 volts. 1/2 of 65535 (which is 32767) will output 1/2 of 5 volts, or 2.5 volts. You see? It's just a ratio... like the analog input. An analog input of 2.5 volts would give you around 1/2 of the max value which would be around 511 or 512.
Let's say we wrote a small "for" loop that did "for x = 0; x < 65536; x++;" and sent it to the DAC. On a scope, you would see a straight upward sloping line from 0 volts to 5 volts.
Now, why we split the value for the two ports. Seen as bits, "value" looks like this:
[b]15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0[/b]
...but each port is 8 bits, so we have to chop the 16 bits into 2 blocks of 8 bits, like this:
[b][color=blue][15 14 13 12 11 10 9 8][/color] [color=green][color=red][ 7 6 5 4 3 2 1 0[/color][/color][/b]]
Now, putting the bottom half (the red part) into an 8 bit port is easy... just put it in. But what about the top half? Even it's minimum value is bigger than 8 bits. So, what we do is "bit shift" it (that is, slide all the bits down, and if we slide it 8 bit places, the top half lines up the same as the bottom half. So, watch this:
[b][color=blue][15 14 13 12 11 10 9 8][/color] [color=green][color=red][ 7 6 5 4 3 2 1 0[/color][/color][/b]] ---> put low bits into low port
[color=red][b][color=blue]slide 8 bits to the right ----->[/color] [/b][/color][b][color=blue][ 7 6 5 4 3 2 1 0]
[/color][/b]
[b][color=blue] [ 7 6 5 4 3 2 1 0][/color] [/b]---> put hi bits into hi port
Do you get it?
Finally, here's a small example of how you would use the code:
void setDac (uint16_t value)
{
// the ">> 8" is what slides the bits down 8 places
PORTK = (value >> 8); // set in top 8 bits
// the "& 0xFF" just makes sure none of the top sneaks into the bottom
PORTF = (value & 0xFF); // set in lower 8 bits
}
void setup (void)
{
Serial.begin (9600); // maybe you want serial comm?
DDRF = 0xFF; // set all port f bits as outputs
DDRK = 0xFF; // set all port k bits as outputs
}
void loop (void)
{
float volts; // the user will put in a voltage value
uint16_t value; // this var will control the DAC
// hypothetical function you wrote that asks user to enter a number
volts = userPrompt ("Enter desired DAC voltage: ");
value = (65535 * (volts / 5.0)); // convert volts into a "percentage" of 16 bits
// example: user types 2.5 volts, value is 32767
// example: user types 1.8 volts, value is 23592
setDac (value); // send value to DAC
// then loop around and ask for another value
}
Make sense? Notice that this example is very primitive. For example, it doesn't check if the user entered a legal value. They could type "minus 1000 volts", but they sure as heck wouldn't GET that! 
