R2R Ladder DAC

This post is about an R2R Ladder DAC that I build one evening. I wanted to play around with the Arduino Mega 2560 board I have and this seemed like a perfect way to try it out. Below is the schematic for the DAC…

This is an 8-bit DAC built-up from just resistors, each bit is connected to a GPIO pin of the Arduino. I built the circuit on a breadboard and connected it to the Arduino using jumpers.

The first thing I did with the software is toggle the MSB bit using the Ardino digitalWrite() function, however using these functions makes writing to the DAC far too slow. So I needed to write directly to the ATmega GPIO registers and to avoid jitter I use a Timer interrupt. At the moment the software just increments PORTA each interrupt and lets it overflow, this creates a sawtooth wave on the output of the DAC and exercises each bit in the DAC.

int timer1_reload;
 
void setup() {
   pinMode(22, OUTPUT);
   pinMode(23, OUTPUT);
   pinMode(24, OUTPUT);
   pinMode(25, OUTPUT);
   pinMode(26, OUTPUT);
   pinMode(27, OUTPUT);
   pinMode(28, OUTPUT);
   pinMode(29, OUTPUT);
   PORTA = 0;
 
   noInterrupts();
   TCCR1A = 0;
   TCCR1B = 0;
 
   timer1_reload = 65535 - 5;
   TCNT1 = timer1_reload;
   TCCR1B |= (1 << CS10);
   TCCR1B |= (1 << CS11); // /644
   TIMSK1 |= (1 << TOIE1);
   interrupts();
}
 
ISR(TIMER1_OVF_vect)
{
  TCNT1 = timer1_reload;
  PORTA++;
}
 
void loop() {
}

Next I will generate a sine wave using the DAC.

   

Playing with the Arduino UNO

Recently I ordered an Arduino UNO and two shields from Adafruit and two weeks ago it arrived… YAY! As soon as I signed for it I rushed to unpacked it! I was amazed at how small it is, it looked bigger in photos. So far over this couple of weeks I’ve only used a few times but those few times I have used I have been very impressed with it.

The first thing I did of course is get it flashing an LED, which only took about 10 minutes to get going. This was including the time to install the IDE on my laptop running Ubuntu 11.04. After a while I had an issue with the Ardunio IDE hanging whenever I click on the Tools menu and also an issue with the IDE reporting the serial port was locked by another application. I haven’t yet found a fix for this other than using my Fiancée’s laptop instead, which is running Windows Vista… I love cross platform software, it can be so handy sometimes.

After that I built the two shields, the first was a Protoshield with screw terminals, and the second was the data logging shield. These were both pretty easy to build and the instructions where excellent. The next thing I did was get the arduino sampling with the ADC, I did this by simply hooking a 10k pot to A0. Then I soldered an LM35 onto the data logging shield and hooked 3V3 to the VREF pin as recommended by Adafruit. At this point I was using the Serial port to get the samples out.

The next day I decided to follow the tutorials on how to use the SD card interface. Thanks to the excellent work by the authors of the various libraries this was very easy. The same for using the RTC. It all just came together and I had the Arduino sampling temperature with a timestamp to the SD card.

In conclusion I think the Arduino is fantastic and I hope to use it much more my future projects. I especially like the idea of the Arduino Pro and Nano because they can be embedded in a system somewhere.