Tutorial: Building cool projects with MCUs (Part 5)

I finally received the circuit boards! And, in this fifth and final part of the microcontroller tutorial, we are going to solder the components to the circuit board and program the MCU using the USB port of a computer.

Just to refresh our memories, so far we have learned:

• Part 1: What is an MCU?
• Part 2: How to choose an MCU
• Part 3: Designing a circuit for the MCU
• Part 4: Making a circuit board

I recently ordered the PCBs from Seeed Studio. In order to expedite their delivery, I used a more expensive shipping option from UPS. I did get the boards pretty fast – but I also got an unexpected bill from them because they had to take it through customs.

So, even though the boards were only $10, I ended up with paying about $60 in shipping and customs… But luckily, there exists a much cheaper shipping option (about $3-4) – you just have to wait a little bit longer for the boards to arrive.

Let’s solder the board!

I wanted to make this circuit in such a way that it was possible to make it at home. To solder the circuit, I’m going to use my old Ersa soldering iron and some standard solder wire. The tip of the iron is a bit thick, so it’s really not ideal for this job. However, I know many people only have a simple soldering iron like this lying around the house – so it’s the perfect test to see if this is something that anyone can build from the comfort of your home.

The first thing we’re going to solder is the MCU chip. This is also the hardest part to solder. I have to admit – when I looked at my soldering iron, then looked at the chip – I was a bit worried that it was going to be hard. But the main trick here was to be patient!

To solder the surface mount components, we can use the techniques described in this smd soldering article.

First, we solder one corner pin of the chip. When we have managed to solder this one pin correctly – and all the pins are aligned over their pads – we move on to the corner on the other side. With two corners soldered properly, all we need to do is to add a tiny bit of solder to all the other pins and pads.

Don’t rush it. Take your time. Inspect the pins closely to see if they are soldered and that they don’t have a “solder bridges” to their neighbors. And, don’t worry if it looks a bit like a war-zone with solder all over – just look at mine above – it still works!

Now, safe to say that the worst part is over. The other components are pretty straightforward to solder. Just make sure the LED and the polarized capacitor is placed in the correct direction.

Programming the circuit

Once we are confident that the components are soldered properly, it’s time to test it! First, we need to check if the USB interface works. Otherwise, we won’t be able to program the circuit. To test the USB interface, all we need to do is to connect a USB cable and connect the circuit to our computer. From there, we can just check if it pops up as a USB device on the computer.

And… it does!

So, let’s program the MCU. A simple way of testing it is to make an LED-blink program. This is a simple program that, well, makes our LED blink. It looks like this:

#define F_CPU 1000000 // The chip runs at 1 MHz as default (even if you are using a 8MHz crystal)

#include
#include

int main(void)
{
DDRC = (1<<PC7); //Sets the direction of the PC7 to output
PORTC = (1<<PC7); //Sets PC7 high

while(1)
{
_delay_ms(500); //Wait 500 milliseconds
PORTC &= ~(1<<PC7); //Turn LED off

_delay_ms(500); //Wait 500 milliseconds
PORTC |= (1<<PC7); //Turn LED on
}

return 0;
}

We save this code in a file called led-blink.c

Compiling our code

The first thing we need to do is to compile our code into machine code that the MCU can read. One way of doing this is through Atmel Studio. But, since I am a big fan of using the Linux terminal, I’ll show you how to compile and upload a program using Ubuntu.

First, install avr-gcc with the command:

sudo apt-get install avr-gcc

Then, compile the code and make it into the right format with the following commands:

avr-gcc -mmcu=atmega32u2 -Os blink-led.c -o blink-led.out
avr-objcopy -j .text -j .data -O ihex blink-led.out blink-led.hex

The resulting file – blink-led.hex – can now be uploaded to the microcontroller. You can find more information on the commands here.

Uploading the code to the MCU

Time to upload the program and see if it works. One way to do this is by using Atmel’s FLIP software. But, once again, let’s see how we can do it with the Linux terminal.

Install dfu-programmer with the command:

sudo apt-get install dfu-programmer

Then, erase the old flash memory on the MCU and upload the compiled .hex file:

sudo dfu-programmer atmega32u2 erase
sudo dfu-programmer atmega32u2 flash blink-led.hex

Unplug the circuit from your computer, then plug it in again. And what do you know, the LED starts to blink!

Making something cool

Now that we’ve got it working, we’re ready to make something cool. There are so many cool things you can make with a microcontroller. For example, you can connect it to this Wi-Fi module and make the LED blink every time @AtmelMakes posts a new tweet.

Or, how about connecting it to this sound module and this motion sensor, and make it play a Christmas song every time someone goes near your Christmas tree? As Atmel always says, the possibilities are truly endless.

If you missed any of the previous parts of this tutorial – you can find them here:

• Part 1: What is an MCU?
• Part 2: How to choose an MCU
• Part 3: Designing a circuit for the MCU
• Part 4: Making a circuit board

Tutorial: Building cool projects with MCUs (Part 3)

As we proceed onto the third portion of this microcontroller tutorial, let’s first revisit what we have accomplished thus far.

In Part 1, we defined what a microcontroller actually was. I wanted to get everybody on-board (no pun intended) for this, so I started from scratch. Feel free to jump back there if you need a refresher.

Then in Part 2, we looked at the various types of MCUs. And, it turns out that there were a lot! However, by using Atmel’s selection tool, we managed to narrow it down to a few different ones, before choosing a winner — an ATmega32U2. Why? Mainly because it can be programmed through the USB interface without any extra components or tools, and since it did not have too many pins – so we should be able to solder it at home.

Now, the time has come for us to design the circuit diagram. We’ll start with outlining what we need, then we’ll dive into the datasheet to figure out exactly how we can do this.

What Components Do We Need?

If you’ve never done it before, putting all the necessary components onto the circuit diagram is not a super easy task. Well, the process is not that hard; but until you know how to do it, it will seem a bit difficult. So, let me take you through my thought process:

We need to power the chip somehow. There are some pins on the microcontroller that need power. We have to figure out which pins that need power, and what voltage they need.

We also want to program the chip trough USB, so we need a USB connector and learn how to connect it to the chip.

Then, there is extra stuff. Like maybe an LED to play around with, and definitely some connection pins that we can connect other stuff to later when we want to experiment.

So, we need to figure out:

• How to power the circuit
• How to connect the USB part
• How to connect pins to the chip

Using the Datasheet to Find the Details of Our Circuit

The datasheet for the ATmega32U2 is a must when we are designing our circuit. Here, we can find all the necessary technical details.

We don’t have to read it from beginning to end; instead, we simply look up the parts we are curious about, and study them in more depth. We can find a table of contents at the end of the document.

For instance, if we wanted to use the timer of the MCU, we’d look up the part about timers. If we wanted to use the SPI communication part, we would look that up.

Connecting Power and USB

As you probably know, USB devices can be powered through, well, USB. By doing this, we simplify our circuit and require less components. So, let’s do this.

To figure out exactly HOW we can do this, we’re going to read the datasheet. By going to the table of contents, we find that the section that describes the USB part of the microcontroller is on page 186.

Here, there are different images showing how to connect the USB connector to the microcontroller to make it a USB powered device. For our circuit, we’ll use the bus-powered 5V version:

Under Design Guidelines on page 189, we find the exact values we need for the resistors. It’s also there that we learn it’s highly recommended to use a 10µF capacitor on the VBUS, so let’s add that too.

Adding a Crystal

Another interesting thing to note from the image of how to connect the USB, is that there is also a crystal and a couple of capacitors connected to the XTAL1 and XTAL2 pins. This gives us the idea that we probably need a crystal too. But why?

Microcontrollers rely on clock signals to work. Every time the clock signal gives a pulse, something will happen in the processor. Most MCUs come with an internal RC-oscillator that can create this clock signal; however, the USB part of the microcontroller cannot operate from this internal clock — it needs a crystal oscillator to work.

To create a crystal oscillator, we need to connect a crystal and two capacitors to XTAL1 and XTAL2. By looking up “Crystal Oscillator” in the datasheet on page 36, we can also identify exactly what crystal and which capacitors are required.

Connecting an LED And Some Pins

By now, we have everything we need for the microcontroller circuit to work. But, we won’t be able to have much fun with it if we don’t add some connections to the input and output pins. And, it’s also good to add an LED. Everything gets better with an LED!

The LED needs to be connected through a resistor. We call this a current limiting resistor — this is to control the amount of current going through it.

From there, we will need a few physical pins that we can use to connect other stuff to our circuit. There are 22 I/O pins on the ATmega32U2, yet some of the pins are used for other purposes (like XTAL2), so we can’t use them all. For this circuit, we’ll add 16 I/O pins, as it’s a nice and round number.

What Else Do We Need?

On MCU circuits, it’s very common to include a RESET button. This makes it possible to reset the MCU without removing the power connection. This will add a couple of more components to our circuit and isn’t necessary to make it work, but it’s very handy, so let’s add it.

In the datasheet, we can see that pin 24 on the chip is the reset signal. It has a line over itself, which means that it’s activated when pulled low.

To make the reset signal stay in a high-state when the button is not pushed, we’ll add a pull-up resistor. This is a resistor up to VCC. And, we connect the button so that it will pull the reset-pin to ground when pushed.

The pull-up resistor should have a value of around 10k Ohm. For reference, SparkFun has a good article on how to choose a pull-up value.

Our Microcontroller Circuit Diagram

Now that we have figured out the different pieces of the circuit, it’s time to put them all into one circuit diagram. By adding everything into one circuit diagram, we end up with this:

Next, we need to create a circuit board out of this. Creating a circuit board does not have to be very complicated. In the subsequent part of the microcontroller tutorial series, we’ll design and make the circuit board.

Stay tuned for Parts 4-5 in the coming days…