AVR Microcontrollers

Most AVR microcontrollers behave the same where they are an ATtiny variant or ATMEGA variant. We have a tutorial that introduces AVR microcontrollers and what is necessary to get started using them. Pay close attention to this tutorial as we describe how to work with AVR microcontrollers in general. You need to decide whether you would like to use an Arduino UNO as an ISP to program your ATtinyX5, or the Sparkfun Pocket AVR Programmer. The tutorial covers AVR microcontrollers and is mostly split up between ATtiny AVR microcontrollers and ATMEGA AVR microcontrollers - we want to follow the ATtiny specfic parts! After you go through the tutorial, return here and we will dive specifically into the ATtinyX5. Now, we direct you to the tutorial here: AVR Microcontrollers

Returning from the AVR Microcontrollers tutorial. We should have the ATtinyCore installed, know what device we will use to bootload and program the ATtinyX5 (Arduino as ISP or Pocket AVR Programmer), have some familiarity of the board settings (we will explore the ATtinyX5 settings explicitly), and some of the bootloading process (again, explicitly covered here).

In the next module, we explore the ATtinyX5.

Overview of the ATtinyX5

We refer to the ATtinyX5 where the 'X' can be either 2, 4, or 8. Thus, there are three different variants of the ATtinyX5, namely: ATtiny25, ATtiny45, and ATtiny85. They are all identical except for how much flash memory, RAM (Random Access Memory), and EEPROM (Electrically Erasable Programmable Read-Only Memory). This is shown in the specifications table below.

The specifcations of the ATtinyX5 variants are outlined as follows:

The following shows the pinout diagram for the ATtinyX5:

The pins of interest are the following:

In-System Programmer (ISP) Pins paralleled with that of those on the Arduino UNO (or ATMEGA328p):

Even for such a small chip, there is a ton of information that we are not able to cover. Feel free to checkout the complete documentation of the ATtinyX5 from its datasheet linked in the Resources and References at the bottom of the page.

Observe the similarities of the ATtinyX5 to the Arduino UNO (or ATMEGA328p), shown below for reference:

Now that we have familiarized with the ATtinyX5, we are ready to bootload and program the microcontroller now!

Bootloading the ATtinyX5

To bootload the ATtinyX5, we need to use an external master device to program our ATtinyX5 (slave device). There are several ways we can do this. The most common way is to use an Arduino UNO to program the ATtinyX5. This is usually easy because it is assumed that you already have an Arduino UNO on hand. Another common method is to use Sparkfun's Pocket AVR Programmer.

From the AVR Microcontrollers tutorial, configure your Arduino IDE and/or programming device. Assuming the ATtiny board settings in the Arduino IDE are configured appropriately.

From hereon out, we will be using at ATtiny85 to demonstrate the bootloading process. We will still mention ATtinyX5 where it is general, but configure the appropriate settings for the ATtiny85. However, this same process can be applied to any of the ATtinyX5 variants, given that you make the appropriate change at the Chip selection shown later.

Next, we have to burn the bootloader to the ATtinyX5.

Burn the Bootloader to ATtinyX5 using Arduino UNO as an In-System Programmer

If you have not done so yet, make sure your Arduino UNO is configured as an In-System Programmer, which is outlined in the Bootloading AVR Microcontrollers module in the AVR Microcontrollers tutorial.

With your Arduino UNO configured, we need to build the following circuit to connect the Arduino UNO to the ATtinyX5:

Pay close attention to the schematic. The ATtinyX5 physical pins (numbers protruding from the symbol) and Arduino or programming pins (in the description inside the symbol) are labeled on the schematic.

The 10µF capacitor prevents the Arduino UNO from automatically resetting when we bootloading/programming the ATtinyX5.

Observe that the Serial pins of the Arduino UNO are connected to the ISP pins of the ATtinyX5. Furthermore, the ATtinyX5 needs to be powered when we bootload/program it, we can just tap off the Arduino UNO power rail pins with it connected to your computer USB port. We outline these connections in the following table:

Arduino UNO (physical pin)	ATtinyX5 (physical pin)
3.3V or 5V	VCC (pin 8)
SCK (pin 13)	SCK (pin 7)
MISO (pin 12)	MISO (pin 6)
MOSI (pin 11)	MOSI (pin 5)
SS (pin 10)	RESET (pin 1)
GND	GND (pin 4)

Do not confuse the physical pin with the Arduino or programming pin.

Although the schematic shows the connection to 3.3V, 5V will also work just as well.

Please use the Chip Remover when removing ATtiny microchips, the pins are extremely fragile. If you break a pin, the chip practically becomes useless.

With the appropriate settings selected in the Arduino IDE, first select Burn Bootloader to burn the selected settings to the ATtinyX5.

Be sure you have selected Arduino as ISP (ATtinyCore) for the programmer setting. As for clock, we recommend using the default 8 MHz crystal - do NOT select an external clock unless you know what you are doing.

To burn the bootloader, on the Arduino IDE navigate to Tools and click Burn Bootloader.

Selecting Burn Bootloader will act instantly!

You should always Burn Bootloader when first using the ATtiny variant chip. You never know what fuse settings came with the board by default.

Be sure all the settings we have discussed during this tutorial are set appropriately.

For the Clock setting, if you do external on accident, you will brick (effectively disable or render useless) the ATtiny until we connect the corresponding external clock. Do NOT select any external clock rates unless you already have an external oscillator configured.

If your connections are correct, you should receive a successful Burn Bootloader message.

If you do NOT get a successful Burn Bootloader message, please check your circuit, the board settings, and make sure the ArduinoISP sketch has been uploaded to your Arduino UNO.

Burn the Bootloader to ATtinyX5 using Pocket AVR Programmer

If you have not done so yet, make sure you have the Pocket AVR Programmer configured appropriately, which is outlined in the Bootloading AVR Microcontrollers module in the AVR Microcontrollers tutorial.

With your Pocket AVR Programmer configured, we need to build the following circuit to connect the Pocket AVR Programmer 2x3 ISP header to the ATtiny:

You need to use your imagination (since we do not have a physical picture here - only a schematic representation). The Pocket AVR Programmer 2x3 ISP header has a key sticking out of one end of the connector (which is illustrated in the schematic). This is also from a top-view perspective.

Pay close attention to the schematic. The ATtinyX5 physical pins (numbers protruding from the symbol) and Arduino or programming pins (in the description inside the symbol) are labeled on the schematic.

Observe that the Serial pins of the Pocket AVR Programmer are connected to the ISP pins of the ATtinyX5. Furthermore, the ATtinyX5 needs to be powered when we program it, we can just tap off an Arduino UNO power rail pins with it connected to your computer USB port or using the Pocket AVR Programmer itself by toggling the switch between Power Target and No power.

Careful if you chose to power the ATtinyX5 using the Pocket AVR Programmer, it provides 5V when Power Target is selected. This is convenient for breadboard prototyping in general, however it is common to not to use the Pocket AVR Programmer to power the target (e.g. ATtiny) when it is on its own (custom) PCB because you may be using a voltage less than 5V which may damage your custom circuitry.

We outline these connections in the following table:

Pocket AVR Programmer (2x3 ISP header)	ATtinyX5 (physical pin)
5V (pin 2) or NC	VCC (pin 8)
SCK (pin 3)	SCK (pin 7)
MISO (pin 1)	MISO (pin 6)
MOSI (pin 4)	MOSI (pin 5)
RESET (pin 5)	RESET (pin 1)
GND (pin 6)	GND (pin 4)

Do not confuse the physical pin with the Arduino or programming pin for the ATtiny.

The connection to the Pocket AVR Programmer VCC (5V) pin may NOT need to be connected if you will power your microcontroller with an external power source.

Please use the Chip Remover when removing ATtiny microchips, the pins are extremely fragile. If you break a pin, the chip practically becomes useless.

With the appropriate settings selected in the Arduino IDE, first select Burn Bootloader to burn the selected settings to the ATtiny85.

Remember the Pocket AVR Programmer will NOT show up in Port, so it is OK to have nothing selected.

Be sure you have selected USBtinyISP for the programmer setting. As for clock, we recommend using the default 8 MHz crystal - do NOT select an external clock unless you know what you are doing.

To burn the bootloader, on the Arduino IDE navigate to Tools and click Burn Bootloader.

Selecting Burn Bootloader will act instantly!

You should always Burn Bootloader when first using the ATtiny variant chip. You never know what fuse settings came with the board by default.

Be sure all the settings we have discussed during this tutorial are set appropriately.

For the Clock setting, if you select external on accident, you will brick (effectively disable or render useless) the ATtiny until we connect the corresponding external clock. Do NOT select any external clock rates unless you already have an external oscillator configured.

If your connections are correct, you should receive a successful Burn Bootloader message.

If you do NOT get a successful Burn Bootloader message, please check your circuit, and the board settings.

With the bootloader burned to your ATtinyX5, we are finally ready to program it with custom code! You will keep the same circuit configuration used for burning the bootloader to program the ATtiny with a typical Arduino sketch.

Programming the ATtinyX5

We demonstrate a trivial example of programming the ATtinyX5, the typical blink sketch. You may now open a new sketch and write your own code or copy-paste the following code into the sketch:

// Trivial program to test the ATtinyX5.
//
// Toggles LED on and off for a fixed period of time.
//  Typical blink sketch.


#define led 0

void setup() {
  // Initialize ATtinyX5 digital physical pin 5 
  //	(programming pin 0) as an output.
  pinMode(led, OUTPUT);
}

void loop() {
  // Toggle LED on
  digitalWrite(led, HIGH);
  delay(1000);

  // Toggle LED off
  digitalWrite(led, LOW);
  delay(1000);
}

With the same settings from the Burn Bootloader module, instead of burning the bootloader, upload the program to the ATtiny85 like you would with any Arduino device!

You should receive the typical successful programming message.

With the code given above, the ATtinyX5 will toggle one of the digital pins. The following circuit blinks an LED using the appropriate digital pin designated in the uploaded code:

The results should be similar to:

And that is it! Your ATtinyX5 should be blinking an LED. This was a trivial example, but it is the start of many possibilities to reduce your overall hardware footprint and cost. The ATtiny can do a large subset of what the Arduino UNO (ATMEGA328p) can do, so if you have an existing Arduino project that is using only a subset of pins and is relatively simple, consider whether you can free up that Arduino and use an ATtiny instead. If the ATtinyX5 does not have enough GPIO pins, there are several other ATtiny or ATMEGA variants that have more or less pins than the Arduino UNO (ATMEGA328p).