Arduino core for ATtiny 1634, 828, x313, x4, x41, x5, x61, x7 and x8
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Latest commit 8028dc6 Feb 16, 2017 @SpenceKonde committed on GitHub Add link to makefile doc.

ATTiny Core - 1634, x313, x4, x41, x5, x61, x7, x8 and 828 for Arduino 1.6.x

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Basic Wiring

ATtinyCore Universal

This core supports the following processors - essentially every ATtiny processor that makes sense to use with Arduino. Click the processor name for part-specific information:

When uploading sketches via ISP using the Arduino IDE, you must select a programmer marked ATTiny from the programmers menu (or any other programmer added by an installed third party core) in order to upload properly to most supported chips - this is due to a limitation in the IDE.

Bootloader Support (ATtiny 841, 828, 1634, 87, 167 only)

The Optiboot bootloader is included for the ATtiny 841, 1634, 828 and x7 series (87 and 167). This runs at 57600 baud at 8mhz and slower, and 115200 baud above that. By default it uses UART0 or the LIN/UART as appropriate (bootloaders that use UART1 for devices that have a second UART are included, prefixed with "ser1" - you must flash them manually or modify boards.txt if you wish to use them). Once the bootloader is programmed, the target can be programmed over serial; the bootloader will run after reset, just like on a normal Arduino. The standard DTR reset circuit is highly recommended.

The ATtiny841, ATtiny1634, and the ATtiny x7 series do not have hardware bootloader support. To make the bootloader work, the "Virtual Boot" functionality of Optiboot is used. Because of this, the Watchdog Timer interrupt vector will always point to the start of the program, and cannot be used for other functionality. Under the hood, the bootloader rewrites the reset and WDT interrupt vectors, pointing the WDT vector at the start of the program (where the reset vector would have pointed), and the reset vector to the bootloader (as there is no BOOTRST fuse). As a result of this, the Watchdog Timer cannot be used as a software interrupt on these parts, and attempting to do so will cause strange behavior - however, it can still be used to reset the chip. This does not effect the 828 (it has hardware bootloader support), nor does it effect the 1634 or 841 if they are programmed via ISP.

Supported clock speeds:


  • 8 MHz
  • 1 MHz
  • 16 MHz (PLL clock, x5, x61 only)
  • 4 MHz (x313 only)
  • 0.5 MHz (x313 only)
  • 128 kHz

External crystal (all except 828 and x8 series):

  • 20 MHz
  • 16 MHz
  • 12 MHz
  • 8 MHz
  • 6 MHz
  • 4 MHz

External crystal (x41, 1634 only, in addition to above):

  • 18.43 MHz
  • 14.74 MHz
  • 11.056 MHz
  • 9.216 MHz
  • 7.37 MHz

Warning When using weird clock frequencies (ones with a frequency (in MHz) by which 64 cannot be divided evenly), micros() is 4-5 times slower (~110 clocks) (It reports the time at the point when it was called, not the end, however, and the time it gives is pretty close to reality - w/in 1% or so). This combination of performance and accuracy is the result of hand tuning for these clock speeds. For other clock speeds (for example, if you add your own), it will be slower still - hundreds of clock cycles - though the numbers will be reasonably accurate. Millis() is not effected, only micros() and delay() (where it doesn't matter for this core, since it still returns correct results). The "stock" micros() executes equally fast at all clock speeds, but returns wrong values with anything that 64 doesn't divide evenly by.

Link-time Optimization (LTO) support

In version 1.1.2 and later, this core supports Link Time Optimization (lto). This can substantially reduce the compiled size of your sketch. Version 1.6.13 or later of the official AVR boards package (included with 1.6.11 and later of the IDE) is required for this functionality. Link time optimization can be enabled from the option in the tools menu.

Makefile Support

For those who prefer to compile with a makefile instead of the IDE, sketches can be compiled with - See the makefile documentation for more information on specific steps needed for this process.

I2C support

On the following chips, I2C functionality can be achieved with the hardware USI, using a library like TinyWireM. This library has the necessary #defines for use with these parts:

  • ATtiny x5 (25/45/85)
  • ATtiny x4 (24/44/84)
  • ATtiny x61 (262/461/861)
  • ATtiny x7 (87/167)
  • ATtiny x313 (2313/4313)
  • ATtiny 1634

On the following chips, slave I2C functionality is provided in hardware, but a software implementation must be used for master functionality. Use for hardware slave functionality, or for software implementation of I2C master.

  • ATtiny 828
  • ATtiny x41 (441/841)

On the following chips, full master/slave I2C functionality is provided in hardware allowing use of the normal Wire library:

  • ATtiny x8 (48, 88)

SPI support:

On the following chips, SPI functionality can be achieved with the hardware USI, using an SPI USI library.

  • ATtiny x5 (25/45/85)
  • ATtiny x4 (24/44/84)
  • ATtiny x61 (262/461/861)
  • ATtiny x7 (87/167)
  • ATtiny x313 (2313/4313)
  • ATtiny 1634

On the following chips, full SPI functionality is provided in hardware allowing use of the normal SPI library:

  • ATtiny 828
  • ATtiny x7 (87/167) (it has both a USI and full SPI)
  • ATtiny x41 (441/841)
  • ATtiny x8 (48, 88)

Serial Support

On the following chips, full serial (UART) support is provided in hardware, as Serial (and Serial1 for parts with two serial ports):

  • ATtiny x313 (2313/4313)
  • ATtiny x7 (87/167 - LIN support)
  • ATtiny x41 (441/841 - two UARTs)
  • ATtiny 1634 (two UARTs)
  • ATtiny 828

On the following chips, no hardware serial is available, however, a built-in software serial is provided. This uses the analog comparator pins (to take advantage of the interrupt, since very few sketches/libraries use it, while lots of sketches/libraries use PCINTs); the serial is named Serial, to maximize code-compatibility. TX is AIN0, RX is AIN1. This is a software implementation - as such, you cannot receive and send at the same time. If you try, you'll get gibberish, just like using SoftwareSerial.

  • ATtiny x5 (25/45/85)
  • ATtiny x4 (24/44/84)
  • ATtiny x61 (261/461/861)
  • ATtiny x8 (48/88)

These cores are compatible with the usual SoftwareSerial library.

Note that when using the internal oscillator or pll clock, you may need to tune the chip (using one of many tiny tuning sketches) and set OSCCAL to the value the tuner gives you on startup in order to make serial (software or hardware) work at all - the internal clock is only calibrated to +/- 10% in most cases, while serial communication requires it to be within just a few percent. However, in practice, a larger portion of parts work without tuning than would be expected from the spec. There are two exceptions to this: the ATtiny x41 series, 1634R, and 828R have an internal oscillator factory calibrated to +/- 2% - but only at operating voltage below 4v. Above 4v, the oscillator gets significantly faster, and is no longer good enough for UART communications. The 1634 and 828 (non-R) are not as tightly calibrated (so they may need tuning even at 3.3v) and are a few cents less expensive, but suffer from the same problem at higher voltages. Due to these complexities, it is recommended that those planning to use serial (except on a x41, 1634R or 828R at 2.5~3.3v) use an external crystal or other clock source.

ADC Support

All of the supported parts except for the x313 series have an Analog to Digital converter on chip. Single-ended ADC inputs can be read using the pin number or the Ax constant. In addition to the single-ended input channels marked on the pinout diagrams, many also support differential ADC input channels. To use these, simply call analogRead() with the appropriate ADC channel number, as if it were a pin. To get the ADC channel number, refer to the datasheet - it is listed in the Register Description section of the chapter on the ADC, under the ADMUX register.

B. O. D. (brown out detect) Configuration option

All chips have a menu to select the level of Brown-out Detection, if any, to use. Brown-out detection continuously monitors Vcc, and holds the chip in reset state (BOR) if the applied voltage is below a certain threshold. This is a good idea with slow-rising power supplies or where it is expected that the supply voltage could droop below the required operating voltage for the frequency it is running at (see the speed grade specification for the part you're using) - without BOD enabled, this can put the chip into a hung state until manually reset. However, BOD increases power consumption slightly, and hence may be inappropriate in low power applications. The selected BOD option is configured by the fuses, and as such these settings are applied upon burning bootloader, not upon sketch upload.

Pin Mapping

ATtiny 441/841

x41 pin mapping

ATtiny 1634

1634 pin mapping

ATtiny 828

828 Pin Mapping

ATtiny 25/45/85

x5 pin mapping

ATtiny 24/44/84

x4 Pin Mapping

ATtiny 261/461/861

x61 Pin Mapping

ATtiny 87/167

x7 Pin Mapping

ATtiny 88/48

x8 Pin Mapping x8 Pin Mapping

ATtiny 2313/4313

x313 Pin Mapping

Note that two pin mappings are supported for some devices to retain backwards compatibility with other cores - the pin mapping may be chosen from a menu.

Note that analog pin numbers (ex A0 ) cannot be used with digitalWrite()/digitalRead()/analogWrite() - all pins have a digital pin number. Analog pin number should only be used for analogRead() - this represents a departure from the behavior used in the official AVR boards. This enables us to expose the advanced ADC functionality available on some of the ATtiny parts with minimal impact, as clearly written code is unlikely to fall afoul of this anyway.


To work correctly, these parts should be installed with a 0.1uf capacitor between Vcc and Ground, as close to the chip as possible. Where there are more than one Vcc pin (x61, x7, x8) both must have a capacitor. No other specific hardware is needed, though, when designing a custom board, it is incredibly helpful to provide a convenient ISP header. See the pinout diagrams in the datasheet for the location of the ISP/SPI programming pins. A larger value capacitor for power filtering is recommended - if using a

For use with Optiboot, the following additional components and connections are required:

  • Arduino pin 9/PA1/TXD0 to RXI of serial adapter (0/PB0 on 1634)
  • Arduino pin 8/PA2/RXD0 to TXO of serial adapter (1/PA7 on 1634)
  • Diode between Reset and Vcc (band towards Vcc)
  • 0.1uf capacitor between Reset and DTR of serial adapter
  • 10k resistor between reset and Vcc (required)
  • (optional) LED and series resistor from Arduino pin 2/PB2 (on 841) or pin 13 (on 1634/828) to ground (This is the pin optiboot flashes to let you know it's running)

Buy Breakout boards

Except for the x5, x4, x61, and x313 series, these are only available in surface mount packages. Breakout boards are available from my Tindie store (these are the breakout boards used for testing this core), which have the pins numbered to correspond with the pin numbers used in this core. Where applicable, all of these assembled boards have the bootloader installed, and all are set to run at the advertised speed (most are available with several speed/voltage combinations).


  • Some people have problems programming the 841 and 1634 with USBAsp and TinyISP - but this is not readily reproducible ArduinoAsISP works reliably. In some cases, it has been found that connecting reset to ground while using the ISP programmer fixes things (particularly when using the USBAsp with eXtremeBurner AVR) - if doing this, you must release reset (at least momentarily) after each batch of programming operation. This may be due to bugs in USBAsp firmware - See this thread on the Arduino forums for information on updated USBAsp firmware: (Links to the new firmware are on pages 5~6 of that thread - the beginning is largely a discussion of the inadequacies of the existing firmware)
  • At >4v, the speed of the internal oscillator on 828R, 1634R and 841 parts increases significantly - enough that neither serial (and hence the bootloader) does not work. It is recommended to run at 3.3v if using internal RC oscillator as a clock source - however, for these chips, a workaround is provided. This takes the form of a bootloader compiled assuming the chip is running a little fast, so that UART communication will work, and a matching board definition that tries to compensate by assuming the chip is running at 8.2mhz instead of 8. If you wish to handle tuning of the oscillator in your sketch (this means that serial won't work and timing will be off until you do something about it), you can use the 5v workaround as the bootloader but compile assuming 8mhz - select that option when you burn bootloader, but not when compiling and uploading. (New feature added in version 1.1.2)


This core is based on TCWorld's ATTinyCore, which is in turn based on the arduino-tiny core here: The ATtiny841 support is based on shimniok's ATTiny x41 core, and the 1634 support on Rambo's ATtiny 1634 core. And of course - everything is based on the great work of the Arduino development team for creating the Arduino IDE and community which we all use.