Arduino core for the new megaAVR ATtiny series chips. These parts represent the new megaAVR technology (as used in megaAVR chips like the ATmega4809 as used on Nano Every and Uno Wifi Rev. 2) and advanced peripherals in low-cost, small packages of the ATtiny line. All of these parts feature a full hardware UART, SPI and TWI interface, and the 1-series parts have a DAC for analog output as well. Moreover, these parts are cheap - the highest end parts, the 3216 and 3217, with 32k of flash and 2k of SRAM (same as the atmega328p used in Uno/Nano/ProMini!) run just over
These use a UPDI programmer, not traditional ISP like the classic ATtiny parts did. One can be made from a classic AVR Uno/Nano/Pro Mini - see Making a UPDI programmer.
The Optiboot serial bootloader is now supported (as of 1.1.1) on these parts, allowing them to be programmed via a serial port. See the Optiboot section below for more information on this, and the relevant options. Installing the bootloader does require a UPDI programmer. In the near future, I will be selling pre-bootloaded boards on Tindie.
For this core to work when installed manually, and via board manager for 1.0.1 and earlier, you must install the official Arduino megaAVR board package using board manager - this is required to get the compiler and correct version of avrdude installed. If you receive an error similar to "avr-g++: error: device-specs/specs-attiny402: No such file or directory", that indicates that the Arduino megaAVR board package is not installed. As of 1.0.2, this is no longer required if installed via board manager. It is still required for a manual installation.
- ATtiny3217,1617,817,417
- ATtiny3216,1616,816,416
- ATtiny1614,814,414,214
- ATtiny412,212
- ATtiny1607,807
- ATtiny1606,806,406
- ATtiny1604,804,404,204
- ATtiny402,202
- 20MHz Internal (4.5v~5.5v - typical for 5v systems)
- 16MHz Internal (4.5v~5.5v - typical for 5v systems)
- 10MHz Internal (2.7v~5.5v - typical for 3.3v systems)
- 8MHz Internal (2.7v~5.5v - typical for 3.3v systems)
- 5MHz Internal (1.8v~5.5v)
- 4MHz Internal (1.8v~5.5v)
- 1MHz Internal (1.8v~5.5v)
These parts do not support using an external crystal like the classic ATtiny parts do, however the internal oscillator is tightly calibrated enough that the internal clock will work for UART communication.
The pin PA0 is the UPDI or reset pin depending on fuse settings (it could also be set as a GPIO pin, but we already have two very compelling uses for said pin)
This has one very troubling consequence - if we want to program via UPDI, we must not make that pin a Reset pin, otherwise we would need to use an HV programmer to reset the chip to allow UPDI programming again. At present this process is beyond the scope of this guide (apparently on some silicon revs, it is trivially easy to do, while on others it requires careful timing and a magic KEY signal). I hope to be able to point y'all to an affordable re-UPDIFIER in the near future.
There is a solution to the lack of a hardware reset pin when you need to keep UPDI programming, however - and that is that these parts support a software reset. Such a reset can be achieved with the simple command:
_PROTECTED_WRITE(RSTCTRL.SWRR,1);
If you were to set up a "low level" interrupt on a pin, and put that in the ISR, that pin would act as an ersatz reset pin (note that it wouldn't "stay" in reset like a real reset pin - it will reset the chip, and then the sketch (or bootloader) will start running again from the start until the interrupt was enabled, then reset again).
megaTinyCore supports configurations with this pin fused to act as reset with programming over UART serial. See the Optiboot section below for more information
Unlike classic AVRs, on the megaavr parts, the flash is within the same address space as the main memory. This means pgm_read_*_near functions are not needed to read them. Because of this, the compiler automatically puts any variable declared const into progmem, and accesses it appropriately - you no longer need to explicitly declare them as PROGMEM. This includes quoted string literals, so the F() macro is no longer needed either (as of 1.0.6, the F() macro is a no-op).
However, do note that if you explicitly declare a variable PROGMEM, you must still use the pgm_read functions to read it, just like on classic AVRs - when a variable is declared PROGMEM on megaavr parts, the pointer is offset (address is relative to start of flash, not start of address space); this same offset is applied when using the pgm_read_near functions. Do note that declaring things PROGMEM and accessing with pgm_read_near functions, although it works fine, is slower and wastes a small amount of flash (compared to simply declaring the variables const).
WARNING In versions of megaTinyCore 1.0.6 and earlier, the sketch size reported during compilation does not include variables declared const. PROGMEM variables, however, are reported normally. in 1.1.0 and subsequent releases, the flash used by const variables is correctly reported.
The simple matter of how to refer to a pin for analogRead() and digitalRead(), particularly on non-standard hardware, leads to a surprising amount of confusion. It's my opinion that the blame rests with the dubious usability decisions made in design of the original Arduino core. This core uses an extremely simple basic scheme - but with a few additional options for people who are used to other conventions.
For digitalRead(), digitalWrite(), pinMode(), analogWrite() analogRead() and other sundry functions, the "Arduino Pin Number" shown in the pinout charts is the recommended way to refer to pins. digitalRead(1) and analogRead(1) refer to the same pin, and will perform a digital or analog read on said pin. See the charts in the part specific documentation.
Two additional ways are provided to refer to pins:
- The analog channel numbers are available as An defines (ex, A0, A1, A2, etc). These are the analog channel numbers; they are simply #defined as the digital pin number associated with the analog channel in question. See the datasheet for the assignments of analog channel numbers to pins; because we do not recommend referring to pins in this way, we have chosen not to include the analog channel numbers on the included pinout charts. There are also PIN_An defines for compatibility with the official cores - these likewise point to the digital pin number associated with the analog channel. Note that channel A0 is on the UPDI/Reset pin, and hence is typically not usable.
- If you wish to refer to pins by their port and pin number, you can do that too - As of 1.1.1, the core provides PIN_Pxn (for example, PIN_PA1, or PIN_PC0). Not to be confused with the PIN_An defines described above). Again, these just resolve to the digital pin number of the pin in question - they don't go through a different code path or anything (for that - for example if you need higher performance than digitalWrite() et. al. can provide - see direct port manipulation.
All of these parts have a single hardware serial port (UART). It works exactly like the one on official Arduino boards (except that there is no auto-reset, see note above about the lack of a reset pin). See the pinout charts for the location of the serial pins. Serial output for a few milliseconds after calling Serial.begin() seems to be off - if you are printing a message at the start of the sketch, add a small delay between Serial.begin() and your first Serial.print() call.
On all parts, the UART pins can be swapped to an alternate location. This is controlled by the Tools -> UART Pins menu (new in 1.1.0)
#ifdef UARTREMAP
#error "This sketch and hardware assume the the standard UART pin locations, please select that option"
#endifAlternately, you can manually move to alternate pins within the sketch - but this won't automatically get you the TX and RX #defines on the correct pins.
PORTMUX.CTRLB|=(1<<SPI0);Move to default pins:
PORTMUX.CTRLB&=~(1<<SPI0);To maximize the accuracy of the baud rate, from the Tools -> Voltage for UART Baud menu, select whether the voltage is closer to 5v or 3v - the factory calibration supplies an oscillator error adjustment for the purpose of UART baud calculation for 5v and 3v, and using the right one will produce a baud rate closer to the target value. That said, testing has indicated that either setting is almost always good enough.
Note: The UART Serial option selected when you do "burn bootloader" for an Optiboot board definition is the serial port that the uploaded bootloaded will use. You may freely change this when compiling/uploading sketches to use the other pins - the pins used by the bootloader will only change when you do "burn bootloader". If this is your first time bootloading the board in question, and you want to turn UPDI into a Reset pin, burn bootloader first with the UPDI pin left as UPDI, so you can verify that, with the desired UART option, the bootloader really does try to use the pins you want it to - before you turn UPDI into reset and render the part unprogrammable.
Note: UART serial is broken on the x12 and x02 parts before version 1.0.3.
All of these parts have a single hardware SPI peripheral. It works exactly like the one on official Arduino boards using the SPI.h library. See the pinout charts for the location of these pins. Note that the 8-pin parts (412, 212, 402, 204) do not have a specific SS pin.
On all parts except the 14-pin parts, the SPI pins can be moved to an alternate location (note: On 8-pin parts, the SCK pin cannot be moved). This can be configured using the Tools -> SPI Pins submenu; this is set at compile time (reburning bootloader is not required). You can test for whether the alternate pins are in use by checking if SPIREMAP is #defined - you can for example use it to check that the correct options are selected and terminate compilation so you can select the right option if that is the case:
#ifdef SPIREMAP
#error "This sketch and hardware assume the the standard SPI pin locations, please select that option"
#endifAlternatively, you can manually move to alternate pins within the sketch - but this won't automatically get you the SCK, MISO, MOSI, SS #defines on the correct pins.
PORTMUX.CTRLB|=(1<<SPI0);Move to default pins:
PORTMUX.CTRLB&=~(1<<SPI0);All of these parts have a single hardware I2C (TWI) peripheral. It works exactly like the one on official Arduino boards using the Wire.h library. See the pinout charts for the location of these pins.
On all parts with more than 8 pins, the TWI pins can be swapped to an alternate location. This can be configured using the Tools -> I2C Pins submenu; this is set at compile time (reburning bootloader is not required). You can see whether the alternate pins are in use by checking if TWIREMAP is #defined - you can, for example, use it to check whether correct options are selected and terminate compilation, so you can select the right option:
#ifndef TWIREMAP
#error "This sketch and hardware assumes the alternate I2C/TWI pins are used, please select that option"
#endifAlternately, you can manually move to alternate pins within the sketch; this is not guaranteed to work with all libraries because it doesn't set the pin macros to the correct pins - though these pin macros don't look like they are widely used, so it may not matter.
It can be done like this:
PORTMUX.CTRLB|=(1<<TWI0);Move back to default pins:
PORTMUX.CTRLB&=~(1<<TWI0);The core provides hardware PWM (analogWrite) support. On the 8-pin parts (412, 212, 402, 204), 4 PWM pins are available (1.0.5 and later - 1.0.4 and earlier only have 1). On all other parts except the x16 and x17 series, 6 PWM pins are available, driven by Timer A. The type B timers cannot be used for additional PWM pins - their output pins are the same as those available with Timer A - however you can take them over if you need to generate PWM at different frequencies. See the pinout charts for a list of which pins support PWM. The 3216,1616,816,416,3217,1617 and 817 have two additional PWM pins driven by Timer D (pins 10 and 11 on x16, 12 and 13 on x17). Timer D is an async timer, and the outputs can't be enabled or disabled without briefly stopping the timer. This results in a brief glitch on the other PWM pin (if it is currently outputting PWM), and doing so requires slightly longer (in 1.0.0, this delay is 1ms, in 1.0.1 and later, it is around 1us). This applies to digitalWrite() or analogWrite of 0 or 255 while it is currently outputting PWM, and analogWrite of 1~254 while the pin is not currently outputting PWM. This is a hardware limitation and cannot be further improved.
Note that TCA0 (the type A timer) on all parts is configured in split mode to support the most PWM pins possible with analogWrite(). This timer is also used for millis() (all parts on 1.1.2 and earlier, 8 and 14-pin 1-series parts on 1.1.3 and later), so reconfiguring it for different PWM frequencies will break millis(). Use a type B timer if you need to take over a timer if possible, or change millis() to use a different timer - see the section on that below
The usual NeoPixel (WS2812) libraries have problems on these parts. This core includes two libraries for this, both of which are tightly based on the Adafruit_NeoPixel library. See the tinyNeoPixel documentation and included examples for more information.
Support for tone() is provided on all parts using Timer B 0. This is like the standard tone() function; it does not support use of the hardware output compare to generate tones (yet). See caveats below if using specific millis()/micros() settings.
By default, as of 1.1.3, TCD0 will be used by default for millis() on parts without PWM pins that can be driven by TCD0 and not TCA0, otherwise TCA0 will be used (on previous versions, TCA0 is used on all parts). A tools submenu, millis()/micros() provides options to enable with default settings for the part, disable millis and micros entirely (to save flash), or to force it onto a specific timer - as of 1.1.5, TCA0 and TCD0 (1-series parts only, breaks PWM on the two timer D pins on 20 and 24 pin parts), TCB0, TCB1 (where present) and the RTC (no micros, only millis, clocked from internal ULP or external 32,768khz watch crystal) are supported for timekeeping. When using TCB0 (breaks tone and servo on parts with only one type b timer, and tone on all parts); using TCB1 (parts with two type b timers only - 1-series parts with 16k or more of flash - on those parts servo and tone cannot be used simultaneously if this is selected). Support for millis() with the RTC using external watch crystal or internal ULP are planned for a future version (this will only support millis, not micros - the 32kHz clock is not fast enough for microsecond timing). The timer selection menu provides a way to ensure that if you need to take over a specific timer completely, you can move millis()/micros() to a different timer, accepting the consequences outlined above.
When 20/10/5MHz system clock is used, the micros() count will not increase exactly 1000 times faster than millis() count - it is close, but not exact. This was required for performance and flash usage optimization.
If the RTC is selected as the timer for millis timekeeping, micros will not be available. Additionally, this timer is configured to run while in STANDBY sleep mode. This has two important consequences: First, it will keep time while in sleep. Secondly, every 64 seconds, the RTC overflow interrupt will fire, waking the chip - thus, if you are using the RTC for millis and putting the part into sleep, you should declare a volatile global variable that you set in the ISR that is supposed to wake the part, eg 'volatile boolean ShouldWakeUp=0;' - set it to 1 in the ISR, and when you put the ATtiny to sleep, have it check this immediately after waking, going back to sleep if it's not set, and clearing it if it is, e.g.:
void GoToSleep() {
do {
sleep_cpu();
} while (!ShouldWakeUp)
ShouldWakeUp=0;
}This board package also supports using an external 32.768khz crystal as the clock source for the RTC (not supported on 8-pin parts). If this is used, make sure that the crystal is connected between the TOSC1 and TOSC2 pins (these are tbe same as the TX and RX pins), that nothing else is, that no excessively long wires or traces are connected to these pins, and that appropriate loading capacitors per crystal manufacturer datasheet are connected. Since the TOSC1 and TOSC2 pins are the same pins used for serial, you must use the alternate serial pins.
These parts all have ADC channels available on most pins (11 pins on 24 and 20 pin parts, 9 on 14-pin parts and 5 on 8 pin parts - plus the one on the UPDI pin which is not normally usable), they can be read with analogRead() like on a normal AVR. While the An constants (ex, A0) are supported, and refer to the corresponding ADC channel (not the corresponding pin number), using these is deprecated - the recommended practice is to pass the digital pin number to analogRead(). Analog reference voltage can be selected as usual using analogReference(). Supported reference voltages are:
- VDD (Vcc/supply voltage - default)
- INTERNAL0V55
- INTERNAL1V1
- INTERNAL1V5
- INTERNAL2V5
- INTERNAL4V3
- EXTERNAL (1-series - not including 412/212 - only)
The 1-series parts have an 8-bit DAC which can generate a real analog voltage (note that this provides very low current and can only be used as a voltage reference, it cannot be used to power other devices). This generates voltages between 0 and the selected VREF (which cannot be VCC, unfortunately) - select the DAC VREF voltage from the Tools -> DAC Voltage Reference submenu. This voltage must be lower than Vcc to get the correct voltages. Call analogWrite() on the DAC pin to set the voltage to be output by the DAC. To turn off the DAC output, call digitalWrite() on that pin. The Tools -> DAC Voltage menu contains an additional option to disable the DAC output, as this saves a fair bit of flash.
This core provides a version of the Servo library which will select an appropriate timer (Timer B 0, except on the 3216, 3217, 1617, 1616 and 1614, where there is a Timer B 1 available; except on the aforementioned parts, tone cannot be used at the same time as the Servo library. If millis/micros is set to use TCB1 on those parts, servo will use TCB0 instead, making it incompatible with tone there as well). Servo output is better the higher the clock speed - when using servos, it is recommended to run at the highest frequency permitted by the operating voltage to minimize jitter.
Warning If you have installed a version of the Servo library to your /libraries folder (including via library manager), the IDE will use that version of the library (which is not compatible with these parts) instead of the one supplied with megaTinyCore. As a workaround, a duplicate of the Servo library is included with a different name - to use it, #include <Servo_megaTinyCore.h> instead of #include <Servo.h>
Unlike the official board packages, but like many third party board packages, megaTinyCore includes the .printf() method for the printable class (used for Serial and many other libraries that have print() methods); this works like printf(), except that it outputs to the device in question; for example:
Serial.printf("Milliseconds since start: %ld\n", millis());All pins can be used with attachInterrupt() and detachInterrupt(), on RISING, FALLING, CHANGE, or LOW. All pins can wake the chip from sleep on CHANGE or LOW. Pins marked as ASync Interrupt pins on the pinout chart can be used to wake from sleep on RISING and FALLING edge as well.
Advanced users can instead set up interrupts manually, ignoring attachInterrupt and manipulating the relevant port registers appropriately and defining the ISR with the ISR() macro - this will produce smaller code (using less flash and ram) and the ISRs will run faster as they don't have to check whether an interrupt is enabled for every pin on the port. For full information and example, see: Pin Interrupts
Like ATTinyCore, Sketch -> Export compiled binary will generate an assembly listing in the sketch folder; this is particularly useful when attempting to reduce flash usage, as you can see how much flash is used by different functions.
The EESAVE fuse can be controlled via the Tools -> Save EEPROM menu. If this is set to "EEPROM retained", when the board is erased during programming, the EEPROM will not be erased. If this is set to "EEPROM not retained", uploading a new sketch will clear out the EEPROM memory. You must do Burn Bootloader to apply this setting. WARNING In megaTinyCore 1.0.6 and earlier, this setting is backwards - setting it to retained will not retain, and vise versa. This will be corrected in the next release.
These parts support many BOD trigger levels, with Disabled, Active, and Sampled operation options for when the chip is in Active and Sleep modes - Disabled uses the least power, Active uses the most, and Sampled is in the middle. See the datasheet for details on power consumption and the meaning of these options. You must do Burn Bootloader to apply this setting.
This core always uses Link Time Optimization to reduce flash usage - all versions of the compiler which support the 0-series and 1-series ATtiny parts also support LTO, so there is no need to make it optional as was done with ATtinyCore.
When writing code that may be compiled for a variety of target chips (which is particularly easy on the megaAVR parts, as the peripherals are so similar), it is often useful to be able to identify which family of parts is being used, in order to, for example, select which pin numbers to use for each purpose. Each part is identified by a #define:
__AVR_ATtiny***__ where *** is the part number (ex: __AVR_ATtiny3216__).
This core provides two additional #defines for part "families":
__AVR_ATtinyx**__where ** is the last 2 digits of the part number (ex:__AVR_ATtinyx16)__AVR_ATtinyxy*__where * is the last digit of the part number, (ex:__AVR_ATtinyxy6).
This is just shorthand, for convenience - #ifdef __AVR_ATtinyxy2__ is equivilent to #if defined(__AVR_ATtiny212__) || defined(__AVR_ATtiny412__) || defined(__AVR_ATtiny202__) || defined(__AVR_ATtiny402__)
A new version of Optiboot (Optiboot-x) now runs on the Tiny0 and Tiny1 chips. It's under 512 bytes, and works on all parts supported by megaTinyCore, allowing for a convenient workflow with the same serial connections used for both uploading code and debugging (like a normal Arduino Pro Mini).
To use the serial bootloader, select a board definition with (optiboot) after it (note - this might be cut off due to the width of the menu; the second set of board definitions are the optiboot ones).
If the chip you will be programming has not been bootloaded, connect your UPDI programmer, and the desired options for clock rate (20/10/5MHz vs 16/8/4/1MHz is all that matters here. The fuses set the base clock to 20MHz or 16MHz, but the prescaler is set at startup by the sketch - so if you "burn bootloader" with 20MHz selected, but upload sketch compiled for 10MHz, that's fine and will work), Brown Out Detect (BOD), Serial port pins for the bootloader, and whether to leave the UPDI pin configured as such, or reconfigure it as a reset pin, and select Tools -> "Burn Bootloader" WARNING: After doing "Burn Bootloader", if you set the UPDI pin to act as reset, the chip cannot be reprogrammed except via the serial bootloader or using an HV UPDI programmer - it is strongly suggested to first burn the bootloader with the UPDI/Reset pin left as UPDI, verify that sketches can be uploaded, and only then "Burn Bootloader" with the UPDI/Reset pin set to act as Reset
After this, connect a serial adapter to the serial pins (as well as ground and Vcc). On the megaavr ATtiny breakout boards which I sell on Tindie, a standard 6-pin "FTDI" header is provided for this that can be connected directly to many serial adapters.
If the UPDI/Reset pin option was set to UPDI when bootloading, you must unplug/replug the board (or use a software reset - see note near start of readme; you can make an ersatz reset button this way) to enter the bootloader - after this the bootloader will be active for 8 seconds.
If the UPDI/Reset pin option was set to reset, you must reset the chip via the reset pin (or software reset to enter the bootloader, and the bootloader will run for 1 second before jumping to the application). This is the same as how optiboot works on classic AVR boards. The standard DTR-autoreset circuit is recommended (this is how boards I sell on Tindie with Optiboot preloaded are configured).
Serial uploads are all done at 115200 baud, regardless of speed or part.
If using the bootloader with with the UPDI pin set to reset, this is needed for uploading. It will reset the chip when serial connection is opened like on typical Arduino boards. Do not connect an autoreset circuit to the UPDI pin if it is not set to act as reset, as this will not reset the chip, and will just block UPDI programming.
- 1 Small signal diode (specifics aren't important, as long as it's approximately a standard diode)
- 1 0.1uF Ceramic Capacitor
- 1 10k Resistor
- An ATtiny megaavr with the bootloader installed on it (you can't do UPDI programming once the autoreset circuit is connected the the UPDI/Reset pin, even if that pin hasn't yet been set to act as reset, as the autoreset circuit will interfere with UPDI programming).
Connect the DTR pin of the serial adapter to one side of a 0.1uF ceramic capacitor. Connect the other side of the capacitor to the Reset (formerly UPDI) pin. Connect the 10k resistor between Reset and Vcc. Connect the diode between Reset and Vcc with the band towards Vcc.
The breakout boards I sell on Tindie have a pair of pads on the back that can be bridged with solder to reversibly connect the on-board autoreset circuit to the UPDI/Reset pin. If you buy a breakout board from me which has the autoreset circuit, but which is not bootloaded, these pads will not have been connected - use a UPDI programmer to put the bootloader on it with your desired settings, then bridge the pads on the back to connect the auto-reset circuit. If you buy a breakout board from me with Optiboot preloaded, it will come set for 20/10/5MHz, BOD set to "sampled" mode with a threshold of 3.7v (for 5v boards) or 2.6v (for 3.3v boards) or disabled entirely (for no-regulator boardds), and if you ordered a board with the autoreset circuit, the UPDI pin will be set to work as reset (if you need different settings, please mention this when ordering - boards with optiboot preloaded are bootloaded only when the order is packed, so I can use whatever settings you specify).
Note that, if you have UPDI programming enabled, and desire the convenience of autoresetting to enter serial bootloader to upload sketches, or resetting the chip when entering the serial monitor, you can use the technique described near the top of the readme (under the section about there not being a dedicated reset pin) to make an ersatz reset pin that does a software reset when held low - this will enter the bootloader, so if your sketches set up an ersatz reset pin, you can use that to get into the bootloader (this isn't foolproof, because if your sketch chokes before it properly sets up the interrupt, or you choked and forgot to include the ersatz reset code in the last sketch you uploaded - but in this case you can enter bootloader by power cycling as described above). The autoreset circuit works the same way - just connect the parts as described above, only using your ersatz reset pin instead of the UPDI/Reset pin.
- The bootloader is at the beginning of memory, rather than at the end (where it was on older chips). Thus, the start of the application code must be 512b after the start of the memory - this is handled by the core, but you cannot upload a .hex file compiled with a non-optiboot board definition to an optiboot board definition and vise versa.
- If you have set the UPDI/Reset pin to act as a reset pin, you can no longer program the part via UPDI without using an HV programmer to reset the pin to act as UPDI.
- Currently, Optiboot_x resets the reset cause register after saving the contents in R2.
- The new chips have more than one option for Uart Pins. The option selected when you "burn bootloader" determines which version is used. If using the breakout boards I sell on Tindie, the first menu option uses the pins connected to the 6-pin FTDI-style serial header.
- When you "burn bootloader", the base oscillator frequency is set according to the selected clock speed, but the actual operating speed while running the sketch is set in the uploading code. If you initially set it to 16/8/4/1MHz, you may use any of those options when you upload your sketch and it will run at that speed; if you initially set it to 20/10/5MHz, you may use any of those options. If you wish to change between 16/8/4/1MHz and 20/10/5MHz, you must burn bootloader again - failure to do so will result in it running at the wrong speed, and all timing will be wrong.
- The "size" of the sketch as reported by avrdude during the upload process is 512b larger (the size of the bootloader) than the actual sketch size when a bootloader is in use. The fact that the bootloader goes at the start of the flash instead of the end confuses avrdude. The size displayed by the IDE when you "verify" a sketch is correct, the value that avrdude displays is not.
- Tools -> Chip - sets the specific part within a selected family to compile for and upload to.
- Tools -> Clock Speed - sets the clock speed. You must burn bootloader after changing between 16/8/4/1MHz and 20/10/5MHz to apply the changes (ie, changing from 20MHz to 10MHz does not require burning bootloader, changing from 20MHz to 16MHz does). A virgin chip will be set to use the 20MHz internal oscillator as its base clock source, so 20/10/5MHz clock speed options should work without an initial "burn bootloader" - though we still recommend it to ensure that all other fuses are set to the values you expect.
- Tools -> Retain EEPROM - determines whether to save EEPROM when uploading a new sketch. You must burn bootloader after changing this to apply the changes. This option is not available on Optiboot board definitions - programming through the bootloader does not execute a chip erase function.
- Tools -> UART pins - All of these parts have the potential to remap the UART pins. This menu option sets which pins will be used as TX and RX by default. If Optiboot is being used, the selection from this menu when you "burn bootloader" will set which set of pins will be used by Optiboot. However, once the bootloader has been uploaded, you may change this if you need the sketch to use the other set of pins - the pins chosen when the bootloader was burned will still be used for uploads, but the current selection will be used by the sketch.
- Tools -> B.O.D. Voltage - If Brown Out Detection is enabled, when Vcc falls below this voltage, the chip will be held in reset. You must burn bootloader after changing this to apply the changes. Take care that these threshold voltages are not exact - they may vary by as much as +/- 0.3v! (depending on threshold level - see electrical characteristics section of datasheet). Be sure that you do not select a BOD threshold voltage that could be triggered during programming, as this can prevent successful programming via UPDI (reported in #86).
- Tools -> UPDI/Reset pin - This menu is only available on board definitions that use the Optiboot bootloader. If set to UPDI, the pin will be left as the UPDI pin, there will be no hardware reset pin - to enter the bootloader, disconnect and reconnect power to the part, and upload within 8 seconds. If set to Reset, the pin will be configured to act as reset, like a classic AVR, but UPDI programming will no longer work - you must use an HV programmer if you wish to reenable UPDI - if the pin is set to reset, the version of Optiboot uploaded will jump straight to the application after a power-on reset, and will only enter the bootloader if reset by software reset or via the reset pin. The bootloader will also only wait 1 second for a sketch (ie, it behaves like optiboot does on classic AVR microcontrollers), instead of 8.
- Tools -> B.O.D. Mode (active) - Determines whether to enable Brown Out Detection when the chip is not sleeping. You must burn bootloader after changing this to apply the changes.
- Tools -> B.O.D. Mode (sleep) - Determines whether to enable Brown Out Detection when the chip is sleeping. You must burn bootloader after changing this to apply the changes.
- Tools -> DAC Reference Voltage - Determines the voltage reference for the DAC. This should be less than Vcc to get the right voltage. You do not need to use Burn Bootloader to apply changes to this menu.
- Tools -> tinyNeoPixel Port - If using the tinyNeoPixel library (see above), and you are running at 8 or 10 MHz, you must set this option to the port with the pin(s) you are using. Not present on the 8-pin parts, as they only have one port. If not using tinyNeoPixel library or running at 16MHz or more, this option can be ignored.
- Tools -> Voltage for UART Baud - Controls which oscillator error values will be used to maximize the accuracy of the UART baud rate generation - choose whether operating voltage is closer to 5v or 3v.
- Tools -> millis()/micros() - If set to enable (default), millis(), micros() and pulseInLong() will be available. If set to disable, these will not be available, Serial methods which take a timeout as an argument will not have an accurate timeout (though the actual time will be proportional to the timeout supplied); delay will still work. Disabling millis() and micros() saves flash, and eliminates the millis interrupt every 1-2ms; this is especially useful on the 8-pin parts which are extremely limited in flash. Depending on the part, options to force millis/micros onto specific timers are available. A #error will be shown upon compile if a specific timer is chosen but that timer does not exist on the part in question. If RTC is selected, micros() andpulseInLong() will not be available - only millis() will be.
- Sometimes a sketch which is too big to fit will, instead of generating a message saying that, result in the error: 'relocation truncated to fit: R_AVR_13_PCREL against symbol tablejump2'.
I sell breakout boards with regulator, UPDI header, and Serial header in my tindie shop, as well as the bare boards. Buying from my store helps support further development on the core, and is a great way to get started using these exciting new parts with Arduino.