ATtiny 48/88

Specification	ATtiny88	ATtiny88	ATtiny88	ATtiny48	ATtiny48
Bootloader (if any)		Optiboot	Micronucleus		Optiboot
Uploading uses	ISP/SPI pins	Serial Adapter	USB (directly)	ISP/SPI pins	Serial Adapter
Flash available user	8192 bytes	7552 bytes	6588 bytes	4096 bytes	3456 bytes
RAM	512 bytes	512 bytes	512 bytes	256 bytes	256 bytes
EEPROM	64 bytes	64 bytes	64 bytes	64 bytes	64 bytes
GPIO Pins	26 + RESET	26 + RESET	25 + RESET	26 + RESET	26 + RESET
ADC Channels	8 (6 in DIP)	8 (6 in DIP)	8	8 (6 in DIP)	8 (6 in DIP)
PWM Channels	2 (9, 10)	2 (9, 10)	2 (9, 10)	2 (9, 10)	2 (9, 10)
Interfaces	SPI, I2C	SPI, I2C	vUSB, SPI, I2C	SPI, I2C	SPI, I2C
Clocking Options:	in MHz	in MHz	in MHz	in MHz	in MHz
Int. Oscillator	8, 4, 2, 1	8, 4, 2, 1	Not supported	8, 4, 2, 1	8, 4, 2, 1
Int. WDT Oscillator	128 kHz	Not supported	Not supported	128 kHz	Not supported
Internal, with tuning	8, 12	8, 12	Not supported	8, 12	8, 12
External Crystal	Not supported	Not supported	Not supported	Not supported	Not supported
External Clock	All Standard	All* Standard	16,8,4,2,1	All* Standard	All Standard
Default Pin Mapping	Standard	Standard	MH-Tiny	Standard	Standard
LED_BUILTIN

* 20 MHz operation not supported. These parts are not even rated for 16!

USB only available at BOLD clock. Other frequencies when using Micronucleus are generated by prescaling this 16 MHz clock. Notes on this table.

The ATtiny x8-family is intended as a low cost option compatible with the popular ATmega x8 series. As such, they have a nearly identical pinout (with a couple of extra GPIO pins in the TQFP version). Although these have the full hardware I2C and SPI peripherals, they lack both a hardware serial port and the option to use a crystal as a clock source. A Micronucleus board is available with a 16 MHz external CLOCK under the name "MH Tiny" (yes, that is overclocked, maximum is spec'ed at 12 MHz. But they seem to work under typical conditions though). They use a pin numbering scheme that differs significantly from the standard one; a pin mapping is provided which matches the markings on the board.

Programming

Any of these parts can be programmed by use of any ISP programmer. 4k and 8k parts can be programmed over the software serial port using Optiboot, and 8k parts can be programmed via Micronucleus. Be sure to read the section of the main readme on the ISP programmers and IDE versions. 1.8.13 is recommended for best results.

Optiboot Bootloader

This core includes an Optiboot bootloader for the ATtiny88/48, operating using software serial at 19200 baud - the software serial uses the AIN0 and AIN1 pins, marked on pinout chart (see also UART section below). The bootloader uses 640b of space, leaving 3456 or 7552b available for user code. In order to work on the 88/48, which does not have hardware bootloader support (hence no BOOTRST functionality), "Virtual Boot" is used. This works around this limitation by rewriting the vector table of the sketch as it's uploaded - the reset vector gets pointed at the start of the bootloader, while the EE_RDY vector gets pointed to the start of the application.

Micronucleus Bootloader

The Micronucleus bootloader for these parts uses a 16 MHz external clock source. Boards are commercially available (and cheap) under the name MH-Tiny (also MH-ET and several other names). USB is on pins 1 and 2 (PIN_PD1 and PIN_PD2, and the LED is on pin 0; there are slight differences in the numbering of pins. Additionally, the As of 1.4.1, the new entry mode options detailed in Using Micronucleus are available for the Tiny88 (MH-ET). Be aware that there are many bootloaders circulating, including the ones shipped with 1.4.0, which do not actually work on the MH-ET boards; if you have uploaded one of those, you can restore functionality by bootloading using an ISP programmer (provided you hadn't previously disabled reset, of course).

Alternate pinout options

The MH Tiny boards have pins labeled with a different pin mapping. Pins up to 13 (all of PORTD and first 6 pins of PORTB) are the same, PB6 is not available because it is the clock input, and from there on out, order is different as well. The pinout can be selected from the Tools -> Pin Mapping submenu, regardless of which bootloader, if any, is in use. This way if you are (for example) using the MH-Tiny hardware, but programming it via ISP (for example) you can choose to use the pin mapping that matches the numbers printed on the board. The standard pin mapping is the default if not using Micronucleus bootloader; if using that, we of course default to the MH-Tiny pin mapping.

Pin Mapping	Standard x8	MH-Tiny
LED_BUILTIN	PB5 (pin 13)	PD0 (pin 0)
Pins Missing		PB6 (ext clk)
An = ADC chan	Yes	Yes

Clock options

The ATtiny x8-family of microcontrollers, in the interest of lowering costs, does not provide support for using an external crystal as a clock source, only the internal oscillator (at ~8 or ~1 MHz) or an external clock source. The internal oscillator is only guaranteed to be within 10% of the targeted speed across the operating temperature and voltage range. At normal operating conditions (3.3-5.0V, room temperature) they are generally quite a bit closer, usually close enough for Serial (which is software serial here, as noted below) to work. .

Using external CLOCK on 48/88

These parts do not support using an external crystal (cost savings - with 28 I/O pins, hardware I2C and SPI, and a headline price like this, some things had to give). External clock, however, is supported - this requires an external clock generator (not just a crystal) connected to PB6 (CLKI). These usually come in the shiny rectangular metal package (shielding, same as on crystals), only instead of 2 terminals, or 4 terminals of which 2 are unconnected, these are almost universally use all 4 pins - Vcc, Gnd, CLKOUT, and Enable; Enable is generally active-high, and internally weakly pulled up. Be aware that if you "burn bootloader" with an external clock selected, but you have actually connected a crystal, (they are virtually impossible to tell apart visually, except by positively identifying the part and hunting down the specs), the chip cannot be programmed until you give it a clock signal (you will get a signature mismatch, and if you enable verbose uploads, which you should do anyway, it will report the signature as 0x000000). This means removing what you hoped was an external clock, and connecting a clock signal to PB6 (a 2-8 MHz square wave would be appropriate; The included enhanced ArduinoAsISP++ sketch outputs such a signal on PB2 (digital pin 10) while burning the bootloader.

Micronucleus clock options

Micronucleus is supported with an external 16 MHz external clock only. It may optionally be prescaled to 8, 4, or 1 MHz for low power applications; this is generated by prescaling the 16 MHz clock after the application starts, at these lower clock speeds, VUSB functionality is not supported. It is also less power efficient since the oscillator needs to keep running. For the same reason, the power savings possible through sleep are very limited (much of the savings from power down sleep are possible because the oscillator is turned off. Well, that's not happening here!.

PWM frequency

TC0 is always run in Fast PWM mode: We use TC0 for millis, and phase correct mode can't be used on the millis timer - you need to read the count to get micros, but that doesn't tell you the time in phase correct mode because you don't know if it's upcounting or downcounting in phase correct mode.

F_CPU	No PWM from TC0	F_PWMTC1	Notes
1 MHz	-	1/8/256= 488 Hz
2 MHz	-	2/8/256= 977 Hz
<4 MHz	-	x/8/512= 244 * x Hz	Phase correct TC1
4 MHz	-	4/8/512= 977 Hz	Phase correct TC1
<8 MHz	-	x/8/512= 244 * x Hz	Between 4 and 8 MHz, the target range is elusive
8 MHz	-	8/64/256= 488 Hz
>8 MHz	-	x/64/256= 61 * x Hz
12 MHz	-	12/64/256= 735 Hz
16 MHz	-	16/64/256= 977 Hz
>16 MHz	-	x/64/512= 31 * x Hz	Phase correct TC1
20 MHz	-	20/64/512= 610 Hz	Phase correct TC1

Phase correct PWM counts up to 255, turning the pin off as it passes the compare value, updates it's double-buffered registers at TOP, then it counts down to 0, flipping the pin back as is passes the compare value. This is considered preferable for motor control applications, though the "Phase and Frequency Correct" mode is better if the period is ever adjusted by a large amount at a time, because it updates the doublebuffered registers at BOTTOM, and thus produces a less problematic glitch in the duty cycle, but doesn't have any modes that don't require setting ICR1 too.

For more information see the Changing PWM Frequency reference.

Tone Support

Tone() uses Timer1. For best results, use PB1 or PB2, as this will use the hardware output compare to generate the square wave instead of using interrupts. Using tone will disable PWM functionality (as timer1 is the only PWM capable timer on these parts)

Servo Support

The standard Servo library is hardcoded to work on specific parts only, we include a builtin Servo library that supports the Tiny x8 series. As always, while a software serial port (including the builtin one, Serial, on these ports, see below) is receiving or transmitting, the servo signal will glitch. See the Servo/Servo_ATTinyCore library for more details. Like tone(), this will disable PWM on PB1 or PB2. Tone and Servo cannot be used at the same time.

I2C Support

There is full Hardware I2C! It is provided by Wire.h Do not attempt to use third party I2C libraries designed for ATtiny parts, they are designed for parts that have other hardware instead of a proper I2C port, that is not the case for these devices.

SPI Support

There is full Hardware SPI! It is provided by SPI.h. Do not attempt to use third party SPI libraries designed for ATtiny parts, they are designed for parts that have other hardware instead of a proper SPI port, that is not the case for these devices.

UART (Serial) Support

There is no hardware UART. The core incorporates a built-in software serial named Serial - this uses the analog comparator pins, in order to use the Analog Comparator's interrupt, so that it doesn't conflict with libraries and applications that require PCINTs. TX is defaults to AIN0 (PD6), RX is always AIN1 (PD7). Although it is named Serial, it is still a software implementation, so you cannot send and receive at the same time. The SoftwareSerial library may be used; if it is used at the same time as the built-in software Serial, only one of them can send or receive at a time (if you need to be able to use both at the same time, or send and receive at the same time, you must use a device with a hardware UART).

If running off the internal oscillator (since this chip does not support a crystal), you may need to calibrate it to get the speed close enough to the correct speed for UART communication to work, though as noted above, this is rarely necessary under typical operating conditions. While one should not attempt to particularly high baud rates out of the software serial port, there is also a minimum baud rate as well

Though TX defaults to AIN0, it can be moved to any pin on PORTD using Serial.setTxBit(b) where b is the number in the pin name using Pxn notation (only pins on PORTD are valid, and unless it is set for TX only, AIN1 (PD7) is not valid) (2.0.0+ only - was broken in earlier versions).

To disable the RX channel (to use only TX), select "TX only" from the Builtin SoftSerial tools menu. To disable the TX channel, simply don't print anything to it, and set it to the desired pinMode after Serial.begin()

ADC Features

The ATtiny88 ADC is about as boring as they come. It's a standard 8-channel (6 on DIP or 28-pin QFN packages), one reference, temp sensor. Single-ended only. Basically what any x8 atmega has too, which is also incredibly uninspiring.

ADC Reference options

Just two options on the t88/48. We recommend "INTERNAL1V1" to refer to that reference, as parts have multiple references of different voltages.

Reference Option	Reference Voltage	Uses AREF Pin
DEFAULT	Vcc	There is no AREF pin
INTERNAL1V1	Internal 1.1V reference	n/a
INTERNAL	Alias of INTERNAL1V1	n/a

Internal Sources

Voltage Source	Description
ADC_INTERNAL1V1	Reads the INTERNAL1V1 reference
ADC_GROUND	Reads ground - for offset measurement?
ADC_TEMPERATURE	Reads internal temperature sensor

Temperature Measurement

To measure the temperature, select the 1.1v internal voltage reference, and analogRead(ADC_TEMPERATURE); This value changes by approximately 1 LSB per degree C. This requires calibration on a per-chip basis to translate to an actual temperature, as the offset is not tightly controlled - take the measurement at a known temperature (we recommend 25C - though it should be close to the nominal operating temperature, since the closer to the single point calibration temperature the measured temperature is, the more accurate that calibration will be without doing a more complicated two-point calibration (which would also give an approximate value for the slope)) and store it in EEPROM (make sure that EESAVE fuse is set first, otherwise it will be lost when new code is uploaded via ISP) if programming via ISP, or at the end of the flash if programming via a bootloader (same area where oscillator tuning values are stored). See the section below for the recommended locations for these.

Tuning Constant Locations

These are the recommended locations to store tuning constants. In the case of OSCCAL, they are what are checked during startup when a tuned configuration is selected.

ISP programming: Make sure to have EESAVE fuse set, stored in EEPROM

Optiboot used: Saved between end of bootloader and end of flash.

Tuning Constant	Location EEPROM	Location Flash
Temperature Offset	E2END - 3	FLASHEND - 5
Temperature Slope	E2END - 2	FLASHEND - 4
Tuned OSCCAL 12 MHz	E2END - 1	FLASHEND - 3
Tuned OSCCAL 8 MHz	E2END	FLASHEND - 2
Bootloader Signature 1	Not Used	FLASHEND - 1
Bootloader Signature 2	Not Used	FLASHEND

Mironucleus used: Micronucleus boards are locked to the external clock, no oscillator calibration is possible.

Tuning Constant	Location Flash
Temperature Offset	FLASHEND - 1
Temperature Slope	FLASHEND

Interrupt Vectors

This table lists all of the interrupt vectors available on the ATtiny x8-family, as well as the name you refer to them as when using the ISR() macro. Be aware that a non-existent vector is just a "warning" not an "error" (for example, if you misspell a vector name) - however, when that interrupt is triggered, the device will (at best) immediately reset (and not cleanly - I refer to this as a "dirty reset") The catastrophic nature of the failure often makes debugging challenging.

Note: The shown addresses below are "byte addressed" as that has proven more readily recognizable. The vector number is the number you are shown in the event of a duplicate vector error, as well as the interrupt priority (lower number = higher priority), if, for example, several interrupt flags are set while interrupts are disabled, the lowest numbered one would run first. Notice that INT0 is (as always) the highest priority interrupt. All of the parts in this family are 8k or less flash, so they do not need to use 4-byte vectors.

num	Address	Vector Name	Interrupt Definition
0	0x0000	RESET_vect	Not an interrupt - this is a jump to the start of your code.
1	0x0002	INT0_vect	External interrupt request 0
2	0x0004	INT1_vect	External interrupt request 1
3	0x0006	PCINT0_vect	Pin Change Interrupt 0 (PORT A)
4	0x0008	PCINT1_vect	Pin Change Interrupt 1 (PORT B)
5	0x000A	PCINT2_vect	Pin Change Interrupt 2 (PORT C)
6	0x000C	PCINT3_vect	Pin Change Interrupt 3 (PORT D)
7	0x000E	WDT_vect	Watchdog Time-out (Interrupt Mode)
8	0x0010	TIMER1_CAPT_vect	Timer/Counter1 capture event
9	0x0012	TIMER1_COMPA_vect	Timer/Counter1 compare match A
10	0x0014	TIMER1_COMPB_vect	Timer/Counter1 compare match B
11	0x0016	TIMER1_OVF_vect	Timer/Counter1 overflow
12	0x0018	TIMER0_COMPA_vect	Timer/Counter0 compare match A
13	0x001A	TIMER0_COMPB_vect	Timer/Counter0 compare match B
14	0x001C	TIMER0_OVF_vect	Timer/Counter0 overflow
15	0x001E	SPI_STC_vect	SPI serial transfer complete
16	0x0020	ADC_vect	ADC conversion complete
17	0x0022	EE_RDY_vect	EEPROM ready
18	0x0024	ANA_COMP_vect	Analog comparator
19	0x0026	TWI_vect	2-wire serial interface