ATtiny85: DIP-8 pinout, clock limits, pin current and programming

The through-hole part is the ATtiny85-20PU: 20 for the 20 MHz speed grade, P for the 8-lead 0.3 inch PDIP, U for matte tin. ATtiny85-20SU is the same chip in an 8-lead SOIC with the same pin numbers, and -MU is a 20-pad QFN.

A V after the number (ATtiny85V-10PU) marks the low-voltage grade: it starts at 1.8 V instead of 2.7 V but never runs faster than 10 MHz. The ATtiny45 and ATtiny25 are the same chip with 4 KB and 2 KB of flash. All of them print only the part number on top, so the reliable test is the signature a programmer reads: 0x1E 0x93 0x0B for an ATtiny85.

Only Microchip (formerly Atmel) makes the ATtiny85, so the disagreement worth knowing is inside the one datasheet. The maximum ratings allow 6.0 V and 40 mA per pin, but the chip is specified only to 5.5 V, and the output voltages are guaranteed only at 10 mA per pin at 5 V. Note 4 under Table 21-1 goes further: pins are "not guaranteed to sink current greater than the listed test condition." Design to 5.5 V and 10 mA.

RESET is the one pin that tolerates more than the supply, up to 13 V, because a high-voltage programmer uses 12 V on it. Every other pin is held to 0.5 V outside the rails, so a 5 V signal must not reach the chip when it runs from 3 V.

Between the corners the line is straight. From 10 MHz at 2.7 V to 20 MHz at 4.5 V the clock rises 10 MHz over 1.8 V, so the chip's own 16 MHz PLL clock needs 2.7 + (16 − 10) × 1.8 / 10 = 3.78 V. At 3.3 V the limit is 10 + 0.6 × 10 / 1.8 = 13.3 MHz. On a 3.3 V or battery supply, 8 MHz from the internal oscillator stays inside the rating. The V grade is different again: 4 MHz at 1.8 V, 10 MHz from 2.7 V upward.

At 5 V an input reads low below 1.5 V and high above 3.0 V; at 3 V the high threshold drops to 1.8 V. See the logic levels lesson and the 3.3 V to 5 V level shifter guide when the ATtiny85 talks to a board on another supply.

• Small jobs that would waste an Arduino: a blinker, a fan or pump timer, a button that sends one I²C command, an LED effect. Five free pins once RESET is kept.
• Battery gadgets. In power-down at 3 V it draws at most 10 µA with the watchdog running and 2 µA without it.
• Fast PWM. Timer1 can run from the 64 MHz PLL, which gives PWM well above the audio band for LED dimming and small converters, and a dead-time generator for driving half-bridges.
• Reading up to four analog inputs (ADC1 to ADC3, plus ADC0 on the RESET pin).

It is the wrong chip for anything needing a hardware UART, USB, or more than five pins. The USI can do SPI and I²C, but it needs more code than a real TWI port. For more pins in a breadboard-friendly package, see the ATmega328P page.

The smallest working circuit: a 5 V supply, one decoupling capacitor, a reset pull-up and an LED on PB0. No crystal: the chip runs on its internal oscillator.

On batteries the sleeping chip is rarely what empties them. At the 10 µA maximum with the watchdog waking it, it uses 0.24 mAh a day; the LED, a regulator or a sensor left powered usually costs far more. The LED resistor calculator does step 4 for other colours.

The ATtiny85 has no USB and no bootloader as shipped. It is programmed over SPI: hold RESET low and clock instructions in on MOSI (pin 5) and SCK (pin 7), with answers on MISO (pin 6). An Arduino Uno running the ArduinoISP example sketch does this; the sketch uses D10 to drive the target's RESET.

The programmer's clock is limited by the chip's own clock: SCK high and low must each last more than 2 CPU cycles below 12 MHz (p. 151). On the factory 1 MHz that means under 250 kHz; once the chip runs at 8 MHz, up to 2 MHz. Setting the clock fuses (the Arduino IDE's "Burn Bootloader" does this for ATtiny cores) is a separate step from uploading the program, and it has to be done once per chip.

• Keep pins 5, 6 and 7 free of heavy loads if you want to reprogram the chip in place. An LED with 330 Ω is fine; a capacitor or a low-value pull-down on SCK or MOSI can stop the programmer.
• Plan on 10 mA per pin at 5 V and 5 mA at 3 V, the currents the output voltages are specified at. For a relay, motor or LED strip, drive a transistor; the MOSFET guide for Arduino and ESP32 picks one.
• The internal oscillator is ±10% as shipped (±1% after calibration), too loose for reliable serial timing. Calibrate OSCCAL or use a crystal on PB3 and PB4, which costs two of your six pins.
• Use the internal pull-ups (20 to 50 kΩ) for buttons. For I²C on PB0 (SDA) and PB2 (SCL), fit external pull-ups.
• Below 2.7 V the PLL needs its low-speed mode, and the standard ATtiny85 is out of its rating altogether. A falling battery needs the V grade or the brown-out detector set to 1.8 or 2.7 V.

The ATtiny45 is the same chip with half the memory: 4 KB of flash and 256 bytes each of EEPROM and SRAM, against 8 KB and 512 bytes on the ATtiny85. Pins, peripherals, clock limits and pin currents are identical, so every number on this page applies. A program that fits in 4 KB runs unchanged once the programmer is set for the 45 (signature 0x1E 0x92 0x06). The ATtiny25 halves it again, to 2 KB.

1. With power on, measure 5 V between pin 8 and pin 4, and about 5 V on pin 1. RESET sitting low means a shorted button or a missing pull-up.
2. Wire the programmer as in Figure 4 and read the signature: 0x1E 0x93 0x0B for an ATtiny85. On a new chip, slow the programmer below 250 kHz first. No answer at all usually means a loose RESET wire or a load on pin 5, 6 or 7.
3. Load a blink program built for 8 MHz with a one-second delay. If each blink takes 8 seconds, the chip is still on its 1 MHz factory clock: set the fuses.
4. For a direct look, the CKOUT fuse puts the system clock out on PB4 (pin 3), where a frequency counter should read the clock you set, within the oscillator's ±10%.

Buy the ATtiny85-20PU, read its signature, and set the clock fuses to 8 MHz before it goes into a board. Build it as Figure 2: 100 nF across pins 8 and 4, 10 kΩ on RESET, and 10 mA or less per pin. Leave RESET alone, keep pins 5 to 7 lightly loaded so you can reprogram in place, and use the V grade if the supply can drop below 2.7 V.

The DIP ATtiny85-20PU is stocked by most Indian online component shops, and so are small USB boards built around it. Read the print on loose chips (ATtiny85, not 45 or 25) and check the signature when they arrive. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes. Before a first dimmer project, read the duty cycle and PWM lesson.