ATtiny84 Guide

Overview

The ATtiny84 is a compact, low-power 8-bit microcontroller from Microchip's AVR family, designed for embedded systems where space and energy efficiency are critical. Part of the ATtiny series, it bridges the gap between smaller models like the ATtiny85 (with fewer I/O pins) and larger AVRs like the ATmega328P, offering a balanced mix of I/O flexibility, memory, and peripherals. Its RISC architecture executes most instructions in a single clock cycle, achieving up to 20 MIPS (million instructions per second) at 20 MHz. With its robust feature set, the ATtiny84 is a go-to choice for hobbyists and engineers needing a small yet capable MCU for constrained environments. Its low cost (typically under $2 in small quantities) and wide voltage range make it versatile for both prototyping and production.

Specifications

CPU: 8-bit AVR RISC (135 instructions, 32×8 general-purpose registers).

Clock Speed: Up to 20 MHz (at 4.5–5.5V) or 10 MHz (at 2.7–5.5V), with internal 8 MHz oscillator (calibrated to ±10%, tunable to ±2% with OSCCAL register).

Memory:

- 8 KB Flash (10,000 write/erase cycles, self-programmable).

- 512B SRAM (volatile, for runtime data).

- 512B EEPROM (100,000 write/erase cycles, non-volatile storage).

I/O Pins: 12 GPIOs (8 on Port A, 4 on Port B), with 25 mA sink/source per pin (40 mA total per port).

Analog:

- 10-bit ADC (8 channels, 15 kSPS at 200 kHz ADC clock).

- Analog comparator (with programmable interrupt).

Timers:

- Two 8-bit timers (Timer0, Timer1) with PWM (4 outputs total).

- One 16-bit timer (Timer1 in some modes, with prescaler up to 1:16384).

Communication:

- Hardware USART (full-duplex, up to 1 Mbps at 20 MHz).

- SPI (via 4-pin Universal Serial Interface, USI).

- I²C (USI-based, bit-bangable with software libraries).

Voltage: 1.8–5.5V (operational), with brown-out detection (configurable at 1.8V, 2.7V, or 4.3V).

Packages: DIP-14 (through-hole), SOIC-14 (surface-mount), and 15-pad QFN/MLF (4×4 mm, ultra-compact).

Key Features

Ultra-Low Power: Sleep modes include Idle (CPU off, peripherals on), ADC Noise Reduction (quiet ADC sampling), and Power-down (everything off except WDT/interrupts), achieving <0.1 µA current in Power-down at 1.8V.

PWM & Control: Four PWM channels (up to 20 kHz at 8-bit resolution) for motor control, LED dimming, or audio generation (e.g., square wave synthesis).

Analog Sensing: 10-bit ADC with temperature sensor input (internal, ~1°C accuracy after calibration), and internal voltage references (1.1V or 2.56V, ±2% accuracy).

Robust Communication:

- USART: Full-duplex serial for UART communication, ideal for interfacing with GPS modules or serial displays.

- SPI/I²C: Shared via USI module, configurable for master/slave modes; supports devices like OLED screens (I²C) or SD cards (SPI).

Resilience: Watchdog timer (8 timeout options from 16 ms to 8 s), brown-out detection, and internal reset circuitry for reliable operation in harsh conditions.

Arduino Support: Compatible with Arduino IDE via ATTinyCore, enabling easy prototyping with a familiar ecosystem.

Applications

Remote Sensors: Soil moisture detection (ADC with resistive probes), PIR motion sensors (GPIO interrupts), or LoRa-based edge nodes (USART to RF module).

Battery-Powered IoT: Door/window alarms (reed switch on interrupt pins), handheld game controllers (PWM for buzzers), or solar-powered trackers (1.8V operation with coin cells).

Wearables: Heart-rate monitors (ADC for photoplethysmography + PWM for haptic feedback), smart jewelry with Neopixel control (bit-banged SPI).

Automation: Relay controllers (GPIO with transistor drivers), WS2812B LED strips (bit-banged protocol on any pin), or HVAC fan speed regulation (PWM with optocouplers).

DIY Electronics: Breadboard-friendly projects like macro keyboards (GPIO matrix), IR remotes (PWM for 38 kHz carrier), or binary clocks (multiplexed LED displays).

Pinout Deep Dive

Notable Peripherals by Pin:

- PWM: PB1, PB2 (Timer0); PA5, PA6, PA7 (Timer1 in specific modes).

- Interrupts: All PA0–PA3 pins support pin-change interrupts; PB2 can trigger external INT0.

- Clock Options: PA6/PA7 for external crystal (up to 20 MHz), PB0 for external clock signal.

Programming & Development

Tools:

- Arduino as ISP: Use an Uno/Nano with the ArduinoISP sketch to program via 6-pin ISP header.

- USBasp: Affordable USB programmer; supports 10-pin or 6-pin ISP connections.

- PlatformIO: Advanced IDE with debug support (via Atmel-ICE or SimAVR simulator).

- AVR-GCC: Bare-metal programming with avrdude for flashing and full control.

Workflow Tips:

- Fuse Configuration: Set clock source (internal 8 MHz, external crystal, or CLKI) and disable RESET pin (PB3) if needed via RSTDISBL fuse. Use AVR Fuse Calculator or avrdude commands (e.g., -U lfuse:w:0xE2:m for 8 MHz internal).

- Arduino Setup: Install ATTinyCore, select "ATtiny24/44/84" board, choose clock speed (e.g., 8 MHz internal, no bootloader to save Flash).

- Debugging: Serial.print() not natively supported; use SoftwareSerial on PA0/PA1 or toggle pins for LED debugging.

Example Code Snippet (Blink with PWM Dimming):

void setup() {
  pinMode(PB1, OUTPUT); // PB1 = Pin 10 (PWM)
  TCCR0A = (1 << COM0B1) | (1 << WGM00); // Phase-correct PWM
  TCCR0B = (1 << CS01); // Prescaler 8
}

void loop() {
  for (int brightness = 0; brightness <= 255; brightness += 5) {
    OCR0B = brightness; // Set PWM duty cycle
    delay(10);
  }
  for (int brightness = 255; brightness >= 0; brightness -= 5) {
    OCR0B = brightness;
    delay(10);
  }
}

Power Optimization

- Sleep Modes: Use SLEEP_MODE_PWR_DOWN with external interrupts (e.g., pin change on PA0) to wake, reducing idle current to <100 nA at 1.8V.

- Peripheral Shutdown: Disable ADC with ADCSRA &= ~(1<, disable brown-out detection with sleep_bod_disable(); in AVR sleep.h.