Timers by hand
Timer 1 set up register by register: a prescaler, CTC mode and OCR1A for a match every 500 ms, the compare flag polled and cleared by hand, and no millis() anywhere.
Open it in the simulatorAll paths
Under the hood
Take the Arduino wrapper off: DDR, PORT and PIN, Timer 1 by hand, the vector table and interrupts, cycle-counted delays in a .S file, a bug read off a halt in the debugger, and a UART drawn by hand on a pin.
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What you will build
2 projects, built step by step
6 lessons in 4 chapters, about 5 hours on a simulated Arduino Uno R3. Free, in your browser, every step checked by the simulator.
Reading a halt in the debugger
Ten readings of 30 that average 4: stop the Uno on a breakpoint, watch a variable climb, step one instruction and read the carry flag in SREG, then fix the bug the halt showed you.
Open it in the simulatorThe map
Chapter by chapter
Each lesson opens when the one before it is done, and each chapter ends in a project.
Chapter 1
Registers
Pins and a timer, written register by register.
- Open265 XP
DDR, PORT and PIN
The three registers behind every digital pin: count in binary on three lamps with one write to PORTB, turn on a pull-up through PORTD, and read a button straight out of PIND.
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- ProjectLocked530 XP
Timers by hand
Timer 1 set up register by register: a prescaler, CTC mode and OCR1A for a match every 500 ms, the compare flag polled and cleared by hand, and no millis() anywhere.
0 of 6 steps
Chapter 2
Interrupts
The vector table and handlers that run while loop() sleeps.
- Locked265 XP
The vector table and interrupts
The 26 jumps at the start of flash, read out of the chip itself: a Timer 1 interrupt that blinks while loop() sleeps, an INT0 handler that counts presses, volatile, and reading shared data safely.
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Chapter 3
Assembly and the debugger
Count cycles in a .S file, and read a halted board.
- Locked227 XP
Cycle-counted delays in a .S file
Count a loop in CPU cycles from the data sheet and measure it with Timer 1, then write the delay as a function in its own assembly file and call it from C++.
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- ProjectLocked438 XP
Reading a halt in the debugger
Ten readings of 30 that average 4: stop the Uno on a breakpoint, watch a variable climb, step one instruction and read the carry flag in SREG, then fix the bug the halt showed you.
0 of 6 steps
Chapter 4
Speak a protocol
A UART frame drawn by hand on a plain pin.
- Locked265 XP
Bit-banging a protocol
Draw UART frames by hand on a plain pin with counted delays, let the Mega's real USART receiver judge them, read them on the logic analyzer, and break the baud rate on purpose.
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Syllabus
Every lesson in the course
All 6 lessons, in order, each with what it covers. Any of them opens in the editor right away.
Registers
- DDR, PORT and PINThe three registers behind every digital pin: count in binary on three lamps with one write to PORTB, turn on a pull-up through PORTD, and read a button straight out of PIND.
- Timers by handTimer 1 set up register by register: a prescaler, CTC mode and OCR1A for a match every 500 ms, the compare flag polled and cleared by hand, and no millis() anywhere.
Interrupts
- The vector table and interruptsThe 26 jumps at the start of flash, read out of the chip itself: a Timer 1 interrupt that blinks while loop() sleeps, an INT0 handler that counts presses, volatile, and reading shared data safely.
Assembly and the debugger
- Cycle-counted delays in a .S fileCount a loop in CPU cycles from the data sheet and measure it with Timer 1, then write the delay as a function in its own assembly file and call it from C++.
- Reading a halt in the debuggerTen readings of 30 that average 4: stop the Uno on a breakpoint, watch a variable climb, step one instruction and read the carry flag in SREG, then fix the bug the halt showed you.
Speak a protocol
- Bit-banging a protocolDraw UART frames by hand on a plain pin with counted delays, let the Mega's real USART receiver judge them, read them on the logic analyzer, and break the baud rate on purpose.
Keep going
The next course on your track
Under the hood is part of the embedded track, a ladder of courses from zero to expert.
Read datasheets and schematics
The skill every hardware job asks for: schematic symbols, a first page, absolute maximum ratings, timing diagrams, register maps and pinouts, each checked on real parts. Ends with a sensor brought up from its datasheet alone.
Open the next courseEmbedded track
Arduino, ESP32, Raspberry Pi Pico and STM32: pins, timing, buses, motors, WiFi, registers and debugging, every lesson checked on a simulated board.
See every course on the trackKits
Build it on your bench too
Every lesson runs in the simulator with nothing to buy. When you want the real thing, these kits have the parts.
Find your kit
Pick the starter kit you own and run every lesson in its manual with the same parts on the same pins.
See every kitQuestions
About Under the hood
Is Under the hood free?
Yes. All 6 lessons of Under the hood are free and run in your browser, with nothing to install and no parts to buy before you start.
Do I need a real Arduino Uno R3?
No. Every lesson runs on a simulated Arduino Uno R3 with its parts on the canvas, and the simulator checks each step as you go. When you want to build it for real, the same wiring and code work on a real board.
How long does Under the hood take?
About 5 hours: 6 lessons in 4 chapters, each one a few short steps. Your progress is kept, so you can stop after any lesson and pick up where you left off.
What will I build?
Each chapter ends in a project that puts it together: Timers by hand and Reading a halt in the debugger.
Where do I start?
With "DDR, PORT and PIN", and each lesson builds on the one before it. New to circuits? Electronics basics is the course that starts from zero.