Bootloaders and ISP

Learning Goals 5 min

Every Arduino board ships with a tiny program called the bootloader already in its flash. It runs at power-up, listens for a few seconds for an upload request, and either accepts a new sketch or jumps to the existing one. Today you learn what the bootloader does, and how to use one Arduino to program another (or a bare AVR chip) — "Arduino as ISP". By the end of this lesson you will:

1. Explain in your own words what the bootloader does and why it exists.
2. Use the Arduino IDE's "Arduino as ISP" example to turn a UNO into a programmer.
3. Burn a fresh bootloader onto a target ATmega328P chip (or a second UNO) — the prep for tomorrow's standalone-ATmega328 build.

Warm-Up 10 min

Power up your UNO without uploading anything. The on-board LED on pin 13 blinks three times rapidly — that's the bootloader signalling "I'm here, waiting". After ~2 seconds it gives up and runs whatever sketch is in flash.

Two questions

1. Why does the bootloader exist? Why not just upload sketches directly to the chip?
2. Could you fit a 32 KB sketch on a UNO? (Flash is 32 KB total.)

New Concept · What the bootloader does 25 min

The bootloader lifecycle

1. Chip resets (power-on, reset button, or auto-reset from USB).
2. Execution jumps to the bootloader's start address (top of flash).
3. Bootloader sets up Serial, blinks the LED briefly.
4. Listens on Serial for the "new sketch" protocol (STK500) for ~1–2 seconds.
5. If a host (Arduino IDE via avrdude) starts the upload protocol, bootloader accepts the new sketch into flash, then jumps to it.
6. If no upload arrives, bootloader jumps to whatever sketch is already in flash.

Why a bootloader is "optional"

The chip can run a sketch without a bootloader at all — provided you have an ISP (In-System Programmer) to load the sketch directly to flash. Without bootloader, the chip jumps straight to the sketch on reset (no 2-second delay), and the saved bootloader space (~0.5 KB) is free for sketch use.

Trade-off:

ISP — what it is

In-System Programmer: a device that talks the SPI-based programming protocol directly to the AVR chip. It can read fuses, write the bootloader, write the sketch, set the chip's clock source.

Options:

• Dedicated ISP hardware: USBasp (£3 clone), Atmel-ICE (£100 official). Reliable, requires no host MCU.
• Arduino as ISP: load a special sketch onto a UNO, use it as an ISP. Saves money; needs more wires.

Arduino-as-ISP wiring

Then in the Arduino IDE: Tools → Programmer → "Arduino as ISP". Use "Burn Bootloader" or "Upload Using Programmer".

The "burn bootloader" operation

Writes the bootloader (~0.5 KB) to the top of the target chip's flash, sets the fuse bits to point to it as the boot vector, and configures the chip's clock source. Required for any factory-fresh ATmega328P (the chip you can buy bare for $2 — comes blank). After burning, the chip can be uploaded to like any UNO.

Worked Example · Use Arduino as ISP to burn a bootloader 25 min

Step 1 — equipment

• Programmer: 1 × UNO.
• Target: 1 × fresh ATmega328P (or a second UNO you can sacrifice the bootloader of and re-burn).
• If you only have one UNO, you can "burn its own bootloader back on" using the Atmel-ICE or a USBasp.
• Jumper wires.

Step 2 — load the ArduinoISP sketch on the programmer

1. File → Examples → 11.ArduinoISP → ArduinoISP.
2. Tools → Board → Arduino UNO.
3. Upload as normal.

After upload, the LED on pin 9 blinks the "heartbeat", pin 8 lights on programming errors, pin 7 lights during active programming.

Step 3 — wire programmer to target

Programmer UNO's D10 → target's RESET. D11 → MOSI. D12 → MISO. D13 → SCK. +5V → VCC. GND → GND. For a target UNO (board form factor), use the ICSP header (6-pin block near the top of the UNO).

Step 4 — burn the bootloader

1. In the IDE, switch the BOARD setting to match your TARGET (Arduino UNO if target is a UNO).
2. Tools → Programmer → "Arduino as ISP". (NOT "Arduino ISP" — they're different.)
3. Tools → Burn Bootloader.
4. Wait ~10 s. Watch the LED on pin 9 (heartbeat) and pin 7 (programming). Pin 8 stays off if all OK.
5. Done. The target now has a fresh bootloader and can be uploaded to via its own USB port (if it has one) or via the programmer again with "Upload Using Programmer".

Step 5 — sanity check

Disconnect the programmer wires. Connect target's USB. Upload the Blink sketch as normal. The on-board LED should blink. Bootloader works.

Step 6 — alternative: upload a sketch directly (no bootloader needed)

If you want to skip the bootloader entirely:

1. Same wiring as Step 3.
2. Switch Programmer to "Arduino as ISP".
3. Sketch → Upload Using Programmer (Ctrl+Shift+U).
4. The sketch is written directly to flash, bootloader is overwritten / unused, chip boots instantly into your sketch on reset.

Useful for production / standalone builds where you don't want the 2-second boot delay.

Try It Yourself 15 min

Mini-Challenge · Sketch a programmer flowchart 10 min

1. Draw the upload-to-Arduino process showing: USB → USB-serial chip (16U2 on UNO) → main MCU's UART → bootloader (in flash) → writes to flash & jumps to sketch.
2. Add a second path: ISP programmer → main MCU's SPI → writes to flash → boots sketch.
3. Note which is faster (ISP) and which is more convenient (USB).

Recap 5 min

The bootloader is the chip's self-update path — listens for USB uploads, then jumps to the sketch. ISP is the alternative — direct SPI programming, no bootloader required, faster boot. "Arduino as ISP" turns a UNO into a programmer for ~£0 instead of buying a USBasp. Tomorrow you'll use the same technique to build a standalone ATmega328 chip on a breadboard — Cluster A's capstone.

Bootloader

Small program in flash that runs first at boot, listens for upload, hands off to user sketch. Optiboot is the modern default on UNO (smaller, faster than the original).

STK500 protocol

The serial protocol the IDE uses to talk to the bootloader during upload.

ISP (In-System Programmer)

A device that talks AVR's SPI-based programming protocol directly to the chip. Can write the bootloader, sketch, fuses.

Arduino as ISP

The technique of loading the ArduinoISP sketch onto one UNO and using it to program another chip via SPI.

Fuse bits

Persistent configuration bits in the chip that select clock source, boot vector, brown-out level. Set during "burn bootloader".

avrdude

The open-source command-line tool the Arduino IDE uses to perform uploads. Speaks both STK500 (bootloader) and SPI (ISP).

Upload Using Programmer

IDE menu item that bypasses the bootloader and writes the sketch directly via ISP.

Brick

Slang for "render unusable by setting wrong fuses or corrupting bootloader". Usually recoverable with an ISP.

Homework 5 min

1. Try the Arduino-as-ISP wiring + burn bootloader sequence on a spare target.
2. Read ahead to ARD-L04-06 (Standalone ATmega328). Bring a bare ATmega328P chip + 16 MHz crystal + 2 × 22 pF caps if you have them.