Know the board

A computer on one board

A Raspberry Pi is a single-board computer (SBC): a processor, memory, storage interface, video output, USB, networking and a row of input/output pins, all on one printed circuit board. It runs a complete Linux operating system with a desktop, a terminal, Python, a web server and everything else a computer can run.

That is what separates it from the Arduino boards you programmed in Robotics I and II. An Arduino or ESP32 is a microcontroller board: it runs one program that you compile and upload, with no operating system underneath. Both are useful. In this course they work as a team, and the division of labour is explained below.

Six blocks that explain the whole board

1. SoC. The processor chip: several 64-bit ARM CPU cores, a GPU for the desktop and video, and the controllers for USB, HDMI and the GPIO. Pi 4 and Pi 400 use the BCM2711; Pi 5 uses the faster BCM2712.
2. RAM. Working memory for running programs: 1 to 8 GB on Pi 4, 4 GB on Pi 400, 2 to 16 GB on Pi 5. Emptied when power goes away.
3. Boot firmware. A small program in an EEPROM chip. At power-on it looks for a bootable device in a set order (microSD, then USB by default on recent firmware) and loads the operating system from it.
4. Storage. Where the OS and your files live: a microSD card in the dedicated slot, or a USB SSD in a blue port. Both work; the SSD is faster, the card is smaller. If you boot from an SSD, leave the card slot empty, because the card is tried first.
5. Interfaces. USB for keyboard, mouse and an SSD if you use one; micro-HDMI for the screen; Ethernet and Wi-Fi for the network; Bluetooth; a USB-C socket for power.
6. GPIO header. Forty pins that let Python programs switch LEDs, read buttons and talk to chips over I²C, SPI and UART. Digital only, 3.3 V only.

The three models used in D-221

Learn to tell them apart at a glance. Power supplies and port positions differ; the software you install is the same.

Connectors you must be able to identify

Where the operating system lives

Students often confuse RAM with storage because both are "memory". Keep the two apart:

When you press power, this chain runs in about ten seconds:

1. EEPROM bootloader

   Firmware on the board wakes up, checks the boot order, and looks for a device with a boot partition: the microSD slot first, then USB. On the Pi 4, Pi 400 and Pi 5 it can boot from USB. Some early Pi 4 boards need a bootloader update first (see Troubleshoot).

2. Boot partition

   The first, small partition on the boot drive (FAT format, mounted later as /boot/firmware). It holds the GPU firmware, the Linux kernel and config.txt.

3. Linux kernel

   The kernel is copied from the drive into RAM and started. It detects the hardware and mounts the second, large partition as the root file system /.

4. Raspberry Pi OS

   Services start, the network comes up, the login screen or desktop appears. From now on every file you save is written back to the drive.

This is why the last rule matters: shut down before removing power. A clean shutdown tells the kernel to finish writing and unmount the drive. Pulling the plug while a write is in progress leaves half-written files behind.

Power and cooling

The Pi 5 runs hot under sustained load and throttles its clock when it reaches 80 °C; a fan case or the active cooler keeps it at full speed. The Pi 4 benefits from a heatsink. The Pi 400 uses its keyboard's metal plate as a heat spreader and needs nothing. You can check both at any time:

$ vcgencmd measure_temp
$ vcgencmd get_throttled   # 0x0 means no power or heat problem since boot

The 40-pin header

The header is where the Raspberry Pi meets electronics. You will not wire anything today, but you should understand the map before Week 4. Two numbering schemes exist and every diagram or program must say which one it uses:

• Physical (BOARD) numbering counts header positions 1 to 40. Pin 1 is the corner pin nearest the microSD slot; it is usually marked with a square pad.
• BCM numbering uses the SoC signal names, GPIO2 to GPIO27. Python libraries such as gpiozero use BCM numbers. Example: physical pin 11 is BCM GPIO17.

Odd pins are the row nearest the board edge on a Pi 4 and Pi 5 when the header is at the top; on the Pi 400 the header is mirrored on the back edge, so confirm pin 1 on the physical board before trusting any diagram. Pins 27 and 28 are reserved for HAT identification and should not be used as ordinary GPIO.

Rules of the header

Raspberry Pi and ESP32: complementary, not interchangeable

The term project needs a light measurement. A photocell (light-dependent resistor) in a voltage divider produces an analog voltage that changes smoothly with light. The Raspberry Pi header cannot read that. The ESP32 can: it has a 12-bit analog-to-digital converter. So in this course the sensor node is the ESP32 and the Raspberry Pi is the brain and the face of the system.

Handling the board

• Hold the board by its edges. Avoid touching the pins of the header and the small components.
• Set it on a non-conductive surface, never on a metal desk or on top of a bag of resistors.
• Connect screen, keyboard, mouse and the boot drive first; power last. Disconnect power first when you take the station apart, but only after a clean shutdown.
• The Pi 5 power button: one short press starts it; a short press while running opens the shutdown dialog; holding it forces power off (only when the OS is frozen).

Check yourself