Rpi-logic-analyzer: a simple stand-alone logic analyzer

RPi is very much just a computer with couple GPIO (general purpose input/output) pins exposed. RPi runs pretty much the same Linux as your PC would. Indeed RPi originally was designed to allow school pupils to have cheap access to Ubuntu and LibreOffice and whatnot, the maker community thing was kinda just a sideproduct of that. This class of devices are commonly called single-board computers.

Arduino on the other hand is designed to run your code on bare metal and typically does not involve operating system of any sort. That means that you got very direct access to the HW, and also that you generally have fairly solid hard realtime guarantees. Arduinos also have orders of magnitude less resources (memory/compute/storage) available than RPi but also consume order(s?) of magnitude less power. I'd call this class devboards.

One relatively important distinction between the two is that RPi class devices can be considered self-hosting in the sense that you can program the devices on the device itself without another computer, whereas Arduino development generally always requires a host computer that programs ("flashes") the device.

RPi is closer to a vanilla laptop/PC than it is to an Arduino.

Arduino describes an ecosystem of microcontrollers, traditionally 8-bit AVR and more recently 32-bit ARM. Microcontrollers are designed specifically (physically, instruction set, cost) to control low-level digital and analog IO, and not to run a general Operating System.

Raspberry Pi etc are general purpose computers. They only differ from your general laptop etc in form factor, and that they're built around a System on a Chip (SoC) rather than discrete CPU and peripherals. In that regard it's much like your cell phone, tablet, Chromebook and other budget laptop/desktop computers. But general computers are moving in that direction too — on-chip graphics, ethernet etc. Because RPi targets education and hobbyists, its form factor exposes some general purpose IO (GPIO) pins, much like a general computer parallel port might.

As far as I know, Raspberry Pi doesn't have ADC on-board, but you can wire an external one into its digital GPIO/SPI pins. You could also plug a USB or serial port ADC into your laptop, or your RPi.

With regard to “physical computing”, there's generally a real-time requirement, which cannot be achieved with a multi-tasking OS. Microcontrollers generally run at less then 100 MHz (often 8 or 16 MHz), but because of their instruction set design, timer and interrupt systems, and lack of multitasking, they can reliably and rapidly sample inputs or trigger outputs. A 48 MHz microcontroller could perhaps come close to 1 MHz sample rate.

SoC devices and general purpose computers are designed to run a multi-tasking OS at GHz speed with heaps of RAM. But despite that, this example shows an inability to reliably control I/O at more than 100 KHz when running a general (multitasking) OS. IO tasks that do require real-time characteristics (USB, ethernet, etc) are delegated to separate controller chips.

It's possible to write “bare-metal” code to run directly on a Raspberry Pi (or any computer) without an operating system which brings it closer to behaving like a microcontroller, but the instruction set isn't optimized for this, nor are the compilers/toolchains, and nor is the I/O and on-chip peripherals of the SoC.

Hope that meandering ramble helps.