Your First Bare-Metal Blink
Blinking an LED from an Arduino sketch takes two lines and teaches nothing about the machine. Blinking one with no HAL, no IDE and no library takes about ninety lines spread over five files, and by the end you know where every byte of the image came from, what the processor did before your first instruction, and which two writes in the whole program actually made the light change.
Register-Level Programming
A peripheral is a piece of digital logic sitting on the same bus as your RAM. It has no API, no calling convention and no way to be invoked. The only interface it exposes is a small block of addresses: write a word to one of them and some flip-flops change state; read from another and you get the current state of some wires. That is the whole model.
volatile and the Compiler
volatile has a reputation for being either a magic word that makes hardware access work or a deprecated relic that nobody should use. Both readings come from the same place: people learn what it does by observing that adding it fixed a bug, and never learn the boundary of the guarantee. The boundary is narrow, it is written down precisely in the C standard, and knowing exactly where it stops is what separates code that works from code that works on your desk.
CMSIS and Vendor HALs
"Bare metal" does not have to mean "type every address yourself". Between raw pointer casts and a full vendor framework there are three or four distinct layers, each with a different bargain, and the useful skill is knowing which one you are standing on and why — not picking a side.
A GPIO Driver from Scratch
The blink program drove one pin with two #defines and it was the right amount of code for one pin. The second pin costs another two, the first alternate-function pin costs four, and by the tenth you have shift arithmetic scattered across the codebase with the pin number written out by hand in each place. The fix is not a HAL. It is about eighty lines that name what the hardware already does.
Configuring the Clock Tree
Out of reset the STM32F411RE runs at 16 MHz on an internal RC oscillator, which is a deliberately conservative choice: it works with no crystal, no configuration, and no risk. It is also one sixth of what the part can do, it is accurate to about ±1 % over temperature rather than the ±20 ppm a crystal gives, and it cannot produce the 48 MHz that USB requires. Somewhere in the first week of a real project you will need to change it.
Interrupt Handlers in C
An interrupt handler is the only function in your program that nothing calls. You write it, you never reference it, and yet it runs — sometimes millions of times a second, at a moment you did not choose, on top of whatever the main program happened to be doing. That inversion is the whole difficulty. Every rule below follows from it: you cannot pass arguments to something nobody calls, you cannot return a value to nobody, and you cannot assume anything about the state of the code you interrupted.
Critical Sections and Atomicity
A bare-metal program with interrupts enabled is a concurrent program. There is one core and no scheduler, but there are still two threads of control — main and whatever handler just fired — and they share memory. Everything that makes concurrency hard is already present:atomic you can assume is lock-free, no kernel to block against.
The Superloop
Every embedded program is an infinite loop. The interesting question is what is inside it. A while(1) that calls three functions in order is the simplest architecture that can run a device, it ships in an enormous number of products, and it is entirely capable of being the right answer for the lifetime of a project. It is also the architecture that fails most quietly when it stops being the right answer, because nothing breaks — the loop just gets slower, and one day a button press is dropped.
Static Memory, No malloc
Most embedded coding standards ban dynamic allocation after startup, and most engineers meet the rule before they meet the reason. Stated as a rule it sounds like superstition — malloc works, it is in the standard library, the vendor examples call it. Stated as a consequence it is obvious: a device that runs for three years without restarting cannot use a memory strategy whose correctness depends on restarting.
Stack Usage and Overflow
Stack overflow on a hosted operating system is an event. A guard page is hit, the process receives a signal, the debugger stops on the offending frame, and you have a stack trace pointing at the recursion you forgot to bound. Stack overflow on a bare-metal Cortex-M is not an event. It is a silent write to a variable that belongs to something else, discovered later, in code that is innocent.
Embedded C Idioms
Embedded C is the same language as any other C. What differs is which of its underspecified corners you are standing on. On a desktop, int is 32 bits, structs are laid out the way you expect, unaligned access works, and the byte order matches whatever produced the file. In firmware you are parsing a protocol written by someone else's compiler, laying a struct over a hardware register, and running on a part where int might be 16 bits and an unaligned load might be a fault.