The reference platform

The preferred design strategy for embedded CPU cores is reduced instruction set computer (RISC). Among all the RISC CPU architectures, several reference designs are used as guidelines by silicon manufacturers to produce the core logic to integrate into the microcontroller. Each reference design differs from the others in several characteristics of the CPU implementation. Each reference design includes one or more families of microprocessors integrated into embedded systems, which share the following characteristics:

• Word size used for registers and addresses (8-bit, 16-bit, 32-bit, or 64-bit)
• Instruction set
• Register configurations
• Endianness
• Extended CPU features (interrupt controller, FPU, MMU)
• Caching strategies
• Pipeline design

Choosing a reference platform for your embedded system depends on your project needs. Smaller, less feature-rich processors are generally more suited to low energy consumption, have a smaller MCU packaging, and are less expensive. Higher-end systems, on the other hand, come with a bigger set of resources and some of them have dedicated hardware to cope with challenging calculations (such as a floating-point unit, or an Advanced Encryption Standard (AES) hardware module to offload symmetric encryption operations). 8-bit and 16-bit core designs are slowly giving way to 32-bit architectures, but some successful designs remain relatively popular in some niche markets and among hobbyists.

ARM reference design

ARM is the most ubiquitous reference design supplier in the embedded market, with more than 10 billion ARM-based microcontrollers produced for embedded applications. One of the most interesting core designs in the embedded industry is the ARM Cortex-M family, which includes a range of models scaling from cost-effective and energy-efficient, to high-performance cores specifically designed for multimedia microcontrollers. Despite ranging among three different instruction sets (ARMv6, ARMv7, and ARMv8), all Cortex-M CPUs share the same programming interface, which improves portability across microcontrollers in the same families.

Most of the examples in this book will be based on this family of CPUs. Though most of the concepts expressed will apply to other core designs as well, picking a reference platform now opens the door to a more complete analysis of the interactions with the underlying hardware. In particular, some of the examples in this book use specific assembly instructions from the ARMv7 instruction set, which is implemented in some Cortex-M CPU cores.

The Cortex-M microprocessor

The main characteristic of the 32-bit cores in the Cortex-M family are as follows:

• 16 generic-purpose CPU registers
• Thumb 16-bit only instructions for code density optimizations
• A built-in Nested Vector Interrupt Controller (NVIC) with 8 to 16 priority levels
• ARMv6-M (M0, M0+), ARMv7-M (M3, M4, M7), or ARMv8-M (M23, M33) architecture
• Optional 8-region memory protection unit (MPU)
• Optional TEE isolation mechanism (ARM TrustZone-M)

The total memory address space is 4 GB. The beginning of the internal RAM is typically mapped at the fixed address of 0x20000000. The mapping of the internal flash, as well as the other peripherals, depends on the silicon manufacturer. However, the highest 512 MB (0xE0000000 to 0xFFFFFFFF) addresses are reserved for the System Control Block (SCB), which groups together several configuration parameters and diagnostics that can be accessed by the software at any time to directly interact with the core.

Synchronous communication with peripherals and other hardware components can be triggered through interrupt lines. The processor can receive and recognize several different digital input signals and react to them promptly, interrupting the execution of the software and temporarily jumping to a specific location in the memory. Cortex-M supports up to 240 interrupt lines on the high-end cores of the family.

The interrupt vector, located at the beginning of the software image in flash, contains the addresses of the interrupt routines that will automatically execute on specific events. Thanks to the NVIC, interrupt lines can be assigned priorities so that when a higher-priority interrupt occurs while the routine for a lower interrupt is executed, the current interrupt routine is temporarily suspended to allow the higher-priority interrupt line to be serviced. This ensures minimal interrupt latency for these signal lines, which are somewhat critical for the system to execute as fast as possible.

At any time, the software on the target can run in two privilege modes: unprivileged or privileged. The CPU has built-in support for privilege separation between system and application software, even providing two different registers for the two separate stack pointers. In Chapter 10, Parallel Tasks and Scheduling, we will examine how to properly implement privilege separation, as well as how to enforce memory separation when running untrusted code on the target, in more detail. This is, for example, used to hide secrets such as private keys from direct access from the non-secure world. In Chapter 11, Trusted Execution Environment, we will learn how to properly implement privilege separation, as well as how to enforce memory separation within an OS when running application code on the target with a different level of trust.

A Cortex-M core is present in many microcontrollers, from different silicon vendors. Software tools are similar for all the platforms, but each MCU has a different configuration to take into account. Convergence libraries are available to hide manufacturer-specific details and improve portability across different models and brands. Manufacturers provide reference kits and all the documentation required to get started, which are intended to be used for evaluation during the design phase, and may also be useful for developing prototypes at a later stage. Some of these evaluation boards are equipped with sensors, multimedia electronics, or other peripherals that extend the functionality of the microcontroller. Some even include preconfigured, third-party “middleware” libraries such as TCP/IP communication stacks, TLS and cryptography libraries, simple filesystems and other accessory components, and modules that can be quickly and easily added to a software project.