Overview

Informative

What the Chiisai HAL is, why it exists, and what it deliberately leaves out. For who should read the specification and how it is organized, see How to Read This Specification.

What the Chiisai HAL is

The Chiisai HAL is a standardized, open, resource-conscious hardware-abstraction layer for small, low-power microcontrollers with tight memory budgets — the kind used in cost-sensitive embedded systems such as sensors and actuators in automotive E/E architectures.

It abstracts hardware-dependent functionality — especially communication peripherals — so that middleware and application code run across different MCU platforms without tight hardware coupling or restrictive licensing. Each driver encapsulates hardware-specific access behind a hardware-independent API and configuration, giving upper layers a unified interface for the core functionality while staying flexible enough to expose platform-specific features.

What every driver provides

Across modules, a Chiisai HAL driver typically provides:

  • initialization and de-initialization of the underlying hardware;

  • configuration of module- and channel-specific parameters;

  • data I/O relevant to the hardware domain (transmit/receive, pin control, memory read/write/erase/blank-check, …);

  • status reporting, error indication, and optional notification (event or polling).

Module-specific capabilities and constraints are defined in the respective driver chapters under Chiisai HAL Driver Modules.

Scope

To balance generalisation with extensibility, the specification covers only the hardware elements commonly required by middleware layers — GPIO, memory, and the common communication interfaces (SPI, I2C, UART, CAN, LIN). This keeps the standard small while remaining flexible enough to accommodate platform-specific features without imposing rigid constraints.

Out of scope

The specification is strictly limited to the HAL layer. It deliberately excludes:

  • application logic, operating-system components, and low-level driver internals;

  • specialised peripherals such as ADC and PWM, left to silicon vendors as differentiators.