Hardware Abstraction Layer

Principle

The Hardware Abstraction Layer (HAL) library features API that target IO devices, such as GPIOs, analog to/from digital converters (ADC / DAC), etc. The API are very basic in order to be as similar as possible to the BSP drivers.

Functional Description

The MicroEJ Application configures and uses some physical GPIOs, using one unique identifier per GPIO. The HAL implementation made for each MicroEJ Platform has the responsibility of verifying the veracity of the GPIO identifier and the valid GPIO configuration.

Theoretically, a GPIO can be reconfigured at any time. For example a GPIO is configured in OUTPUT first, and later in ADC entry. However the HAL implementation can forbid the MicroEJ Application from performing this kind of operation.

Identifier

Basic Rule

MicroEJ Application manipulates anonymous identifiers used to identify a specific GPIO (port and pin). The identifiers are fixed by the HAL implementation made for each MicroEJ Platform, and so this implementation is able to make the link between the MicroEJ Application identifiers and the physical GPIOs.

  • A port is a value between 0 and n - 1, where n is the available number of ports.
  • A pin is a value between 0 and m - 1, where m is the maximum number of pins per port.

Generic Rules

Most of time the basic implementation makes the link between the port / pin and the physical GPIO following these rules:

  • The port 0 targets all MCU pins. The first pin of the first MCU port has the ID 0, the second pin has 1; the first pin of the next MCU port has the ID m (where m is the maximum number of pins per port), etc. Examples:

    /* m = 16 (16 pins max per MCU port) */
    mcu_pin = application_pin & 0xf;
    mcu_port = (application_pin >> 4) + 1;
    
    /* m = 32 (32 pins max per MCU port) */
    mcu_pin = application_pin & 0x1f;
    mcu_port = (application_pin >> 5) + 1;
    
  • The port from 1 to n (where n is the available number of MCU ports) targets the MCU ports. The first MCU port has the ID 1, the second has the ID 2, and the last port has the ID n.

  • The pin from 0 to m - 1 (where m is the maximum number of pins per port) targets the port pins. The first port pin has the ID 0, the second has the ID 1, and the last pin has the ID m - 1.

The implementation can also normalize virtual and physical board connectors. A physical connector is a connector available on the board, and which groups several GPIOs. The physical connector is usually called JPn or CNn, where n is the connector ID. A virtual connector represents one or several physical connectors, and has a name; for example ARDUINO_DIGITAL.

Using a unique ID to target a virtual connector allows you to make an abstraction between the MicroEJ Application and the HAL implementation. For exmaple, on a board A, the pin D5 of ARDUINO_DIGITAL port will be connected to the MCU portA, pin12 (GPIO ID = 1, 12). And on board B, it will be connected to the MCU port5, pin0 (GPIO ID = 5, 0). From the MicroEJ Application point of view, this GPIO has the ID 30, 5.

Standard virtual connector IDs are:

ARDUINO_DIGITAL = 30;
ARDUINO_ANALOG = 31;

Finally, the available physical connectors can have a number from 64 to 64 + i - 1, where i is the available number of connectors on the board. This allows the application to easily target a GPIO that is available on a physical connector, without knowing the corresponding MCU port and pin.

JP3 = 64;
JP6 = 65;
JP11 = 66;

Configuration

A GPIO can be configured in any of five modes:

  • Digital input: The MicroEJ Application can read the GPIO state (for example a button state).
  • Digital input pull-up: The MicroEJ Application can read the GPIO state (for example a button state); the default GPIO state is driven by a pull-up resistor.
  • Digital output: The MicroEJ Application can set the GPIO state (for example to drive an LED).
  • Analog input: The MicroEJ Application can convert some incoming analog data into digital data (ADC). The returned values are values between 0 and n - 1, where n is the ADC precision.
  • Analog output: The MicroEJ Application can convert some outgoing digital data into analog data (DAC). The digital value is a percentage (0 to 100%) of the duty cycle generated on selected GPIO.

Dependencies

Installation

HAL is an additional module. In the platform configuration file, check HAL to install the module.

Use

A classpath variable named HAL-1.0 is available.