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Hardware Abstraction Layers – What are they, and why use them?
A young coder recently asked me about the HAL I wrote for an embedded project. I was shocked that she wasn't familiar with HALs. I guess our educational system focuses mainly on web technologies. After explaining the purpose of the HAL in our project, I decided it might be a good topic for an article. I've been so busy that I have neglected to post for several months. So, to reboot my commitment to posting articles, I decided a short introduction to HALs would be a good topic to cover. So if you are not familiar with HALs, what they are, or when to use one. Then this article is for you!
Hardware Abstraction Layers (HALs) are an essential part of modern computing systems. They provide a standardized interface to hardware devices, allowing higher-level software to interact with the hardware in a consistent and portable manner. In this article, we’ll take a closer look at what hardware abstraction layers are, why they are used, and how to write one in MicroPython.
What is a Hardware Abstraction Layer (HAL)?
A hardware abstraction layer (HAL) is a layer of software that sits between the hardware and the operating system (OS) or application software. It provides a consistent interface to hardware devices, regardless of the specific hardware details. This allows higher-level software to interact with the hardware devices in a consistent and portable manner, without having to worry about the specific details of the hardware.
Why are Hardware Abstraction Layers used?
Hardware abstraction layers are used for several reasons. First, they provide a standardized interface to hardware devices, which allows software developers to write code that works on multiple hardware platforms. This reduces development time and makes it easier to port software to new hardware platforms.
Second, hardware abstraction layers allow the software to be written at a higher level of abstraction, which makes it more portable and easier to maintain. This is particularly important for embedded systems, where the hardware is tightly integrated with the software.
Third, hardware abstraction layers provide a layer of insulation between the hardware and software. This makes it easier to modify the hardware or replace it with a newer version without having to modify the software.
How to Write a Hardware Abstraction Layer (HAL) in MicroPython
MicroPython is a version of the Python programming language that is designed to be run on microcontrollers and other embedded systems. Writing a hardware abstraction layer in MicroPython involves defining a set of functions or methods that provide a standardized interface to hardware devices. Here’s an example of a hardware abstraction layer for a GPIO pin in MicroPython:
class GPIO_HAL:
def __init__(self):
# Do hardware initialization here!
pass
def set_direction(self, pin, direction):
# Code to set the direction of the GPIO pin
pass
def set_value(self, pin, value):
# Code to set the value of the GPIO pin
pass
def get_value(self, pin):
# Code to get the value of the GPIO pin
return 0
# Example usage of the GPIO hardware abstraction layer
if __name__ == '__main__':
# Create an instance of the GPIO hardware abstraction layer
gpio_hal = GPIO_HAL()
pin = 17
# Set the direction of the GPIO pin to output
gpio_hal.set_direction(pin, direction="output")
# Set the value of the GPIO pin to high
gpio_hal.set_value(pin, value="high")
# Get the value of the GPIO pin
value = gpio_hal.get_value(pin)
In this example, we define a GPIO_HAL class that provides methods to set the direction, set the value, and get the value of the GPIO pin. We then create an instance of the GPIO_HAL class and use it in our main function to set the direction and value of the pin and get its value. This code can be used with any hardware platform that implements the GPIO hardware abstraction layer interface.
It’s worth noting that the example of a hardware abstraction layer provided in this article is a very simple one, dealing only with GPIO pins. In practice, hardware abstraction layers can be much more complex, providing interfaces to a wide variety of hardware devices, such as sensors, actuators, communication interfaces, and more. As such, designing and implementing hardware abstraction layers can be a complex and challenging task. However, the benefits of using HALs, such as increased portability, maintainability, and scalability, make them valuable tools for software developers working with embedded systems.
As an example, the project that inspired this article required the application software to set several control lines, each, to one of four states. These states were controlled by three I2C I/O expanders. The HAL took care of all the nasty details and allowed the application program to simply use one of four enumerated values to set the state. The values were something like HV2, HV1, HI, and LO. From the application’s point of view, the application simply called the HAL function:
hal.set_ctrl_level(ctrl1, HV1)In the application, this line replaced 30+ lines of low-level code in the HAL. But the most powerful feature of the HAL is that it allowed the application developer to write the application without knowing any of the details about the hardware. She only needed to know what needed to happen, when it should happen, and understand the HAL calls to make it happen.
Conclusion
Hardware abstraction layers are an essential part of modern computing systems. They provide a standardized interface to hardware devices, allowing higher-level software to interact with the hardware in a consistent and portable manner. Writing a hardware abstraction layer in MicroPython involves defining a set of functions or methods that provide a standardized interface to hardware devices. This allows software developers to write code that works on multiple hardware platforms, reducing development time and making it easier to port software to new platforms.