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mbed SDK porting

Table of Contents

  1. Build System
  2. CMSIS Module
  3. mbed Library Drivers

The porting of the mbed SDK to a new target is divided in three steps:

  1. Add the new target to the build system
  2. Add a CMSIS module for the given target
  3. Implement the mbed library drivers for the given target

The source code of the mbed SDK (tools + libraries) is available in this repository: https://github.com/mbedmicro/mbed

Before starting the mbed SDK porting, you might want to familiarize with the mbed library internals first.

Build System

Adding a new target to the build system is as easy as adding a new Target class to this python module: workspace_tools/targets.py

For example, this is the Target class for the LPC1768:

class LPC1768(Target):
    def __init__(self):
        Target.__init__(self)
        
        self.core = "Cortex-M3"
        self.vendor = "nxp"
        
        self.supported_toolchains = ["ARM", "GCC_ARM", "GCC_CS", "GCC_CR", "IAR"]

An instance of this new Target class has to be added to the TARGETS list:

TARGETS = [
    LPC2368(),
    LPC1768(),
    LPC11U24(),
    KL25Z()
]

CMSIS Module

Each target has its standalone CMSIS directory under its vendor directory, for example:

The content of the CMSIS folder for a given target is usually provided by the CMSIS-CORE distributed by the silicon vendor itself.

The addition/modification that should be provided by the mbed SDK are:

  • Add a startup file and linker script for the specific target and toolchain, properly configuring the memory model.
  • Implement the cmsis_nvic API for dynamically setting the vector table.

mbed Library Drivers

Currently, all the target dependent code, implementing the mbed library drivers API, is divided in "vendors" directories, for example:

The reason behind this choice is that, very often, a silicon vendor is reusing the same digital IP for implementing the peripherals of its SoC, therefore its drivers usually share large part of the code.

Each target does have a standalone directory containing the headers defining many of its specific features, for example:

Probably the most important file, at the beginning of a port to a new target, is device.h.

device.h contains all the defines that enable/disable the inclusion of a certain set of peripherals in the mbed library.

When you start a port for a new target, setting all these defines to 0 will quickly allow you to compile the whole mbed library and start running tests.

Currently, the bare minimum you should implement to start running the first tests is:

Having this initial block in place will allow you to run simple programs like the mbed hello world:

#include "mbed.h"

DigitalOut myled(LED1);

int main() {
    while(1) {
        myled = 1;
        wait(0.2);
        myled = 0;
        wait(0.2);
    }
}

This is the full list of the mbed drivers API that could be potentially implemented for a target:

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