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Basics of ARM design on mbed IDE (Part 1)

Posted: 12 Nov 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Microprocessors? embedded systems? ARM? mbed? IDE?

Microprocessors are ubiquitous, providing 'intelligence' in cars, mobile phones, household and office equipment, televisions and entertainment systems, medical products, aircraft: the list seems endless. Those everyday products, where a microprocessor is hidden inside to add intelligence, are called embedded systems.

Not so long ago, designers of embedded systems had to be electronics experts, software experts, or both. Now, with user-friendly and sophisticated building blocks available for our use, both the specialist and the beginner can quickly engage in successful embedded system design.

Figure 1: The ARM mbed. (Image reproduced with permission of ARM Holdings)

One such building block is the mbed, launched by ARM Ltd. The mbed takes the form of a 2 inch by 1 inch (53 mm by 26 mm) PCB, with 40 pins arranged in two rows of 20, with 0.1 inch spacing between the pins. This spacing is a standard in many electronic components.

Figure 1 shows different mbed views. Looking at the main features, labelled in figure 1b, we see that the mbed discussed here is based around the LPC1768 microcontroller, from NXP semiconductors, and contains an ARM Cortex-M3 core.

Program download to the mbed is achieved through a universal serial bus (USB) connector; this can also power the mbed. Usefully, there are five light-emitting diodes (LEDs) on the board, one for status and four that are connected to four microcontroller digital outputs. These allow a minimum system to be tested with no external component connections needed. A reset switch is included, to force restart of the current program.

The mbed pins are clearly identified in figure 1c, providing a summary of what each pin does. In many instances the pins are shared between several features to allow a number of design options.

Top left we can see the ground and power supply pins. The actual internal circuit runs from 3.3 V. However, the board accepts any supply voltage within the range 4.5 to 9.0 V, while an onboard voltage regulator drops this to the required voltage. A regulated 3.3 V output voltage is available on the top right pin, with a 5 V output on the next pin down.

The remainder of the pins connect to the mbed peripherals. These are almost all the subject of later chapters; we will quickly overview them here, though they may have limited meaning to you now. There are no fewer than five serial interface types on the mbed: I2C, SPI, CAN, USB and Ethernet.

Then there is a set of analogue inputs, essential for reading sensor values, and a set of PWM outputs useful for control of external power devices, for example DC motors. While not immediately evident from the figure, pins 5 to 30 can also be configured for general digital input/output.

The mbed is constructed to allow easy prototyping, which is of course its very purpose. While the PCB itself is very high density, interconnection is achieved through the very robust and traditional dual-in-line pin layout.

Background information for the mbed and its support tools can be found at the mbed home page. While this book is intended to give you all information that you need to start work with the mbed, it is inevitable that you will want to keep a close eye on this site, with its cookbook, handbook, blog and forum. Above all else, it provides the entry point to the mbed compiler, through which you will develop all your programs.

Figure 2: Block diagram of mbed architecture.

The mbed architecture
A block diagram representation of the mbed architecture is shown in figure 2. It is possible, and useful, to relate the blocks shown here to the actual mbed. At the heart of the mbed is the LPC1768 microcontroller, clearly seen in figures 1 and 2.

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