Fast and effective embedded systems design: Applying the ARM mbed, Part 1
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Adapted from "Fast and Effective Embedded Systems Design, 1st Edition, Applying the ARM mbed" by Rob Toulson and Tim Wilmshurst (Newnes)
2.1 Introducing the mbed
Chapter 1 reviewed some of the core features of computers, microprocessors and microcontrollers. Now we are going to apply that knowledge and enter the main material of this book, a study of the ARM mbed.
In very broad terms, the mbed takes a microcontroller, such as we saw in Figure 1.4, and surrounds it with some very useful support circuitry. It places this on a conveniently sized little printed circuit board (PCB) and supports it with an online compiler, program library and handbook. This gives a complete embedded system development environment, allowing users to develop and prototype embedded systems simply, efficiently and rapidly. Fast prototyping is one of the key features of the mbed approach.
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 2.1 shows different mbed views. Looking at the main features, labeled in Figure 2.1b, we see that the mbed is based around the LPC1768 microcontroller. This is made by a company called 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.
Figure 2.1. The ARM mbed. (Image reproduced with permission of ARM Holdings)
The mbed pins are clearly identified in Figure 2.1c, 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.