Microcomputer Organisation and SBC Introduction

A simplified representation of a microcomputer is shown in Figure 94.

**Figure 94:** Microcomputer organisation.
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The diagram shows

1.: the CPU (central processing unit)
2.: a memory unit
3.: an interface unit, for memory mapped i/o
4.: the address bus
5.: the data bus
6.: the control bus

Such a microcomputer system might consist of a number of IC chips mounted and interconnected on a circuit board, or it might be fabricated on a single IC chip.

The items in the diagram may be familiar to you from other courses. The CPU controls the operation of the whole system and executes instructions. Programs are loaded in memory for execution by the CPU, and program instructions are fetched using the three buses. Memory locations are specified using the address bus, and the data bus is used to transfer instructions. Data upon which programs act is also stored in memory, and transferred via the buses. In memory mapped systems, peripheral devices appear as memory locations and are accessed as such.

In this course we will be using a microcomputer system built around the Motorola Coldfire MCF5206 microcontroller, Figure 95, called the Arnewsh Single Board Computer (SBC5206).

**Figure 95:** SBC5206 Single Board Computer.
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A microcontroller chip, as opposed to a microprocessor chip, includes modules (timers, serial communication ports, parallel ports) to ease the design of embedded systems, i.e. systems controlled by a computer such as a microwave oven, VCR, fuel injection system, etc. The SBC contains a MCF5206 microcontroller chip, plus additional resources and peripherals such as serial ports, expansion buses, etc. Notice that the board includes a programmable logic device and other discrete components.

We will use one of the serial ports on the SBC to communicate with a host PC. A simple monitor program (i.e. operating system) called dBug is stored in ROM (read only memory), and we interact with it using a terminal program running on the host PC. We will develop our software programs on the host PCs, and download to the SBC RAM (random access memory) when compiled. Our software may use modules in the MCF5206 chip.

A simplified memory map of the SBC is shown in Figure 96.

**Figure 96:** SBC5206 memory map.
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Before we start using the SBC, we will learn some details about assembly and C programming using a simulator called BSVC. The BSVC simulates a Motorola 68000 microprocessor, the initial processor in the 68000 series from which the Coldfire series evolved. Figure 97 shows a 10 year old 68000 single board computer based on the 68000; note the relatively large size and number of components. Programming the two machines is very similar, although as we shall see there are some differences. We use the BSVC as a simulator of a raw68000 processor, with RAM, and no peripherals and no monitor/operating system.

The textbook (Clements) and the notes to follow discuss mainly the 68000 processor, and this discussion applies (with some modification) to the 5206 as well.

**Figure 97:** Applix 68000 computer board (c. 1991).
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