Design With Microprocessors

DESIGN WITH MICROPROCESSORS:

System Architecture

We know that an architecture is a set of elements and the relationships between them that together form a single unit. The architecture of an embedded computing system is the blueprint for implementing that system—it tells you what components you need and how you put them together.

The architecture of an embedded computing system includes both hardware and software elements.

Let’s consider each in turn.

The hardware architecture of an embedded computing system is the more obvious manifestation of the architecture since you can touch it and feel it. It includes several elements, some of which may be less obvious than others.

CPU An embedded computing system clearly contains a microprocessor. But which one? There are many different architectures, and even within an architecture we can select between models that vary in clock speed, bus data width, integrated peripherals, and so on. The choice of the CPU is one of the most important, but it cannot be made without considering the software that will execute on the machine.

Bus The choice of a bus is closely tied to that of a CPU, since the bus is an integral part of the microprocessor. But in applications that make intensive use of the bus due to I/O or other data traffic, the bus may be more of a limiting factor than the CPU. Attention must be paid to the required data bandwidths to be sure that the bus can handle the traffic.

Memory Once again, the question is not whether the system will have memory but the characteristics of that memory. The most obvious characteristic is total size, which depends on both the required data volume and the size of the program instructions. The ratio of ROM to RAM and selection of DRAM versus SRAM can have a significant influence on the cost of the system. The speed of the memory will play a large part in determining system performance.

Input and output devices The user’s view of the input and output mechanisms may not correspond to the devices connected to the microprocessor. For example, a set of switches and knobs on a front panel may all be controlled by a single microcontroller, which is in turn connected to the main CPU.

For a given function, there may be several different devices of varying sophistication and cost that can do the job. The difficulty of using a particular device, such as the amount of glue logic required to interface it, may also play a role in final device selection.

Hardware Design

The design complexity of the hardware platform can vary greatly, from a totally off-the-shelf solution to a highly customized design.

At the board level, the first step is to consider evaluation boards supplied by the microprocessor manufacturer or another company working in collaboration with the manufacturer. Evaluation boards are sold for many microprocessor systems; they typically include the CPU, some memory, a serial link for downloading programs, and some minimal number of I/O devices. Figure 2.11 shows an ARM evaluation board manufactured by Sharp.

The evaluation board may be a complete solution or provide what you need with only slight modifications. If the evaluation board is supplied by the microprocessor vendor, its design (netlist, board layout, etc.) may be available from the vendor; companies provide such information to make it easy for customers to use their microprocessors. If the evaluation board comes from a third party, it may be possible to contract them to design a new board with your required modifications, or you can start from scratch on a new board design.

The other major task is the choice of memory and peripheral components. In the case of I/O devices, there are two alternatives for each device: selecting a component from a catalog or designing one yourself. When shopping for devices from a catalog, it is important to read data sheets carefully it may not be trivial to figure out whether the device does what you need it to do.