How Do You Read a Chip Block Diagram?

• Processor
• ROM
• RAM
• Input port
• Output port

What you will learn

• Say what a block diagram shows and what it leaves out.
• Recognise the blocks that usually appear in a microcontroller block diagram.
• Read what a shared line and a separate line mean.

Before you start

No prior knowledge is needed. The article uses a microcontroller as the example because the University of Texas at Austin textbook describes its diagram clearly.

What does a block diagram show?

A block diagram of a system shows the flow of information, and arrows point from source to destination, according to the University of Texas at Austin interfacing textbook. Each block is one part, and each line shows two parts exchanging information.

A microcontroller is a very small computer containing all the components of a computer on one chip: processor, memory and I/O, according to the introductory chapter of the same textbook. A block diagram shows these three groups and how they connect.

Reading takes three steps:

1. Find each block and name it.
2. Follow the arrow from the source block to the destination block.
3. Check which line is shared by several blocks.

• Starting point
• Blocks in between
• Destination block

Which blocks usually appear?

There are three groups of blocks: the processor, memory and ports. The processor (CPU) contains an arithmetic logic unit (ALU), a control unit and registers.

Memory comes in two kinds. RAM loses its information when power is interrupted and restored. ROM keeps its information in that case.

Ports are where information enters and leaves: information enters through input ports and leaves through output ports.

• Arithmetic logic unit
• Control unit
• Registers
• RAM
• ROM

What do the connecting lines tell you?

A shared connecting line is a bus, a collection of wires used to pass information between blocks. How many buses a chip has and how blocks use them differs from chip to chip.

The textbook describes two styles. In the Harvard style, the chip has one bus for data and a separate bus for instructions, so instructions are fetched from flash ROM at the same time as data is exchanged with memory. In the von Neumann style, a single bus carries both data and instructions, so data has to flow one at a time.

The same set of blocks therefore gets two ways of drawing the lines, for two ways of working.

• Two separate lines
• One shared line

Real-world example

Think of the floor plan of a restaurant kitchen. Each box is an area: receiving, storage, main kitchen, serving counter. Arrows show where goods go from and to. One shared corridor may serve both the kitchen and storage, or each area may have its own way in.

This is only a comparison to help you remember. A real chip diagram has the number of blocks and buses that the chip defines.

Return to the illustrative microcontroller diagram: start at the input port and follow its connection to the processor. Next, find RAM to identify the block exchanging data with the processor, then find ROM to locate the instructions. Finally, trace the connection to the output port. If instruction and data paths are separate, compare them with Harvard architecture; if they share a bus, compare them with von Neumann architecture. This reading reveals relationships between blocks, rather than their physical positions.

Common misunderstandings

A block diagram does not draw real positions. Blocks that sit side by side on paper may not sit side by side on the chip.

A block diagram does not tell speed or size. It tells which part connects to which.

A thick line is not automatically a shared bus. Check how many blocks it joins.

Frequently asked questions

What parts does a microcontroller have?

A processor, memory and input/output ports on one chip.

How does RAM differ from ROM?

RAM loses its information when power is interrupted and restored, while ROM keeps it in that case.

What is a bus?

A bus is a collection of wires used to pass information between blocks.

Summary

• A block diagram draws parts as blocks and joins them with lines; arrows point from source to destination.
• A microcontroller has a processor, memory and ports on one chip; RAM loses information without power and ROM keeps it.
• A shared line is called a bus; the Harvard style uses separate buses and the von Neumann style uses one shared bus.