To get started with embedded system development, you often need to interact with hardware. This requires a basic understanding of both digital and analog circuits, which allows you to dive deeper into the field. Below, we briefly introduce some essential hardware concepts in embedded systems.
**Level**
In digital circuits, signals are typically represented as high (1) or low (0). A digital pin is always in one of these two states, or it may transition between them. However, there's also a third state called "high impedance," which will be discussed later.
**Bus**
Every embedded system has a processor, along with various peripheral chips that work together to perform specific functions. Connecting each peripheral directly to the processor using separate signal lines is impractical due to the large number of required connections. Instead, a shared communication pathway known as a **bus** is used. Think of a bus like a main road connecting multiple houses—each house (peripheral) connects to the main road (bus), allowing communication without needing individual roads for every pair.
There are two main types of buses: the **address bus** and the **data bus**. The address bus carries information about where data should go, while the data bus transfers actual data between the processor and peripherals. The width of the bus determines how much data can be transferred at once, influencing the system’s performance.
**Chip Select (CS)**
To ensure only the correct peripheral is accessed, a **chip select** signal is used. This signal tells a specific peripheral to “open the door†and communicate with the processor. If all peripherals shared the same chip select line, they would all respond simultaneously, causing conflicts. Therefore, each peripheral has its own CS signal, usually controlled by a **decoder** that translates an address into a specific selection signal.
**Decoder**
A decoder takes an input address and activates the corresponding peripheral. For example, a 3-to-8 decoder can activate one of eight different peripherals based on a 3-bit address. This allows the processor to access a large number of devices using fewer address lines.
**High Impedance State**
When a peripheral is not selected, its data bus is in a **high-impedance state**, meaning it doesn’t drive the bus. This prevents interference with other active peripherals. It’s similar to closing a door so that no data is sent through it until it’s opened again.
**Tri-State Gates**
Peripheral pins can be in one of three states: high, low, or high impedance. These are known as tri-state gates, enabling multiple devices to share the same bus without conflict.
**Signal Validity**
The level at which a signal is considered active (e.g., high or low) is referred to as its **validity**. For example, a chip select signal might be active low, meaning it must be low to enable the peripheral.
**Timing**
Communication between the processor and peripherals follows strict timing rules. A **timing diagram** shows the sequence of events, such as when the address is placed on the bus, when the chip is selected, and when data is transferred. Proper timing ensures reliable communication.
**Read/Write Signals**
These signals inform the peripheral whether the processor is reading from or writing to it. They help differentiate between read and write operations.
**I/O Ports**
Peripherals may have **I/O ports**, which are memory-mapped addresses used to read from or write to registers within the peripheral. Some ports are read-only, others are write-only, and some allow both.
**Interrupts**
An interrupt is a signal that alerts the processor when an event occurs, such as data being ready from a peripheral. This allows the processor to handle tasks efficiently, avoiding unnecessary waiting.
**Tools**
In embedded development, tools like **multimeters**, **oscilloscopes**, and **logic analyzers** are essential. A multimeter checks voltage and resistance, while an oscilloscope captures and displays signal waveforms. A logic analyzer monitors multiple digital signals simultaneously, making it useful for debugging complex interactions.
By understanding these hardware concepts, you'll be better equipped to design and debug embedded systems effectively.
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