Local Bus Interfaces

The communication between an embedded system and other systems in its vicinity is enabled by a few protocols. Most microcontrollers designed for embedded systems support the most common interfaces that control and discipline the access to serial lines. Some of these protocols are so popular that they have become the standard for wired inter-chip communication among microcontrollers, and for controlling electronic devices, such as sensors, actuators, displays, wireless transceivers, and many other peripherals. This chapter describes how these protocols work, specifically focusing on the implementation of the system software, through examples running on the reference platform. In particular, we will cover the following topics in this chapter:

• Introducing serial communication
• A UART-based asynchronous serial bus
• An SPI bus
• An I2C bus

By the end of this chapter, you will learn how to integrate the common serial communication protocols.

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Technical requirements

You can find the code files for this chapter on GitHub at https://github.com/PacktPublishing/Embedded-Systems-Architecture-Second-Edition/tree/main/Chapter7.

Introducing serial communication

All the protocols that we will analyze in this chapter manage the access to a serial bus, which may consist of one or more wires, transporting the information in the form of electrical signals corresponding to logic levels zeros and ones, when associated with specific time intervals. The protocols are different in the way they transmit and receive information on the data bus lines. To transmit a byte, the transceiver encodes it as a bit sequence, which is synchronized with a clock. The logic values of the bit are interpreted by the receiver reading its value on a specific front of the clock, depending on the clock’s polarity.

Each protocol specifies the polarity of the clock and the bit order required to transmit the data, which can start with either the most significant or the least significant bit. For example, a system transmitting the ASCII character D over a serial line regulated by raising clock fronts, with the most significant bit...

UART-based asynchronous serial bus

Historically used for many different purposes, thanks to the simplicity of its asynchronous nature, UART dates back to the origins of computing, and it is still a very popular circuit used in many contexts. Personal computers up to the early 2000s included at least one RS-232 serial port, realized with a UART controller and the transceivers allowing to operate at higher voltages. Nowadays, the USB has replaced serial communication on personal computers, but host computers can still access TTL serial buses using USB-UART peripherals. Microcontrollers have one or more pairs of pins that can be associated with an internal UART controller and connected to a serial bus to configure a bidirectional, asynchronous, full-duplex communication channel toward a device connected to the same bus.

Protocol description

As previously mentioned, asynchronous serial communications rely on implicit synchronization of the bit rate between the transmitter and the...

SPI bus

The SPI bus provides a different approach, based on master-slave communication. As the name suggests, the interface was initially designed to control peripherals. This is reflected in the design, as all the communication is always initiated by the master on the bus. Thanks to the full-duplex pin configuration and the synchronized clock, it may be much faster than asynchronous communication, due to the better robustness to clock skews between the systems sharing the bus. An SPI is widely used as a communication protocol for several different devices, due to its simple logic and the flexibility given by the fact that the slave does not have to be preconfigured to communicate at a predefined speed that matches the one on the master. Multiple peripherals can share the same bus, as long as media access strategies are defined. A common way for a master to control one peripheral at a time is by using separate GPIO lines to control the slave selection, although this does require an...

I2C bus

The third serial communication protocol analyzed in this chapter is I2C. From the communication strategy point of view, this protocol shares some similarities with SPI. However, the default bit rate for I2C communication is much lower, as the protocol privileges lower-power consumption over throughput.

The same two-wire bus can accommodate multiple participants, both masters and slaves, and there is no need for extra signals to physically select the slave of the transaction, as slaves have fixed logic addresses assigned:

Figure 7.6 – I2C bus with three slaves and external pull-up resistors

One wire transports the clock generated by the master, and the other is used as a bidirectional synchronous data path. This is possible thanks to the unique mechanism of arbitration of the channel, which relies on the electronic design of the transceivers and may deal with the presence of multiple masters on the same bus in a very clean way.

The...

Summary

This chapter has given us the necessary information to start programming system support for the most popular local bus communication interfaces available on embedded targets. Accessing peripherals and other microcontrollers in the same geographical location is one of the typical requirements of embedded systems interacting with sensors, actuators, and other devices in proximity of the embedded system.

Several implementations providing a higher level of abstraction to the transceivers analyzed here already exist. The serial communication protocols covered in this chapter, namely UART, SPI, and I2C, are usually accessible through drivers that are part of the board support kit and do not need to be reimplemented from scratch. This chapter, however, purposely focused on studying the behavior of the components from the closest possible point of view, to better understand the interface provided by the hardware manufacturer, and possibly provide the tools to design new ways of...