Microprocessor 2

Communication in a Digital System

Philippe Darche

• Publisher: Wiley-ISTE
• Year: 2020
• ISBN:
  9781119788348
• Edition: 1
• Topic: Computer Science
• Subtopic: Computer Engineering
• Index: Computer Science

Calculation is the main function of a computer. The central unit is responsible for executing the programs. The microprocessor is its integrated form. This component, since the announcement of its marketing in 1971, has not stopped breaking records in terms of computing power, price reduction and integration of functions (calculation of basic functions, storage with integrated controllers). It is present today in most electronic devices. Knowing its internal mechanisms and programming is essential for the electronics engineer and computer scientist to understand and master the operation of a computer and advanced concepts of programming. This first volume focuses more particularly on the first generations of microprocessors, that is to say those that handle integers in 4 and 8-bit formats. The first chapter presents the calculation function and reminds the memory function. The following is devoted to notions of calculation model and architecture. The concept of bus is then presented. Chapters 4 and 5 can then address the internal organization and operation of the microprocessor first in hardware and then software. The mechanism of the function call, conventional and interrupted, is more particularly detailed in a separate chapter. The book ends with a presentation of architectures of the first microcomputers for a historical perspective.

The knowledge is presented in the most exhaustive way possible with examples drawn from current and old technologies that illustrate and make accessible the theoretical concepts. Each chapter ends if necessary with corrected exercises and a bibliography. The list of acronyms used and an index are at the end of the book.

1
Basic Definitions

In order to describe communication between components and electronic subunits, first we must cover general notions such as the direction of communication and connection topology, as well as the concepts of exchange synchronization and information coding, finishing off with the concept of a protocol, which defines the rules that have to be followed. A protocol also defines access arbitration and cycles.

1.1. General points regarding communication

The direction of communication between two systems (Figure 1.1) can either be one-way (simplex) or bidirectional, and this can be either a full-duplex or alternating (half-duplex). Note here that the communication protocol (i.e. the link layer) cannot provide more than what the physical layer permits.

Figure 1.1. Direction of transmission

The entity from which the communication originates and which is generating the address and control signals is called the Master (M) or Initiator (I), and is represented by a square in Figure 1.2. The entity that replies and follows the commands is traditionally called the Slave (S), or Target (T), and this is represented by a square in the figure. If the bus can only take a single master, it is referred to as a single master system. If it can take several it is called a multi-master system (cf. § 2.2.5). If the medium is shared during emission, there can be a conflict of access to the resource (i.e. the bus or slave unit); this is called a collision. The collision can be either logical or physical. A physical collision can result in material damage if the electronic output stage is not designed for it. For this reason, access arbitration is needed (cf. § 1.6).

Figure 1.2. Model of a multi-master bus

To access the bus, each entity requires an interface called I/F (Figure 1.3).

Figure 1.3. Shared bus

In a one-way bus (simplex transmission), an emitter Tx can emit towards one or several receivers Rx. This is a divergent bus, which can broadcast information (Figure 3.16, for example, and cf. § 2.2). Another case that must be considered is where several emitters can only communicate towards a single receiver. This is a convergent bus that allows for the broadcall of information (cf. § 2.2). The existence of several masters can result in an issue of contention when multiple access requests are made to the communication carrier. The bus can be bidirectional (Figure 3.14, for example), with simultaneous transmission (full-duplex), or alternating transmission (half-duplex).

There are several topological variations, including the MUX-based bus (multiplex) and the AND–OR structure. Both are preferred to the SoC (System on (a) Chip). They are shown in Figure 4.28(a) and (b) respectively.

Since there are three main types of information (address, data and control) to be passed around the nodes of a bus in a microprocessor system, there are several ways for them to be transported: there are three combinations with one element, three combinations with two and one with three possibilities. These combinations specialize the bus, resulting in address buses, data buses, control buses, address– control buses, address–data buses, control–data buses and address–control–data buses (only one bus!). During an exchange of several types of information between two entities, for example, between an address and a piece of data, the transfer can make use of separate media (non-multiplexed bus), or they can use the same medium (multiplexed bus). The choice to multiplex is often one of cost: a bus takes up physical space on the Printed Circuit Board (PCB), which is expensive. The number of output connection points for each electronic component and even for the connectors must be taken into account, as the cost of an Integrated Circuit (IC) or a connector is directly linked to this amount. The first approach is better in terms of bitrate, as the buses are separate, with one for each information type. Time-division multiplexing¹ is a solution that allows several different types of information to travel through the same bus, but at different times. The initial philosophy at Intel was that of multiplexing the address and data buses. An example is the 8088 microprocessor made by Intel for the IBM PC (Personal Computer, cf. § V5-3.2.1). This was in contrast to Motorola, which did not multiplex its address and data buses. Another example is the PCI (Peripheral Component Interconnect, cf. § 4.2.4). It should be noted that the information required for the transaction does not need to be presented all at the same time. For example, the information to be written can be presented after the address (“late write”, cf. § 4.4.1 in Darche (2012)). The flipside of multiplexing is that the information transfer time is usually longer as the information has to be (de)multiplexed before it can be accessed, resulting in delays in propagation. This can be done either through a process that is external to the communicating elements, or internally, and thus transparently. In the former option, the peripheral circuits communicate specifically with a microprocessor, usually belonging to the same commercial family, for example, the MPU (MicroProcessor Unit, µP for short) 8085 from Intel and its parallel interface circuit 8155, where the former’s (de)multiplexers were integrated into the latter. In the case of a bus with different pieces of information spread over different moments in time, multiplexing does not slow down ...