Microprocessors

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Microprocessor and Application

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Microcomputer Architecture

In this week, we will do a short general general introduction to: (1) computer and computer architecture, architecture, (2) microprocessor architecture, (3) microcontroller, and (4)PIC 18 microcontroller microcontroller

1.1 Introduction to Computer and Computer Architecture Architecture 1.1.1 Introduction to computer What is a computer?

There are many different definitions, more or less equivalent, of what a computer is. The definition from Wikipedia says “ A computer is a programmable programmable machine designed to automatically carry out a sequence of arithmetic or logical operations”. For any computer to function properly, it must have the following three functions: (1) it accepts input (usually numerical); (2) it can perform computational functions, such as addition, subtraction etc; (3) it can communicate communicate results with the person using it.

(Ancient) History of Computer:

“Computer”, broadly interpretated, has a very long history. It is widely believed that Sumerian (today‟s (today‟s Iraq area) people designed the first machine to help arithmetic computation – abacus – abacus – as as early as 2500 BC. The Abacus they designed are so effective that in 1946, it won a speed competition against a “modern” desk calculation machine in Japan. The first mechanical calculator was invented by Blause Pascal in 1642. The Pascaline Pascaline (aka Arithmatique) can perform all four arithmetic operations without human intelligence. A central concept of modern computer is the ability to store programs. This concept is first developed by Joseph Marie in 1801 in his Jacquard Loom. Jacquard Loom is not a computing machine – machine – it it is used to manufacture rugs and clothes. It uses a punched card to store programs to control the machine‟s machine‟s operations to fabricate complex fabricate complex patterns. Charles Babbage, considered as a “father of the computer”, invented the first mechanical computer. He started to work on a machine called Difference Engine from 1822, used to compute values of polynomial functions. Later, he started designing a different, more complex machine called Analytical Engine. The main difference between between the two engines is that the Analytical Engine could be programmed using punched cards. This is the first fully programmable programmable mechanical computer, and served as the basis of early computers. Unfortunately, the Difference Engine was never completed, and the Analytic Engine was completed in 1888 long after hi s death by his son. Although Babbage's machines were mechanical and unwieldy, their basic architecture was very similar to a modern computer. The data and program memory were separated, operation was instruction-based, the control unit could make conditional jumps, and the machine had a separate I/O unit. In 1941, Konrad Zues developed the first program controlled computer computer – – Z3. Z3. Z3 is an electromechanical electromechanical computer, using 2000 relays.

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The first general purpose electronic computer – Electronic Numerical Integrator and Computer (ENIAC) - was developed in 1945-1946. Eniac weighs 27 tons and uses 1800 sqft space. It is programmable, but rewiring is required to program the machine, hence is rather inflexible. In 1946, Electronic Discrete Variable Automatic Computer (EDVAC) was developed. EDVAC can store program using punch card, and is the first stored program electronic computer. Based on EDVAC, John von Neumann draft a report which lies the foundation of modern “von Neumann Machine”.

Generations of (Electronic) Computer:

Since the appearance of ENIAC/EDVAC, computers have evolved for four generations – each generation uses different components to make the computer less expensive, less power consumption, more reliable and more powerful. The first generation of computers (1947-1958) used vacuum tubes (ie. valves) as the main components. Because of this choice, at most a few tens of thousands of switches (i.e., logic gates) are used for one computer hence its computational power is limited. Also, the first generation computer consumed much electricity, produced lots of heat and occupied large space (e.g., ENIAC). The second generation of computers (1959-1965) used transistors as the main component. One computer typically has many tens of thousans of switches, and is hence faster than the first generation. Also, it is less expensive, use less power, smaller, and more reliable. The third generation of computers (1966-1985) appeared following the inventionof Integrated Ciruit (IC). IC, also refered as a chip, or a microchip, is a set of electronic circuits on one small plate of semiconductor material. The third generations of computers used Small Scale Intergration (SSI, several transistors on one chip), and later Medium Scale Intergration (MSI, hundreds of transistors on one chip) and Large Scale Integration (LSI, tens of thousands of transistors on one chip). Some other features of the third generation computers includes the use of Magnetic-core memory (later replaced with solid-state memory) and batch processing operations systems. One remarkable invention during the third generation is the micro-processor. A micro-processor is one IC that contains the whole CPU. The first microprocessor appeared in 1973. The fourth generation of computers (1986-current) use Very Large Scale Intergration (VLSI, millions of transistors on one chip). Other features include the use of solid-state memory, timesharing operation systems, and supporting high level programming language (C, Java, etc). Many techniques have been developed during the fourth generation. For example, microcomputer - a computer that uses microprocessor as its CPU – was invented by IBM. Also, computer networks, email, graphical user interfact, among many others, are invented in t he fourth generation. Because of the wide use of VLSI, the fourth generation computers are small, light, and cheap. This makes embedded system possible. Embeded system is a system that contains a computer to control its operation.

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Classes of Computers:

Depends of the difference in speed, instruction repertoire, number of CPU registers, word length, main memory size, complexity of I/O modules, operating system complexity, physical size, cost, virtual address space, secondary memory size, degree of multiprogramming, modern computers can be catergorized in to three clases: Mainframe, Minicomputer, Microcomputer. Notice that speed is not the only difference, and often microcomputer can be faster than minicomputer or even mainframe. Also, division between the computer classes are increasingly unclear. Mainframe is a powerful computer used for critical application. The difference of mainframe with other classes is not so much of spped, but high reliability and security. Minicomputer is a mid-range computer. It is originally designed to connect microcomputer with mainframes. It uses own Unix operating system, and is often incompatible with other machines. Minicomputers are giving ways to microcomputers. Microcomputer is a computer whose Central Processing Unit (CPU) is a microprocessor. Recall notion of microprocessor: a processor that contrains the whole CPU (including control unit , arithmetic and logic unit , and register ) one one single IC chip. Compare with the notion of micro-controller: the whole CPU and all peripheral functions (i.e., the whole computer system) that is implemented on one IC chip.

1.1.2 Introduction to Computer Architecture Computer architecture is the art that specifies the relations and part of a computer system. One early example was von Neumann‟s First Draft of a Report on the EDVAC , which described an organization of logical elements. The machines following such organization is called the von Neumann Machine. The von Neumann Machine architecture is still dominant – most currently used machines are von Neumann machines. Below is a diagram of the von Neumann machine.

Figure 1: Diagram of von Neumann machine

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Components of von Neumann Machine:    

A memory for holding both instructions and data required by the instructions A control unit for fetching the instruction from memory An arithmetic processor for performing the specific operation Input/output and peripheral devices for transferring data to and from the system

Architechture of Microcomputer:

A von Neumann Machine can be a mainframe, a minicomputer or a microcomputer. We now discuss the architechture of microcomputer. Almost all microcomputers are von Nermann machines. A microcomputer is made up of three fundamental elements: a central processor, memory and input/output device - minimum structure. Notice that compare with the general von Neumann machine structure, the control unit and the arithmetic processor are integrated together as the central processor. (Why?)

Figure 2: Diagram of Microcomputer Arcitecture Central Processor Unit (CPU) refers to the main processor in t he system. Memory refers to any component that stores data and programs used by the processor. Input/Output (I/O) refers to any subsystem that has the responsibility of receiving data for the processor (input) and sending data out from the processor (output). We will discuss each component in detail later.

System interconnection: In microcomputer, system interconnection is done with the Bus Interconnection Scheme, see below figure. Here, Bus refers to a set of 2 or more conductors grouped together to form a parallel

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information path to and/or from the processor. In a microcomputer, typically three buses coexist, namely, data bus, address bus and control bus. Data bus is responsible for transferring data between processor and memory or processor and I/O. Data bus width (in terms of number of bits) correlates with the default processing capacity of the processor. For example, PIC 18 has a 8-bit data bus, and the processing capacity is one byte (=8 bit). Hence, PIC 18 is a 8-bit processor. Address bus is responsible for transferring addresses from processor to memory or to I/O. The width of address bus is hence correlated with the size of memory that the computer supports. For example, a 16-bit address bus can support 65k memory. Control bus is responsible for the control signals necessary to interface the various devices within the microcomputer.

Figure 3: Bus interconnection Scheme

Memory: Memory refers to any component that stores data and programs used by the processor. Since 1970‟s, the use of semiconductor memory (aka solid-state memory) has been dominant. Semiconductor memory has been through many generations: 1K, 4K, 16K, 256K, 1M (in late 1980s) and now 1G bits on a single chip. To access the memory, memory address and memory map are needed. Memory address is an n-bit number that the processor uses to select a specific memory location. Hence, the number of unique addresses =2n. A memory map designates the memory addresses that are connected to physical memory locations and indicates which locations are unused. That is, it maps the address with the physical memory. See below diagram.

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Types of memory: There are two types of memory: RAM (Random access memroy) and ROM (Read only memory). RAM is volatile, i.e., it needs power to sustain data. It is called RAM because same amount of time is required to access any location on the same chip. There are two types of RAM: Dynamic random-access memory (DRAM) which needs period refersh, and Static random-access memory (SRAM) which doesn‟t. DRAM is typically used for primary memory and SRAM for cache. ROM can only be read but not written to by the processor. Mask-programmed read-only memory (MROM) can only be programmed when being manufactured, while Programmable read-only memory (PROM) can be programmed by the end user via a special equipment. PROM can only be programmed once. EPROM can be programmed many time, but it must be done in a bulk manner – ie the whole chip in one erasure operation. EEPROM can be programmed many times, either one location, one row or the whole chip. Flash memory can be programmed many times, in bulk manner (either a block or the whole chip).

Input/output: Input/output refers to any subsystem that has the responsibility of receiving data for the processor (input) and sending data out from the processor (output). A port is an I/O connection that allows the movement of data between computer and I/O device. A serial port allows two-way transfer of data as a serial data stream (ie one bit a time). A parallel port transfers data in parallel (multiple bit simultaneously). Analog interface is needed to cnvert analog to digital data or vice versa.

Central Processing Unit (CPU): CPU refers to the main processor of the computer. It has three main components: Arithmetic logic unit (ALU), register and control unit . ALU performs computation (both mathematical and logical operations) on data, and return result of operation to register or memory. Register (aka register file) is used to temporarily store data values and memory addresses, and to contain status and

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control information. The control unit is t he specific component of CPU that controls the CPU operations, by issueing control signals or instructions.

The internal CPU interconnection is illustrated in the below figure:

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Early computers have many chips for each different components of a CPU. Later on, microprocessor contains all three units of the CPU in one chip. For modern microprocessor (especially those general purpose ones), the CPU also contains memory on chip (aka cache memory) for faster access. A special type of microprocessor is microcontroller. Microcontroller contains all components of a computer system (i.e., CPU, memory, I/O) in one single chip. See below table for comparison between microprocessor and microcontroller.

1.2 Processor Design 1.2.1 Processor Microarchitecture We now briefly processor microarchitecture – the art to specify how the processor is built. Recall the CPU has three components – control unit, register and ALU. The control unit performs two basic tasks: sequencing and execution. Sequencing refers to the control unit causes the CPU to step through a series of micro-operations in the proper sequence, based on program being executed. (instruction sequencing) Execution refers to the control unit causes each micro-operation to be performed. (instruction interpretation). Within the control unit, there are some special components: (1) Memory Address Register (MAR) which contains address of current data word that is being addressed; (2) Memory Data Register (MDR) that buffer and control the movement of data into/out of the processor; (3) Instruction Register (IR) which contains the opcode for the current instruction; and (4) Instruction Decoder that decodes the instruction in IR and controls the execution of the instruction. Notice that although these component is called “register”, then do not belong to the registers of the CPU since they can not be directly accessed by the program. Below is a diagram. Example: to execute the instruction “Fetch(address)”, three steps are needed – 1. Load address into MAR. 2. Decode address in MAR. 3. Copy memory location contents into MDR. To execute “Store(address, value)”, four steps are needed – 1. Load address into MAR. 2.Load value into MDR. 3.Decode address in MAR. 4.Store contents of MDR into that memory location

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There are two ways to implement the control unit design. The hardwired control unit views the control unit as a sequential logic circuit to generate fixed sequences of control signals. The advantage is its speed of operation. The disadvantage is very inflexible to changes after built, and costly to design and debug. The microprogrammed control unit uses microprograms to interest and execute instruction, and hence easy to design and modify. ALU is the unit to perform mathematical and logical operations on the data. Modern processors often contain separate units for integer and floating point computation (floating point unit). Also, modern processors use multiple execution units to execute instructions in parallel (e.g., dual-core, quad-core etc). Register is a small storage area to temporarily store data that the microprocessor is using. There are some special regiserts which we elaborate. Accumulator is a special register that is directly linked to ALU to assist with arithmetic operation (i.e, store one operand and the result of the operation). Program counter (or instruction counter) keeps track of address of the next location in memory that will be accessed. Status register contains bits to indicate certain results/status of the last operation. Other special registers include stack pointer, address register etc.

1.2.2 Instruction Set Architecture Instruction set architecture considers designing machine instruction and instruction set for the processor. Typically, an instruction needs to provide the following information – where are the data located (register, memory, etc) – what to do with the data – where to store the result. The general format for instruction Z = X „op‟ Y is as follows: Operator 1st operand 2nd operand Result op X Y Z

The various methods to identify locations of the operands are called “addressing modes”. Each processor has its own addressing modes. We will discuss in detail the ones for PIC18 later this course. In general, there are the following types of address modes:

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– Immediate addressing: when the operand is part of the instruction. e.g., acc+„3‟-> acc, here number „3‟ is immediate addressed – Absolute addressing: when the address of an operand is held in the instruction. Eg acc + [0x0010] -> acc, here [0x1000] is absolute addressed, and the processor will fetch the data store in memory address 0x1000 – Register direct addressing: when the operand is held in an addressed register. Eg acc + [0x0010] -> acc, here acc is register direct addressed. The processor will use the data in accumulator – Register indirect addressing: when the address of the operand location is held in a register. Eg, fetch [acc]. Here the address of the data is stored in accumulator, and the machine will find the data stored in this address – Relative addressing: when the address of the operand is computed by adding an offset held in the instruction to the contents of specific register. Eg, fetch [acc+1]. Here the address to fetch is the address stored in accumulator plus 1. Instruction set design: There are three types of instruction set – Minimal Instruction Set, Reduced Instruction Set (RISC), and Complex Instruction Set (CISC). The difference are increasingly unclear. Instruction Execution and Pipeline To execute an instruction, the processor must perform the following: – Fetch the instruction – read the instruction opcode from the memory (opcode is the code indicates which instruction to execute). – Advance program counter (PC) – PC is updated to pointing to the address of the next instruction. – Decode the instruction – Opcode is decoded by the instruction decoder – Perform the instruction – executed the instruction based on built-in sequence for the specific instruction – The status register is updated to reflect the status of the operation Thus, to execute one instruction, the process needs to do several tasks sequentially, and may take some time. To reduce the execution time, pipelining is widely used. Pipelining overlap the execution of several instructions to reduce the total time. In particular, the execution of one instruction is divided into sections, and place latches between them. As an example, suppose there is a processor which needs five steps to execute an instruction, namely: fetch instruction (IF), decode instruction (ID), read registers (RR), execute instruction (EE), write result into register (WR). Each step takes the same time, and the total time is 100ns. Now consider the pipelined version, it divides this 100ms into 5 stages, as follows: – Fetch instruction + latch (one clock cycle) – Decode instruction + latch (one clock cycle) – Read register + latch (one clock cycle) – Execute instruction + latch (one clock cycle) – Write result into register (one clock cycle) Suppose the latch latency is 3ns, then each execution cycle time = 100ns/5+3ns =23ns. In one clock cycle, the processor is IF, ID, RR, EX and WR for 5 different instruction, and thus in the long run, each instruction is executed with in one clock cycle. The below i s a diagram illustrating this:

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1.3 Microcontroller Microcontroller (µC or MCU) is an integration of all of the computer components (i.e., one or more microporcessors (µP‟s), memory and I/O devices) on one chip. it is typically intended as a single chip solutions for systems requiring low to moderate processing power Examples of early microprocessors include the following: • Intel 4044 (1971): first chip to contain all components of a CPU – Born of microprocessor • Intel 8008 (1972), Intel 8080 (1974) – 8-bit microprocessors built by Intel • Other 8-bit microprocessors built in Mid 70s – Motorola: M6800, Signetics: 6502, Zilog: Z80, Texas Instruments: T9900, National Semiconductor: IMP-8 etc. Example of microprocessors developed since late 70s and beyond: • in late 70‟s – Intel 8086 • Microcomputer XT – Intel 8088 – Motorola 68000 – Zilog Z8000 etc • Later - CISC – Intel 80X86 • Microcomputer AT – Intel 80286 – Motorola 680X0 • Motorola 68020

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Later - RISC – Power PC – SUN Microsystems

Examples of micro-controllers: • Intel: 8031, 8051, 80188, 8096 etc • Motorola: MC6805, MC68HC11, MC68HC12 (16-bit) etc • Microchip: PIC16, PIC18

Classification of Microcontrollers: Microcontrollers are typically classified according to the processing capacity of the processors – equivalently the width of the data bus. It can be catergorized as 4-bit microcontroller, 8-bit microcontroller, 16-, 32-, 64-bit microcontroller. The larger the data bus width, the more powerful the microcontroller (and more expensive). Hence, low end microcontrollers are used for cheap applications (eg toys, intelligent consumer product etc), while high end ones are used for complex machines, industrial controllers etc.

1.4 PIC18 Microcontroller We now give an introduction of PIC18 microcontroller as well as its properties. Below is a list of features of PIC18 • • • • • • • •

8-bit CPU 16-bit instruction width, 70+ instructions 2 MB program memory space 256 bytes to 1KB of data EEPROM Up to 3968 bytes of on-chip SRAM 4 KB to 128KB flash program memory Operates at up to 40 MHz crystal oscillator Instruction pipelining

• • • •

Sophisticated timer functions that include: input capture, output compare, PWM, real-time interrupt, and watchdog timer Serial communication interfaces: SCI, SPI, I2C, and CAN 10-bit A/D converter

A simpliefied block diagram see below. You should be able t o tell from the diagram the data bus width, instruction width, address bus width, and the maximum amount of memory supported.

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1.4.1 Memory organization of PIC 18: A very important topic to understand is the memory organization of PIC18. In general, memory consistes a sequence of directly addressable information units called memory locations. A memory location has two components – an address and its contents, it can be used to store data, instruction, and the status of peripheral devices. In PIC 18, the data memory (aka data registers for PIC 18) and program memory are separated (this is called Harvard Architecture), see below figure. The adavantage of separating the data memory and program memory is that it is possible to access data and instruction simultaneously. Data memory includes general purpose registers and special function registers.

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A PIC 18 MCU can have up to 4096 bytes of data memory (recall 12-bit register address bus). Data memory is implemented in SRAM and consists both general purpose registers (used to hold dynamic data) and spcial-function registers (used to control the operation of peripheral functions). A special property of PIC 18 data memory is that it is divided into banks. This property has very profound impact on programming with PIC 18. Data memory is divided into 16 banks, each bank has 256 bytes. At any time, only one bank is active. Which bank is active is specified by the BSR register. Thus, to access data from another bank, one need to do bank switching – this is an overhead and can be error-prone. To reduce bank switching, PIC implemented the access bank. Access bank is a special region of the data memory – it consists of the lowest 96 bytes and highest 160 bytes of the data memory space. One can access memory locations within the access bank regardless of which bank is active.

The group of registers from The group of registers from 0xFD8 to 0xFFF are dedicated to the general control of MCU operation in CPU. Two examples are WREG and STATUS register. The WREG register is involved in the execution of many instructions. The STATUS register holds the status flags for the instruction execution

In terms of the program memory – the program memory address bus and the program counter (PC) are 21 bit long, hence the maximum amount of program memory supported is 2MB. PIC 18 also has a 31-entry stack to hold the return address of subrounting call. When power on, PIC18 starts to executre instruction from address 0. The location 0x08 is reserved for high-priority interrupt service routine, 0x18 is reserved for low-priority interrupt service routine. Part of the program memory is inside the chip (depends on the models, currently up to 128kb) and part of the program memory is outside the side.

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1.4.2 Instruction Format There are 5 different types of instructions for PIC 18. They have different formats. Most instruction are 16 bit long, some of them are 32bit (i.e., use two locations of the program memory). 1. Byte oriented instruction : eg. movf 0x20, W, A

2. Byte-to-byte operations: eg. movff 0x100, 0x200. Here, no bank selection is needed since 12 bits are used to address a memory location

3. Bit-oriented file register operations: eg. bcf 0x20, 2, A

4. Literal operations (literal is a number to be operated upon) e.g., movb 3

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5. Control operations – instructions that change the program exection sequence and make subroutine calls – they all have different formats

Addressing mode: There are five addressing modes (ways to identify locations of the operands) in PIC 18: 1. Register direct: Use an 8-bit value to specify a data register. Eg. movf 0x20, W, A. Here 0x20 is a register that is directly addressed 2. Immediate Mode: A value in the instruction to be used as an operand. Eg., movlb 3. Here number 3 is a number that is immediately addressed 3. Inherent Mode: an implied operand. Eg, movf 0x20, W, A, here the register WREG is inherently addressed 4. Indirect Mode: A special function register (FSRx) is used as a pointer to the actual data register. Eg. movwf INDF0. This command copies the contents of the WREG register to the data memory whose location is stored in FSR0. 5. Bit direct: deal with individual bit. E.g., BCF PORTB, 3, A. This command Clears bit 3 of the data register PORTB Because in most instruction, only 8 bit is used to identify a file register, while we have 4096 bytes of file register (212 bytes), we have to divide into banks- each bank have 256 bytes (28 bytes) . Refer to page 14 for the information of bank and access bank.

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1.4.3 Pipelining The PIC18 divide most of the instruction execution into two stages: instruction fetch and instruction execution. Hence, up to two instructions are overlapped in their execution - One instruction is in fetch stage while the second instruction is in execution stage, see below diagram. Notice that this is made possible since the fetch stage accessess program memory and the execution stage accesses data memory (i.e., file registers). Since the two memory are separated with individual buses, pipelineing is possible. Because of pipelining, each instruction appears to take one instruction cycle to complete. For example, suppose for unpiplined version, one instruction takes 60ns, and assume latch requires 3ns, then for the pipelined version, each time cycle = 60/2+3=33 ns.

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Microprocessors

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