Mechatronics Module 3.13 Microprocessor II

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Mechatronics Module 3.13Microprocessor II

• Dr Vesna Brujic-Okretic

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Brief Recap

• Embedded microprocessors are those which form a
  sub-system of a larger device for example
  microprocessors in consumer products.
• The microprocessor architecture consists of three
  fundamental units memory, CPU and I/O devices
• To convey digital information, the microprocessor
  uses three buses. These are the data bus, the
  address bus and the control bus.

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• The CPU consists of control structures, memory in
  the form of registers and the arithmetic and
  logic unit
• The memory map describes how addressable memory
  in the CPU is organised into physical units. An
  address decoder is used to select the correct
  device by peeking at certain parts of the address
  bus (details in Lecture 4)
• There are two broad families of memory devices
  RAM and ROM and each has its advantages and
  disadvantages. ROM tends to be used for
  boot-strapping memory resident programs while RAM
  is used for volatile data storage

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Details on the mP operation
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mP architecture
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CPU

• CPU - is the section of the processor that
  processes the data by
• fetching instructions (from the program) stored
  in the memory,
• decoding them and
• executing them.
• it consists of
• Control Unit
• Arithmetic and Logic Unit (ALU)
• Registers

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CPU control unit

• The control unit controls the timing and sequence
  of operations
• all actions within mP are synchronised to the CPU
  via a clock signal
• clock signal logic square wave to drive all the
  circuitry in the mP. Operations are reckoned in
  terms of the number of cycles they take. Typical
  clock frequencies 1 to 30 MHz
• timing signals are used to fetch program
  instructions from memory, to decode and to
  execute them

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CPU control unit

• supplies control signals to
• read and write data into registers,
• control ALU
• and to control external control signals
• CPU is responsible for the control of
• address,
• data and
• control buses
• it acts as a 'master'

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ALU

• arithmetic and logic unit - responsible for data
  manipulation
• it performs
• arithmetic operations, such as add and subtract,
  and
• logic operations (AND, OR, etc.)
• it also performs
• bit shifting,
• rotating,
• incrementing,
• decrementing and
• testing

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Registers

• Internal data that the CPU currently uses -
  stored in special memory locations on the CPU
• These are called - registers
• There are various types of registers
• The number, size and types of registers used vary
  from one microprocessor to another
• Some types are common and will be explained on
  the following slides

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A selection of typical registers
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Accumulator Register

• accumulator register - where data for the input
  to ALU is temporarily stored
• First, the CPU needs to be supplied with the
  address of the required memory location where an
  instruction, or data, is stored so that it can
  access it via address bus
• When this is done, the instruction, or data, is
  read into the CPU via data bus
• Since only one memory location can be accessed at
  any one time, temporary storage has to be used
  when, for example, numbers have to be manipulated
  (added, subtracted etc.)
• So, if 2 numbers are to be added, one number is
  fetched from its memory location and placed into
  an accumulator register while the CPU fetches the
  other number from another memory location
• Once they are added to each other, the result is
  placed to the accumulator register for temporary
  storage

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Flag register

• The flag register (or, status register, or
  condition code register) - contains the result of
  the latest process carried out by ALU
• it contains individual bits, each having special
  significance. The bits are called flags.
• The status of the latest operation is indicated
  by a flag
• each flag may be set (e.I.1) or reset (e.I. 0)
  depending on the status

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Program counter register

• It keeps track of the CPUs position in the
  program
• it contains the address of the memory location of
  the next program instruction, hence the
  alternative name instruction pointer
• as each instruction is executed, the program
  counter register is updated
• the program counter is incremented each time so
  that the CPU executes instructions sequentially,
  unless some special commands (e.g. JUMP) are
  given to change it out of the sequence
• not accessible by the programmer

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Some other registers

• memory address register (MAR) contains the
  address of the data (like an 'address book' of
  all the addressed where various data are stored)
• instruction register (IR) stores an instruction.
  After fetching an instruction from the memory,
  the CPU stores it in the IR. It can then be
  decoded and used to execute an operation
• general purpose register - temporary storage for
  data or addresses also for transfers between
  various other registers
• stack pointer register (SP) - holds the address
  of the top of the stack in RAM. Stack - special
  area of RAM where program counter values can be
  stored when a subroutine of a program is being
  executed

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Memory organisation
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Memory organisation

• The memory unit stores binary data
• The size of the memory is determined by the
  number of wires in the address bus
• For data permanently stored - a read-only memory
  (ROM) device is used
• If the content of ROM can be altered (somehow) it
  is referred to as erasable programmable ROM
  (EPROM)
• Temporary data - I.e. the data currently being
  operated on - is stored in read/write memory
  called random-access memory (RAM)
• When switched ON, the program from keyboard or
  other input device is loaded in RAM

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Memory devices

• memory takes the form of an IC
• the family tree of the memory devices is shown on
  the following tree-structure

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Memory devices

• typical EPROM a series of small electronic
  circuits - cells - which can store charge. The
  program is stored by producing a pattern of
  charged/uncharged cells
• The pattern is erasable using UV light (through a
  quartz window on the top of the device)
• EEPROM is electronically erasable, which is
  easier - but, the chip itself is more expensive
• Static RAM (SRAM) - based on bistable circuit.
  The output remains in its state until a
  subsequent valid input is issued. The bit cell of
  a SRAM is relatively large so it cannot be
  densely packed within a given area of silicon,
  which is a disadvantage

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Memory devices

• The Dynamic RAM (DRAM) bit cell is a capacitor
  capable of storing charge. A single data line is
  used both to write data into the bit cell and to
  read data from it. The charge tends to leak out
  of the capacitor causing its voltage to drop, so
  DRAM needs to be periodically refreshed. This is
  why it is called 'dynamic' RAM.
• Refreshing is done by reading data and writing it
  back to the same cell.
• usually, circuitry external to the memory chip is
  used for refreshing
• Packing density is higher for DRAMs than SRAMs,
  so more memory can be implemented in the given
  area.
• Modern DRAM have their refresh control logic
  on-chip

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Memory requirements

• there is a considerable difference in memory
  requirements between embedded and computing
  applications
• in both classes the 'system memory' term is used
  to refer to the part which is directly accessible
  to the microprocessor, as opposed to the storage
  media such as a magnetic disc or tape drive etc.
• in embedded systems, the memory consists of
  varying amounts of non-volatile memory (ROM), the
  contents of which will not be lost in the case of
  power loss, and volatile memory (RAM) which loses
  its content if power supply is removed.
• Once information (program and constant data) is
  written into non-volatile memory it can be
  considered permanent and it is referred to as
  firmware

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Memory requirements

• the programs in embedded systems are typically
  small.
• For example, a washing machine control program
  may require only 2k bytes of memory.
• For more demanding applications, such as
  communication controllers, several hundreds of
  kilobytes of ROM may be required

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Input/Output Devices
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Input/Output devices

• Input and output devices provide the means by
  which a microprocessor system can convey
  information between itself and the outside world.
• Microprocesor has to accept input information,
  respond to it and produce output signals to
  implement required control
• There may be inputs from sensors to provide data
  to the microprocessor and outputs such as relays
  or motors
• The term peripheral is used for a device
  connected to a microprocessor. Such devices add
  specific functions, like timers and interrupt
  controllers to the mP system

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Input/Output devices

• But, they cannot be, in general, directly
  connected to a microprocessor due to a lack of
  compatibility with the bus system in signal forms
  and levels
• A circuit, called an interface, is used between
  the peripheral devices and the microprocessor to
  overcome this problem - to perform the required
  conversion
• n general, I/O devices contain 2 types of
  registers
• control, or status register - through which the
  program can control the mode of operation of the
  I/O device
• the second type of register provides the data
  path to enable the microprocessor system to
  read/write information to the outside world

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Buses

• Data bus. To transfer the data associated with
  the processing function of the microprocessor.
  Word lengths may be 4,8,16 or 32 bits. Each wire
  in the bus carries a binary signal (0 or 1). The
  more wires the data bus has the longer the word
  length that can be used. Thus, for the word
  length of 4 bits, the number of values that can
  be transferred is 2416

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Buses

• Address bus which contains the address of a
  specific memory location for accessing stored
  data.It carries signals which indicate where data
  is to be found so that certain memory locations
  can be selected. When a particular address is
  selected by its address being placed on the
  address bus, only that location is open for
  communication with CPU. The CPU communicates with
  only one address at a time. Usually address bus
  contains 16 wires
• Control bus. This carries the control signals to
  the memory and the I/O devices. It is used to
  synchronise separate elements. The system clock
  signal is carried by the control bus, for
  example.

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Number representation - a brief reminder

• binary and hexadecimal number representations are
  commonly used in programming
• to convert a binary number into a hexadecimal
  number it is handy to group digits in fours,
  because 2416 and each block (of 4) can be
  represented by a single hexadecimal character
• For example a binary number
• 1011100100011110
• grouped in fours gives
• 1011 1001 0001 1110
• B 9 1 E

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Conversion table

• Hexadecimal Decimal Binary
• 0 0 0
• 1 1 1
• 2 2 10
• 3 3 11
• 4 4 100
• 5 5 101
• 6 6 110
• 7 7 111
• 8 8 1000
• 9 9 1001
• A 10 1010
• B 11 1011
• C 12 1100
• D 13 1101
• E 14 1110
• F 15 1111

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Memory mapping
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Memory mapping

• The memory map designed to meet requirements of
  the application
• It will be used by a hardware designer to
  partition the address space so that the address
  range of the memory devices in the system
  corrwsponds to the address range specified by the
  memory map
• This is achieved my means of a address decoder
• An example is shown in the following figures

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RAM and decoder for 4k memory device
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Chip Select signal

• when the correct address appears on the address
  bus the output from the decoding circuit changes
  to the logic state necessary to activate the
  device to supply/receive the data
• the signal is called Chip Select signal (CS/)
  often set as active low
• a decoder is a combinational logic circuit which
  will decode a binary code and activate output
  signals according to the states of the lines
  applied at the input

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Address decoder logic gates
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Address decoding

• lets have a look at a typical 8-bit data bus
  whose wires (8) are numbered
• D0 - first wire least significant bit (LSB)
• D7 - eighth wire most signif. bit wire (MSB)
• and the 16-bit address bus (see Figure)
• in the control bus therell be a line dedicated
  to READ/WRITE
• notation RD/ or WR/ (slash means active low)
• a logic circuit decodes the address bus signal
  and selects the appropriate device

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Address decoder logic gates

• the least significant bit is at A12 it remains
  in a logic state '1'
• A12 is passed through an inverter, so that the
  chip select signal at the output of the decoder
  is '0' only when A12 is '1'
• note that in this case, the chip select signal is
  set to be 'active low', I.e. CS/
• similar decoding logic for each chip occupying
  the same amount of memory
• if more complicated memory mapping is required,
  decoder functions are implemented using
  programmable logic array

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Read/Write Cycles
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Timing diagrams - read/write cycle
Figure 8.2
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Read cycle

• It lasts 2 cycles of the clock signal
• 1. address of required memory location put on
  address bus (by CPU), at rising edge
• 2. while device held at tristate level -
  control bus issues read signal (active low) to
  the device (2nd cycle begins)
• 3. after delay - valid data placed on data bus
• 4. levels on the data bus sampled by CPU at
  falling edge of the 2nd cycle

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Write cycle

• 1. CPU places address at rising edge
• 2. decoding logic selects correct device
• 3. 2nd cycle - rising edge CPU outputs data
  onto data bus sets WRITE control bus signal
  active (LOW)
• Note
• memory devices other I/O components have static
  logic - so do not depend on clock signal they
  read data from data bus when write signal high
  (inactive) - data must be valid for transition

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The development of a microprocessor system
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A microprocessor system
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Choosing a microprocessor systems

• the microprocessor system will be originally
  conceived from a functional requirement. For
  example, to control a robot arm, or to monitor
  some process etc.
• based on the requirements the system
  specification will be made
• Input/Output requirements
• the number and type of input/output devices will
  be based on the number of sensors and actuators
  needed for the function
• communications with other systems in order to
  provide remote control will be chosen to be
  compatible with these systems in terms of both
  hardware and protocols used

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Choosing an mP system

• complexity of the function to be performed will
  influence the choice of the processor, the CPU in
  particular
• performance is the most critical factor to be
  considered and most difficult to assess
• number of operations per second IS NOT a
  sufficiently good indicator of the performance
• benchmarking is better - running a representative
  piece of code to determine the speed of execution
• simulator is another good way of assessing the
  performance

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Development environment

• the set of tools with which the designer can
  verify the hardware design , write and test the
  software and test the complete system are
  presented in the figure below

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A microcontroller various peripherals
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Development environment

• the prototype hardware is referred to as target
  system
• the software for the target system is written on
  a computer referred to as the host as it hosts
  the development tools during the development
  (nowadays, it is usually a PC)
• when the program functions correctly it may be
  programmed into a ROM device and be permanently
  installed on a target system
• the interface between the host and the target
  system is a hardware emulator for the target
  microprocessor. It has the ability to control the
  execution of the application program
• typically, the emulator is a standalone unit

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Development environment

• alternatively, emulator can be an add-in card to
  the personal computer (the host)
• communication link is usually a simple serial
  RS232 interface
• through that link the host downloads the machine
  code application program to the emulator and
  controls and monitors its execution
• to the target system emulator appears as would
  the real microprocessor

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Development cycle

• there is a well defined development cycle typical
  of any product development
• The basic cycle is shown schematically in the
  following diagram
• First design
• Implement design
• Test design against the spec.
• Does it meet specifications?
• Review design
• Manufacture a product

No
Yes
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Development cycle

• the designer enters the application program into
  the host system using an editor (similar to a
  word processor). The programming language can be
  either a high-level (e.g. C) or a low-level
  (assembler) language
• the next step is to convert the source code into
  the machine code instructions understood by a
  specific microprocessor. This is done by a
  language compiler or by an assembler, depending
  on which language has been used
• the output of a compiler will be a file
  containing the machine code, which when executed
  by the target microprocessor will perform the
  functions defined by the source code. This
  machine code is called object code

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Common practice

• large programs are often developed as a
  collection of smaller and more manageable
  programs.
• if a frequent use is made of a particular routine
  the routine can be placed into a library of
  commonly used routines for use in any subsequent
  program. A library consists of the relocatable
  object code of such routines
• nowadays, high-level languages are often used.
  The benefits include
• improved productivity
• less prone to errors
• allows more complex data manipulation
• program more portable
• source program more easily readable
• disadvantages
• it generates more object code than an equivalent
  assembly lang. prog.
• compiled object code runs more slowly

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Programming Languages

• microprocessors perform certain actions as a
  result of the so called instructions given to the
  mP.
• the collection of these instructions constitute s
  an instruction set
• the form the instructions take is dependent on
  the type of mP (the manufacturer)
• a series of instructions necessary to complete
  certain task is known as a program.
• microprocessors 'understand' only a binary code,
  which is referred to as a machine code.

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Programming Languages

• writing a program in binary code is very
  'unfriendly' and instructions are not easily
  identifiable
• alternatively, a form of comprehensible shorthand
  code for the patterns of 'zeros' and 'ones' can
  be used. Such codes are referred to as mnemonic
  codes
• the term assembly language is used for such a
  code
• it is easier to use than binary code
• but, they still have to be translated into the
  machine code
• the conversion can be done by hand using data
  sheets from the manufacturer, which give binary
  code for each mnemonic

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Programming Languages

• there are, however, computer programs that do the
  conversion - the so called assembler programs
• even more easily comprehended are so called
  high-level programming languages, such as C,
  BASIC, FORTRAN etc.
• they also have to be converted into a machine
  code by specific computer programs so that mP may
  be able to use.
• high-level languages require more memory to store
  them when they have been converted to a machine
  code. Consequently, they take longer to run than
  the programs written in assembly language

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Instruction sets
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Instruction sets

• the set of instructions given to the mP to
  execute a task is called an instruction set
• Generally, instructions can be classified into
  the following categories
• Data transfer
• Arithmetic
• Logical
• Program control
• We shall address briefly each category in turn.
  They differ depending on the manufacturer, but
  some are reasonably common to most mP's.

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Data transfer

• 1. Load
• It reads the content of a specified memory
  location and copies it to the specified register
  location in the CPU
• 2. Store
• copies the current contents of a specified
  register into a specified memory location.

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Arithmetic

• 3. Add
• Adds the contents of a specified memory location
  to the data in some register
• 4. Decrement
• subtracts 1 from the content of a specified
  location.
• 5. Compare
• indicates whether the contents of a register are
  greater than, less than or same as the contents
  of a specified memory location. The result
  appears as a flag in the status register.

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Logical

• 6. AND
• carries out the logical AND operation with the
  contents of a specified memory location and the
  data in some register
• 7. EXCLUSIVE OR - (similar to 6, but for
  exclusive OR)
• 8. Logical shift
• moving the pattern of bits in the register one
  place to he left or right by moving zero (0) to
  the end of the number
• 9. Arithmetic shift
• moving the pattern of bits one place left/right
  but with copying of the end number into the
  vacancy created by shift

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Logical

• 10. Rotate
• moving the pattern of bits one place left/right
  but the bit that spills out is written back into
  the other end

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Program control

• 11. Jump
• changes the sequence in which the program is
  executed. So the program counter jumps to some
  specified location (other than sequential)
• 12. Branch
• a conditional instruction which might be 'branch
  if zero' or 'branch if plus'. It is followed if
  the right conditions are met.
• 13. Halt
• stops all further microprocessor activities

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Example of a flow chart with a branch
Decrement the accumulator
is accumulator zero??
No
Yes
Start new program segment
Copy accumulator to register X
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Flow chart shapes

• common meaning of certain shapes used in flow
  charts

subroutine
Start/End
Input/Output
Decision
Process
program flow
connector
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Features Use of microcontrollers
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State of the art technology
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Features Use of microcontrollers

• Advanced high level code generating tools for
  efficient code generation.
• Developers can now automatically build device
  drivers, boot and glue code that meet their
  precise specifications through an easy-to-use
  point and click interface.
• Control of complex mechanical system such as
  multi-axis robotics. The intelligent timer system
  is capable of monitoring multiple sensors and
  driving multiple actuators with minimum CPU
  servicing.
• Industrial networking using the industry
  standards.
• Control requiring simultaneous analog signal
  sampling, such as electric motor control.
• Analogue to digital converter systems are capable
  of timed and synchronous sampling of analogue
  inputs.

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A microcontroller
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Features Use of microcontrollers

• Modern microprocessors, feature extensive on-chip
  peripherals like
• Universal Serial Bus (USB)
• Host controller
• USB Function and
• colour LCD controller

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A microcontroller
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State of the art technology

• The high-performance CPU core combines
• 32-bit RISC CPU and 16-bit integer DSP unit into
  a powerful, multitasking core
• four-bus structure,
• 16-kilobyte (KB) cache and 16-KB X/Y random
  access memory (RAM).
• Processing performance is 208 million
  instructions per second (MIPS) at 160-MHz
  operating frequency.
• Memory management unit (MMU)
• other peripheral functions required for system
  configuration such as
• a timer, a real time clock, an interrupt
  controller, and a serial communication interface.

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Instruction Set Complexity CISC vs. RISC

• The primary objective of processor designers is
  to improve performance. Performance is defined as
  the amount of work that the processor can do in a
  given period of time. Different instructions
  perform different amounts of work.
• To increase performance, you can either have the
  processor execute instructions in less time, or
  make each instruction it executes do more work.
  Increasing performance by executing instructions
  in less time means increasing the clock speed of
  the processor. Making it do more work with each
  instruction means increasing the power and
  complexity of each instruction. Ideally you'd
  like to do both, of course, but it is a design
  tradeoff it is hard to make more complex
  instructions run faster.
• This tradeoff in basic instruction set design
  philosophy is reflected in the two main labels
  given to instruction sets. CISC stands for
  complex instruction set computer and is the name
  given to processors that use a large number of
  complicated instructions, to try to do more work
  with each one.
• RISC stands for reduced instruction set computer
  and is the generic name given to processors that
  use a small number of simple instructions, to try
  to do less work with each instruction but execute
  them much faster.

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A microcontroller