Chapter 4 The Embedded Computing Platform 金仲達教授 清華大學資訊工程學系 (Slides are taken from the textbook slides)

Computing Platform-1 Outline  CPU Bus and DMA  Memory and I/O Devices  Component Interfacing  Designing with Microprocessors  Development, Debugging, Testing  Design Example: Alarm Clock

Computing Platform-2 CPU bus  Connects CPU to memory and device  Bus protocol controls communication between entities decides who gets to use bus at any particular time governs length, style of communication  Four-cycle handshake: Basis of many bus protocols 2 wires: enq (enquiry) and ack (acknowledgment) dev1dev2 enq ack data

Computing Platform-3 Four-cycle example time enq ack data

Computing Platform-4 Typical bus signals  Clock  R/W’: true when bus is reading  Address: a-bit bundle  Data: n-bit bundle  Data ready’ CPU Device 1 Device 2 Memory Clock R/W’ Address Data ready’ Data

Computing Platform-5 Timing diagrams time A B C zero one rising falling stable changing 10 ns timing constraint

Computing Platform-6 Typical bus timing

Computing Platform-7 Transaction types  Wait state: state in a bus transaction to wait for acknowledgment  Disconnected transfer: bus is freed during wait state request and response are separate  Burst: multiple transfers need an extra line called burst’

Computing Platform-8

Computing Platform-9 CPU DMAC Device Memory bus request bus grant DMA  Direct Memory Access: a bus operation not controlled by CPU Controlled by DMA controller (a bus master)  2 additional wires Bus request & Bus grant

Computing Platform-10 DMA Operation  CPU controls DMA operation with 3 registers in DMAC Starting address register Length register Status register  DMAC operation mode: Burst mode: CPU stalls until I/O completes Cycle-stealing mode: DMAC releases bus after each unit of transfer

Computing Platform-11 System Bus Configuration  The bridge: A slave on the fast bus The master of the slow bus high-speed bus low-speed bus CPU Memory High-speed device Low-speed Device Bridge Low-speed Device

Computing Platform-12 ARM Bus ARM CPU High-speed I/O device Low-speed I/O device SRAM Bridge Low-speed I/O device External DRAM controller System-on-Chip AMBA high-performance bus (AHB) AMBA peripherals bus (APB)

Computing Platform-13 Outline  CPU Bus and DMA  Memory and I/O Devices  Component Interfacing  Designing with Microprocessors  Development, Debugging, Testing  Design Example: Alarm Clock

Computing Platform-14 CE’ R/W’ Adrs Data SRAM CE’ R/W’ Adrs Data DRAM RAS’ CAS’ CE’ R/W’ Adrs Data SDRAM RAS’ CAS’ Clock RAM: Random-Access Memory  SRAM (Static RAM) and DRAM (Dynamic RAM) DRAM: values must be periodically refreshed; addressed by row and column addresses Page mode, synchronous DRAM, video RAM

Computing Platform-15 Page mode

Computing Platform-16 ROM: Read-Only Memory  Factory-programmed ROM  Field-programmed ROM (with ROM burners) Antifuse-programmable ROM (programmed once) UV-erasble PROM (aka UV-EPROM) (multiple times)  Flash PROM: modern form of EPROM Old time: need to be removed from the system and must be erased in entirety Current time: can be upgraded inside the system and erased in blocks (aka boot-block flash)

Computing Platform-17 Timers and counters  Very similar: a timer is incremented by a periodic signal; a counter is incremented by an asynchronous, occasional signal.  Rollover causes interrupt  Watchdog timer: Periodically reset by system timer If is not reset, an interrupt to reset the host host CPU watchdog timer interrupt reset

Computing Platform-18 A/D and D/A converters  Analog/digital converter (ADC) Sampling the analog input before converting it to digital form Triggered by a control signal  Digital/analog converter (DAC) R 2R 4R 8R bnbn b n-1 b n-2 b n-3 V out encoder V in...

Computing Platform-19 Keyboards  An array of switches  Switch debouncing: A switch must be debounced to multiple contacts caused by eliminate mechanical bouncing

Computing Platform-20 Encoded keyboard  An array of switches is read by an encoder row address and column output used for encodong  N-key rollover remembers multiple key depressions. row scan

Computing Platform-21 LED current-limiting resistor LED digital logic  Must use resistor to limit current: An on LED has only 0.7V voltage drop

Computing Platform-22 7-segment LCD display  May use parallel or multiplexed input.

Computing Platform-23 Types of high-resolution display  Cathode ray tube (CRT)  Liquid crystal display (LCD)  Plasma, etc.

Computing Platform-24 Touchscreen  Includes input and output device.  Input device is a two-dimensional voltmeter for position sensing: ADC voltage

Computing Platform-25 Outline  CPU Bus and DMA  Memory and I/O Devices  Component Interfacing  Designing with Microprocessors  Development, Debugging, Testing  Design Example: Alarm Clock

Computing Platform-26 Example interfacing memory

Computing Platform-27 Example interfacing device

Computing Platform-28 Outline  CPU Bus and DMA  Memory and I/O Devices  Component Interfacing  Designing with Microprocessors  Development, Debugging, Testing  Design Example: Alarm Clock

Computing Platform-29 Designing with microprocessors  Architectures and components: software; hardware.  Debugging.  Manufacturing testing.

Computing Platform-30 Hardware platform architecture  There are several components in HW CPU bus memory I/O devices: networking, sensors, actuators, etc.  How to implement an embedded system using these components?

Computing Platform-31 Software architecture  Functional description must be broken into pieces – partitioning division among people conceptual organization performance testability maintenance  Software doesn’t run without hardware  How much hardware you need is determined by the software requirements speed memory

Computing Platform-32 Evaluation boards  Designed by CPU manufacturer or others  Includes CPU, memory, some I/O devices  May include prototyping section  CPU manufacturer often gives out evaluation board (e.g., EV board) can be used as starting point for your custom board design.

Computing Platform-33 Adding logic to a board  Programmable logic devices (PLDs) provide low/medium density logic  Field-programmable gate arrays (FPGAs) provide more logic and multi-level logic  Application-specific integrated circuits (ASICs) are manufactured for a single purpose

Computing Platform-34 The PC as a platform  Advantages: cheap and easy to get rich and familiar software environment  Disadvantages: requires a lot of hardware resources not well-adapted to real-time

Computing Platform-35 Typical PC hardware platform CPU CPU bus memory DMA controller timers bus interface bus interface high-speed bus low-speed bus device intr ctrl

Computing Platform-36 Typical PC busses  ISA (Industry Standard Architecture) original IBM PC bus, low-speed by today’s standard  PCI: standard for high-speed interfacing 33 or 66 MHz 264 MB/sec or 524 MB/sec  USB (Universal Serial Bus), Firewire, 1394 relatively low-cost serial interface with high speed

Computing Platform-37 Software elements  IBM PC uses BIOS (Basic I/O System) to implement low-level functions: boot-up; minimal device drivers.  BIOS has become a generic term for the lowest- level system software.

Computing Platform-38 Example: StrongARM SA-1100 (1/2)  StrongARM SA-1100 system includes: CPU chip (3.686 MHz clock) system control module ( kHz clock)  Real-time clock  operating system timer  general-purpose I/O  interrupt controller  power manager controller  reset controller

Computing Platform-39 Example: StrongARM SA-1100 (2/2) ARM CPU core System control module Bridge MHz clock kHz clock system bus peripheral bus

Computing Platform-40 Outline  CPU Bus and DMA  Memory and I/O Devices  Component Interfacing  Designing with Microprocessors  Development, Debugging, Testing  Design Example: Alarm Clock

Computing Platform-41 Host system Target system serial port CPU Host vs. Target  Host: a PC or workstation for development  Target: the HW on which the code will run  Cross-compiler: one that runs on host but generates code for target

Computing Platform-42 Debugging embedded systems  Challenges: target system may be hard to observe target may be hard to control may be hard to generate realistic inputs setup sequence may be complex

Computing Platform-43 Software debuggers  A monitor program residing on target provides basic debugger functions  Debugger should have a minimal footprint in memory  User program must be careful not to destroy debugger program, but, should be able to recover from some damage caused by user code

Computing Platform-44 Breakpoints  A breakpoint allows the user to stop execution, examine system state, and change state.  Replace the breakpointed instruction with a subroutine call to the monitor program.  Breakpoint handler actions: Save registers. Allow user to examine machine. Before returning, restore system state.  Safest way to execute the instruction is to replace it and execute in place.  Put another breakpoint after the replaced breakpoint to allow restoring the original breakpoint.

Computing Platform-45 ARM breakpoints 0x400 MUL r4,r6,r6 0x404 ADD r2,r2,r4 0x408 ADD r0,r0,#1 0x40c B loop uninstrumented code 0x400 MUL r4,r6,r6 0x404 ADD r2,r2,r4 0x408 ADD r0,r0,#1 0x40c BL bkpoint code with breakpoint

Computing Platform-46 In-circuit emulators (a.k.a. ICE)  A microprocessor in-circuit emulator is a specially- instrumented microprocessor Inside ICE, there is a special version of the microprocessor that allows its internal registers to be read out when stopped This special CPU provides as much debugging functionality as a debugger (SW) but does not take out any memory  Disadvantage: one ICE (expensive) is specific to one particular microprocessor (down to pinout)

Computing Platform-47 Logic analyzers  A logic analyzer is an array of low-grade oscilloscopes:

Computing Platform-48 Logic analyzer architecture UUT sample memory microprocessor controller system clock clock gen state or timing mode vector address display keypad

Computing Platform-49 Code verification  Instruction-level simulator a.k.a. CPU simulator down to the details in the programming model NOT simulate the actions of bus or I/O devices ARM and SHARC have such simulator  Cycle-level simulator To simulate HW operation of a computer  Hardware/software co-simulator Most common type of co-verification Consists of both HW and SW simulator

Computing Platform-50 How to exercise code  Run on host system.  Run on target system.  Run in instruction-level simulator.  Run on cycle-accurate simulator.  Run in hardware/software co-simulation environment.

Computing Platform-51 Manufacturing testing  Goal: ensure that manufacturing produces defect-free copies of the design.  Can test by comparing unit being tested to the expected behavior. But running tests is expensive.  Maximize confidence while minimizing testing cost.

Computing Platform-52 Testing concepts  Yield: proportion of manufactured systems that work. Proper manufacturing maximizes yield. Proper testing accurately estimates yield.  Field return: defective unit that leaves the factory.

Computing Platform-53 Faults  Manufacturing problems can be caused by many thing.  Fault model: model that predicts effects of a particular type of fault.  Fault coverage: proportion of possible faults found by a set of test. Having a fault model allows us to determine fault coverage.

Computing Platform-54 Software vs. hardware testing  When testing code, we have no fault model. We verify the implementation, not the manufacturing. Simple tests (e.g., ECC) work well to verify software manufacturing.  Hardware requires manufacturing tests in addition to implementation verification.

Computing Platform-55 Hardware fault models  Stuck-at 0/1 fault model: output of gate is always 0/

Computing Platform-56 Combinational testing  Every gate can be stuck-at-0, stuck-at-1.  Usually test for single stuck-at-faults. One fault at a time. Multiple faults can mask each other.  We can generate a test for a gate by: controlling the gate’s input; observing the gate’s output through other gates.

Computing Platform-57 Sequential testing  A state machine is combinational logic + registers.  Sequential testing is considerably harder. A single stuck-at fault affects the machine on every cycle. Fault behavior on one cycle can be masked by same fault on other cycles.

Computing Platform-58 Scan chains  A scannable register operates in two modes: normal; scan---forms an element in a shift register.  Using scan chains reduces sequential testing to combinational testing. Unloading/unloading scan chain is slow. May use partial scan.

Computing Platform-59 Test generation  Automatic test pattern generation (ATPG) programs: produce a set of tests given the logic structure.  Some faults may not be testable---redundant. Timeout on a fault may mean hard-to-test or untestable.

Computing Platform-60 Boundary scan  Simplifies testing of multiple chips on a board. Registers on pins can be configured as a scan chain.

Computing Platform-61 Outline  CPU Bus and DMA  Memory and I/O Devices  Component Interfacing  Designing with Microprocessors  Development, Debugging, Testing  Design Example: Alarm Clock

Computing Platform-62 Alarm clock interface Alarm on Alarm off Alarm ready set time set alarm hourminute light button PM buzzer

Computing Platform-63 Operations  Set time: hold set time, depress hour, minute.  Set alarm time: hold set alarm, depress hour, minute.  Turn alarm on/off: depress alarm on/off.

Computing Platform-64 Alarm clock requirements

Computing Platform-65 Alarm clock class diagram Lights*DisplayMechanism Buttons* Speaker*

Computing Platform-66 Alarm clock physical classes Lights* digit-val() digit-scan() alarm-on-light() PM-light() Buttons* set-time(): boolean set-alarm(): boolean alarm-on(): boolean alarm-off(): boolean minute(): boolean hour(): boolean Speaker* buzz()

Computing Platform-67 Display class Display time[4]: integer alarm-indicator: boolean PM-indicator: boolean set-time() alarm-light-on() alarm-light-off() PM-light-on() PM-light-off()

Computing Platform-68 Mechanism class Mechanism Seconds: integer PM: boolean tens-hours, ones-hours: boolean tens-minutes, ones-minutes: boolean alarm-ready: boolean alarm-tens-hours, alarm-ones-hours: boolean alarm-tens-minutes, alarm-ones-minutes: boolean scan-keyboard() update-time()

Computing Platform-69 Update-time behavior update seconds with rollover update hh:mm with rollover Rollover? T F PM=truePM=false AM->PM PM->AM display.set-time(current time) Time >= alarm and alarm-on? alarm.buzzer(true) T F

Computing Platform-70 Scan-keyboard behavior compute button activations alarm-ready= true alarm-ready= false alarm.buzzer(false) Increment time tens w. rollover and AM/PM Increment time ones w. rollover and AM/PM save button states Alarm-on Alarm-off Set-time and not set-alarm and hours Set-time and not set-alarm and minutes

Computing Platform-71 System architecture  Includes: periodic behavior (clock); aperiodic behavior (buttons, buzzer activation).  Two major software components: interrupt-driven routine updates time; foreground program deals with buttons, commands.

Computing Platform-72 Interrupt-driven routine  Timer probably can’t handle one-minute interrupt interval.  Use software variable to convert interrupt frequency to seconds.

Computing Platform-73 Foreground program  Operates as while loop: while (TRUE) { read_buttons(button_values); process_command(button_values); check_alarm(); }

Computing Platform-74 Testing  Component testing: test interrupt code on the platform; can test foreground program using a mock-up.  System testing: relatively few components to integrate; check clock accuracy; check recognition of buttons, buzzer, etc.