Embedded Hardware Foundation

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CPU Bus Memory I/O Design, develop and debug

1. CPU I/O programming

Busy/wait Interrupt-driven

Supervisor mode, exceptions, traps Co-processor Memory System

Cache Memory management

Performance and power consumption

I/O devices

Usually includes some non-digital component.

Typical digital interface to CPU:

CPU

statusreg

datareg

mec

hani

sm

Application: 8251 UART

Universal asynchronous receiver transmitter (UART) : provides serial communication.

8251 functions are integrated into standard PC interface chip.

Allows many communication parameters to be programmed.

8251 CPU interface

CPU 8251

status(8 bit)

data(8 bit)

serialport

xmit/rcv

Programming I/O Two types of instructions can support

I/O: special-purpose I/O instructions; memory-mapped load/store instructions.

Intel x86 provides in, out instructions. Most other CPUs use memory-mapped I/O.

I/O instructions do not preclude memory-mapped I/O.

ARM memory-mapped I/O Define location for device:DEV1 EQU 0x1000 Read/write code:

LDR r1,#DEV1 ;set up device addressLDR r0,[r1] ;read DEV1LDR r0,#8 ;set up value to writeSTR r0,[r1] ;write value to device

peek and poke (Using C)

int peek(char *location) {return *location;

}

void poke(char *location, char newval) {(*location) = newval;

}

Busy/wait output Simplest way to program device.

Use instructions to test when device is ready.char *mystring="hello, world.";char *current_char;current_char = mystring;while (*current_char != ‘\0’) {

while (peek(OUT_STATUS) != 0);poke(OUT_CHAR,*current_char);current_char++;

}

Simultaneous busy/wait input and output

while (TRUE) {/* read */while (peek(IN_STATUS) != 0);achar = (char)peek(IN_DATA);/* write */while (peek(OUT_STATUS) != 0);poke(OUT_DATA,achar);

}

Interrupt I/O

Busy/wait is very inefficient. CPU can’t do other work while testing

device. Hard to do simultaneous I/O.

Interrupts allow a device to change the flow of control in the CPU. Causes subroutine call to handle

device.

Interrupt interface

CPU

statusreg

datareg

mec

hani

sm

PC

intr request

intr ack

data/address

IR

Interrupt behavior

Based on subroutine call mechanism.

Interrupt forces next instruction to be a subroutine call to a predetermined location. Return address is saved to resume

executing foreground program.

Interrupt physical interface

CPU and device are connected by CPU bus.

CPU and device handshake: device asserts interrupt request; CPU asserts interrupt acknowledge

when it can handle the interrupt.

Example: interrupt-driven input and outputvoid input_handler();void output_handler();main() {

while (TRUE) {if (gotchar) {

while (peek(OUT_STATUS) != 0);poke(OUT_DATA,achar);gotchar = FALSE;

}}

}

Example: character I/O handlers

void input_handler() {

achar = peek(IN_DATA);gotchar = TRUE;poke(IN_STATUS,0);

}void output_handler() {}

Example: interrupt I/O with buffers

Queue for characters:

head tailhead tail

a

Buffer-based input handlervoid input_handler() {

char achar;if (full_buffer())

error = 1;else {

achar = peek(IN_DATA); add_char(achar);

}if (nchars == 1) {

poke(OUT_DATA,remove_char(); poke(OUT_STATUS,1); }}

}

Buffer-based output handlervoid output_handler() {

if (!empty_buffer()) { poke(OUT_DATA, remove_char()); /* send character */ poke(OUT_STATUS, 1); /*turn device on */

}}

Priorities and vectors

Two mechanisms allow us to make interrupts more specific: Priorities determine what interrupt

gets CPU first. Vectors determine what code is called

for each type of interrupt. Mechanisms are orthogonal: most

CPUs provide both.

Prioritized interrupts

CPU

device 1 device 2 device n

L1 L2 .. Ln

interruptacknowledge

Interrupt prioritization

Masking: interrupt with priority lower than current priority is not recognized until pending interrupt is complete.

Non-maskable interrupt (NMI): highest-priority, never masked. Often used for power-down.

Interrupt vectors

Allow different devices to be handled by different code.

Interrupt vector table:

handler 0

handler 1

handler 2

handler 3

Interruptvector

table head

Interrupt vector acquisition

CPU

device

interruputrequest

interruputack. vector

Interrupt vector acquisition

:CPU :device

receiverequest

receiveack

receivevector

Interrupt sequence CPU checks pending interrupt requests and

acknowledges the one of highest priority. Device receives acknowledgement and sends

vector. CPU locates the handler using vector as

index of interrupt table and calls the handler. Software processes request. CPU restores state to foreground program.

Sources of interrupt overhead

Handler execution time. Interrupt mechanism overhead. Register save/restore. Pipeline-related penalties. Cache-related penalties.

ARM interrupts

ARM7 supports two types of interrupts: Fast interrupt requests (FIQs). Interrupt requests (IRQs).

Interrupt table starts at location 0.

ARM interrupt procedure CPU actions:

Save PC. Copy CPSR to SPSR. Force bits in CPSR to record interrupt. Force PC to vector.

Handler responsibilities: Restore proper PC. Restore CPSR from SPSR. Clear interrupt disable flags.

Exception and Trap Exception:

internally detected error. Exceptions are synchronous with instructions b

ut unpredictable. Build exception mechanism on top of interrupt

mechanism. Exceptions are usually prioritized and vectorized.

Trap (software interrupt) an exception generated by an instruction. Call supervisor mode.

Supervisor mode

May want to provide protective barriers between programs. Avoid memory corruption.

Need supervisor mode to manage the various programs.

ARM CPU modes

处理器模式 描述用户模式 (User, usr) 正常程序执行的模式快速中断模式 (FIQ, fiq) 用于高速数据传输和通道处理外部中断模式 (IRQ, irq) 用于通常的中断处理管理模式 (Supervisor, svc) 供操作系统使用的一种保护模式数据访问中止模式(Abort, abt)

用于虚拟存储及存储保护

未定义指令中止模式(Undefined, und)

用于支持通过软件仿真硬件的协处理器

系统模式 用于运行特权级的操作系统任务

异常模式

ARM CPU modes (cont’d)

SWI (Software interrupt) 指令 格式

SWI{< 条件码 >} immed_24 SWI 指令用来执行系统调用，处理器进入管

理模式， CPSR 保存到管理模式的 SPSR 中，并从地址 0x08 开始执行指令。 <immed_24> 由系统所解释。

条 件 码 1 1 1 1 24 (位 符 号 数 偏 移 量 解 释 ）

31 28 27 24 23 0

Co-processor

Co-processor: added function unit that is called by instruction. Floating-point units are often

structured as co-processors. ARM allows up to 16 designer-

selected co-processors. Floating-point co-processor uses units

1 and 2.

Memory System

Cache Memory Management Unit

Cache Small amount of fast memory Sits between normal main

memory and CPU May be located on CPU chip or

module

Cache operation - overview CPU requests contents of memory location Check cache for this data If present, get from cache (fast) If not present, read required block from

main memory to cache Then deliver from cache to CPU Cache includes tags to identify which block

of main memory is in each cache slot

Cache operation Many main memory locations are

mapped onto one cache entry. May have caches for:

instructions; data; data + instructions (unified).

Memory access time is no longer deterministic.

Cache organizations

Direct-mapped: each memory location maps onto exactly one cache entry.

Fully-associative: any memory location can be stored anywhere in the cache (almost never implemented).

N-way set-associative: each memory location can go into one of n sets.

主存 块号

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Cache块号

0

1

2

3

4

5

6

7

主存 块号

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Cache块号

0

1

2

3

4

5

6

7

主存 块号

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

Cache块号

0

1

2

3

4

5

6

7

0第 组

1第 组

2第 组

3第 组

a（ ）全相联映象 b（ ）直接映象

c（ ）组相联映象

Example Cache of 64kByte

Cache block of 4 bytes i.e. cache is 16k (214) lines of 4 bytes

16MBytes main memory 24 bit address (224=16M) 222 blocks; 28 blocks will be mapped

into one cache line on the average

Direct-mapped cache Each block of main memory maps to

only one cache line i.e. if a block is in cache, it must be in one

specific place Address is in two parts Least Significant w bits identify unique

word Most Significant s bits specify one

memory block The MSBs are split into a cache line field

r and a tag of s-r (most significant)

Direct MappingAddress Structure

Tag s-r Line or Slot r Word w

8 14 2

24 bit address 2 bit word identifier (4 byte block) 22 bit block identifier

8 bit tag (=22-14) 14 bit slot or line

No two blocks in the same line have the same Tag field

Check contents of cache by finding line and checking Tag

Direct-mapped cache

valid

=

tag index offset

hit value

tag data

1 0xabcd byte byte byte ...

byte

cache block

Fully-associative cache Set-associative cache

Write operations

Write-through: immediately copy write to main memory.

Write-back: write to main memory only when location is removed from cache.

Memory management units

Memory management unit (MMU) translates addresses:

CPUmain

memory

memorymanagement

unit

logicaladdress

physicaladdress

Memory management tasks

Allows programs to move in physical memory during execution.

Allows virtual memory: memory images kept in secondary

storage; images returned to main memory on

demand during execution. Page fault: request for location not

resident in memory.

Address translation

Requires some sort of register/table to allow arbitrary mappings of logical to physical addresses.

Two basic schemes: segmented; paged.

Segmentation and paging can be combined (x86).

Segments and pages

memory

segment 1

segment 2

page 1page 2

Segment address translation

segment base address logical address

rangecheck

physical address

+

rangeerror

segment lower boundsegment upper bound

Page address translation

page offset

page offset

page i base

concatenate

Page table organizations

flat tree

page descriptor

pagedescriptor

Caching address translations

Large translation tables require main memory access.

TLB: cache for address translation. Typically small.

ARM memory management

Memory region types: section: 1 Mbyte block; large page: 64 kbytes; small page: 4 kbytes.

An address is marked as section-mapped or page-mapped.

Two-level translation scheme.

CPU performance and power consumption

Example: Intel XScale core

2. Bus

总线： CPU 与存储器和设备通信的机制 一组组相关的电线 部件间通信的协议

四周期握手协议 总线主控器

启动总线传输的设备，如 CPU ， DMA 控制器

一个基本的总线连接

DMA

DMA: Direct Memory Access 允许读写不由 CPU 控制的总线操作。 D

MA 传输由 DMA 控制器控制，它从 CPU请求总线控制。得到控制权后， DMA 控制器直接在设备和内存之间执行读写操作。

带 DMA 控制器的总线连接 附加的总线信号

总线请求 总线授权

桥 高速总线和低速总线 总线互连

高速总线提供更宽的数据连接 低速设备降低成本 桥允许总线独立操作。在 I/O 中提供某些并

行性

ARM 总线 -AMBA AMBA: Advanced Microcontroller Bus

Architecture 2.0 版 AMBA 标准定义了三组总线：

AHB(AMBA High-performance Bus) ASB(AMBA System Bus) APB(AMBA Peripheral Bus)

典型的基于 AMBA 的系统 一个典型的基

于 AMBA 的微控制器将使用AHB 或 ASB总线，再加上APB 总线。

ASB 总线是旧版的系统总线；而 AHB 较晚推出，以增强对更高性能、综合及时序验证的支持

3. Memory

RAM SRAM DRAM

ROM PROM ， EPROM ， EEPROM Flash ROM

Flash 在嵌入式系统中的两种作用 (boot ROM 、hard disk)

4. I/O Watchdog timer A/D & D/A Converter LCD LED Touch screen Key board USB ……

Watchdog timer 看门狗定时器是一个用来引导嵌入式微处理器

脱离死锁状态的部件。是嵌入式系统中的特色部件。

在一个较好的系统中，软件将定时监视或重置看门狗定时器。如果软件和设备工作正常，看门狗定时器得到定期重置。当软件和设备无效工作时，看门狗定时器得不到重置，这样它将持续计数，直到溢出，产生中断使 CPU复位。

Watchdog timer

Watchdog timer is periodically reset by system timer.

If watchdog is not reset, it generates an interrupt to reset the host.

host CPU watchdogtimer

interrupt

reset