Building Embedded Systems

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Andreas Knirsch | [email protected]

Building Embedded Systems

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Fachbereich Informatik

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Agenda

• Motivation

• Startup

• Memory

• Optimization

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Motivation

Why is the startup of ES

 

so important?

Building Embedded Systems

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Available „instant on“!

"Embedded software is software integrated

with physical processes."

[Lee, 2006]

However, the software must not have negative effect on

the physical process. This applies in particular to the

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Startup

How does an

 

embedded system startup?

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Startup time is important.

But how …

• … does a system start?

• … to optimize the startup?

• … to implement this?

fundamentals

application

approach

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When is a system up and running?

"When you can use it!“

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How does a System Start?

Reset Vector

Startup Code

OS

Application

1

2

3

4

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Phase #1

Reset Vector

1

power-on, initialize hardware

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Phase #2

Startup Code

2

initialize further HW

 

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Phase #3

OS

3

startup OS-kernel 

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Phase #4

Application

4

the system  

becomes usable

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Startup Overview

• Init HW

• Load* and start OS

• Init (even more) HW

• Load and start App(s)

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Memory

Copy data is an essential part

during system start.

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Hyper

visor

OS 0

OS 1

Startup in detail

RBL

MMC

uboot

OS

UART

USB

NAND

APP

SPL

secondary program loader

ROM boot loader

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Load & Start the Kernel

CPU

RAM

FLASH

Power

Data

Addr

kernel initrd

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IN-CAR MULTIMEDIA LABS, Andreas Knirsch | Building Embedded Systems

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In-Car Multimedia Labs

!

Hochschule Darmstadt www.h-da.de

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Flash != Flash: NAND / NOR

Toshiba NAND vs. NOR Flash Memory Technology Overview Page 2

Fig. 1 Comparison of NOR and NAND Flash

File Storage Use

Code Execution Read Speed Write Speed Capacity NOR NAND Cost-per-bit Active Power (*) Standby Power High Low Hard Easy Easy High Low High High Low High Low Low (*) :

Dependant on how memory is used. NOR is typically slow on writes and consumes more power than NAND. NOR is typically fast on reads, which consume less power.

The History of Flash Memory

As a recognized pioneer in flash technology, Toshiba was a principal innovator of both

NOR-type and NAND-NOR-type Flash technology in the 1980’s. These new memories were developed to

address the need for a non-volatile memory that is easily reprogrammable within a system. Some

kind of non-volatile memory is necessary for computing systems so that the system does not

erase all data every time it is powered down, or following a power failure. Both NOR and

NAND Flash systems are electrically erasable solutions, and can write and erase data many

times, but do not lose stored data when the power is turned off.

NAND and NOR Flash Memory Architecture

In the internal circuit configuration of NOR Flash, the individual memory cells are connected in

parallel, which enables the device to achieve random access. This configuration enables the

short read times required for the random access of microprocessor instructions. NOR Flash is

ideal for lower-density, high-speed read applications, which are mostly read only, often referred

to as code-storage applications.

NAND Flash was developed as an alternative optimized for high-density data storage, giving up

random access capability in a tradeoff to achieve a smaller cell size, which translates to a smaller

chip size and lower cost-per-bit. This was achieved by creating an array of eight memory

transistors connected in a series. Utilizing the NAND Flash architecture’s high storage density

and smaller cell size, NAND Flash systems enable faster write and erase by programming blocks

of data. NAND Flash is ideal for low-cost, high-density, high-speed program/erase applications,

often referred to as data-storage applications.

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IN-CAR MULTIMEDIA LABS, Andreas Knirsch | Building Embedded Systems

icm

In-Car Multimedia Labs

!

Hochschule Darmstadt www.h-da.de

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Flash != Flash: NAND / NOR

[THOSHIBA: NAND vs. NOR Memory Technology Overview. White Paper.]

Toshiba NAND vs. NOR Flash Memory Technology Overview Page 3

NOR vs. NAND Flash Density

For any given lithography process, the density of the NAND Flash memory array will always be higher than NOR Flash. In theory, the highest density NAND will be at least twice the density of NOR, for the same process technology and chip size. In reality, market forces determine the

highest density that will be commercially produced. Today, comparing only single chip memory with one bit per cell (also called Single Level Cell, or SLC), the highest density NOR

commercially available is 256 megabit1 (Mb), while NAND is available in densities of 4 gigabit2 (Gb). Because cost-per-bit, which is closely related to the silicon real estate required, is one of the most important characteristics of memory, the small cell size characteristic of NAND Flash is a significant factor.

Choosing NAND vs. NOR

Toshiba continues to make both types of Flash memory, and recognizes that both forms have their own unique merits.

When should one choose NAND Flash over NOR Flash? The answer depends on the system

requirements. Figure 4 below compares NAND Flash with asynchronous NOR Flash in terms of various operating and performance characteristics:

SLC NAND Flash (x8) MLC NAND Flash (x8)

MLC NOR Flash (x16)

Density 512 Mbits1 – 4 Gbits2 1Gbit to 16Gbit 16Mbit to 1Gbit Read Speed 24 MB/s3 18.6 MB/s 103MB/s

Write Speed 8.0 MB/s 2.4 MB/s 0.47 MB/s Erase Time 2.0 mSec 2.0mSec 900mSec Interface I/O – indirect access I/O – indirect

access

Random access

Application Program/Data mass

storage

Program/Data mass storage

eXecuteInPlace

Figure 4: NAND and NOR Flash Operating Specifications

The characteristics of NAND Flash are: high density, medium read speed, high write speed, high erase speed, and an indirect or I/O like access. The characteristics of NOR Flash are lower

density, high read speed, slow write speed, slow erase speed, and a random access interface.

For a system that needs to boot out of Flash, execute code from the Flash, or if read latency is an issue, NOR Flash may be the answer. However, for storage applications, NAND Flash’s higher density, and high programming and erase speeds make it the best choice. While the benefit of high programming speed in high-density Flash devices is obvious, erase performance is equally important, though less obvious. Unlike magnetic memory systems (hard disk drives and tape drives), Flash memory requires a separate erasing step in order to turn all bits back to the “1” state before the device is programmed.

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Optimization

How accelerate the

startup?

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How to accelerate the startup?

Upgrade

hard- and software

Avoidance

only the necessary

Parallelizing

utilize hardware

Prioritization

essentials first

Optimization

Knowledge

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Approach: architecture

• Identify building blocks  

know the dependencies

!

• Order the tasks  

prioritization & avoidance

!

• Describe and visualize 

UML, SysML etc.

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Approach: hardware

• Direct Memory Access (DMA)  

parallelize

!

• Simultaneous Multi-Threading (SMT)  

and Multicore 

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Approach: operating system

• Customize  

avoid unnecessary initialization/abstraction

• postpone the load of specific driver 

prioritization

• utilize Hyper-Threading and SMP 

parallelization

• Resume from image 

avoid initialization

• exectue in place (XIP) 

avoid copy to memory

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Approach: user application

• Customize libraries 

avoid/disable unnecessary features

• Customize toolchain 

make use of specific compiler and flags

• Multithread/-process 

parallelization

• Resume from image 

avoid initialization

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Approach: system development

Strip-Down

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Approach: system development

• QNX System Builder 

creates a well defined target system

• Linux From Scratch 

step-by-step manual to build a Linux system

• Debian debootstrap 

assembles a root system from prebuilt packages

• Buildroot 

builds a toolchain, rootfs, kernel, bootloader-image

• openEmbedded (YOCTO) 

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Architecture

Hardware

Bootloader

Kernel

Libraries

Framework

Application(s)

Tools

1

2

3

4

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Recap: How does a System Start?

Reset Vector

Startup Code

OS

Application

1

2

3

4

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Analyze

Hardware

Bootloader

Kernel

Libraries

Framework

Application(s)

Tools

1

2

3

4 t

cb wwarby (flickr.com)

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Low Hanging Fruits

• identify where optimization is most efficient 

• do the „easy targets“ first 

„... premature optimization is

the root of all evil ...“

_{(Donald E. Knuth, 1974)}

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References

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