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uClinux on Blackfin BF533 STAMP - A DSP Linux Port

By Jesslyn Abdul Salam

Intel and Analog Devices Inc. (ADI) jointly developed the Micro Signal Architecture (MSA) core and introduced it in December of 2000. ADI's Blackfin processor functions as a DSP and a microcontroller. This device is currently used for multimedia applications.

This article provides an introduction to programming the BF533 STAMP board and is intended for readers with some experience in Linux as well as with embedded devices. It contains a basic description of using the board for embedded programming as I have done without any problem.

BF533 STAMP

The STAMP board features:

The BF533 STAMP runs uClinux - a Linux port for embedded devices. It has soft real-time capabilities, meaning that it cannot guarantee a specific interrupt or scheduler latency.

The STAMP board has been specifically designed to support the development and porting of open source uClinux applications and includes the full complement of memory along with serial and network interfaces. The STAMP uClinux Kernel Board Support Package (BSP) includes a CD with a recent copy of the open source development tools, the uClinux kernel, and the documentation. For more details visit https://blackfin.uclinux.org.

uClinux

The uClinux kernel provides a hardware abstraction layer. The uClinux Application Programming Interface (API) is identical for all processors which support uClinux, allowing code to be easily ported to a different processor core. When porting an application to a new platform, programmers only need to address the areas specific to a particular processor - normally, this means only the device drivers.

Getting Started

We have to note the following components of our development board:

The development workstation is essentially a Linux box which should have

There are two potential interconnections between the workstation and the target:

Connecting the Hardware

  1. Connect the serial port to your host computer from the STAMP board
  2. Connect the Ethernet cable to your network card of your Linux box ( use a straight cable with no cross connections )
  3. Start the terminal emulation program. Minicom is an example of such a program, and is included in most Linux distributions. At the bash prompt type the following command as root: 
    bash# minicom -m -s /dev/ttyS0

    From the configuration menu within Minicom, set the serial port connection to

    • 57600 baud
    • 8 data bits
    • Parity none
    • 1 stop bit
    • H/W flow control off

    Now Save the configuration and choose Exit.

  4. Connect the DC power cord and power up the board
  5. After the STAMP board powers up, you should see a U-boot start-up screen in your terminal window. At this stage, if you allow the boot timer to count down, the board will load the uClinux kernel from flash memory and boot it, finally resulting in a command prompt.

GNU/Linux tool chain for the Blackfin devices

Install the binutils and the GCC tools on your machine:

[ Note: 'rpm' is RedHat-specific. Use the appropriate package manager for your distribution to install the latest 'bfin-gcc' version; if your distribution does not have one, use the 'alien' utility to convert this RPM to your required format. -- Ben ] 

bash# rpm -ivh bfin-gcc-3.4-4.1.i386.rpm

You may not find the same version of GCC tools on the CD. I had to download the latest version since the one provided with CD did not contain the required tools (e.g., the C compiler for building programs on uClinux.) You may download the RPM from here. Or you can visit the Blackfin website to download the toolchain yourself.

In user mode, export the path for the tools:

bash# export PATH=/opt/uClinux/bfin-elf/bin:/opt/uClinux/bfin-uclinux/bin:$PATH

Installing the uClinux Kernel

  1. Copy the kernel source from the CD to your work directory
  2. Uncompress the Kernel source

    bash# bunzip2 uClinux-dist.tar.bz2
    tar -xvf uClinux-dist.tar
Now that the kernel is uncompressed in the work directory, you are ready to start building your own uClinux kernel for the Blackfin processor.

Simple 'Hello World' Application

Traditionally, development environments and programming languages have always begun with the 'Hello World' program; we'll follow the precedent.

Copy the following into a file called 'hello.c':

#include<stdio.h>

int main() {
	printf("Hello, Welcome to Blackfin uClinux!!\n");
	return 0;
}

The next step is to compile 'hello.c' on you host PC:

bash# bfin-uclinux-gcc -Wl,elf2flt hello.c -o hello

The resulting executable is named hello.

Note that the bfin-uclinux-gcc compiler is used; this compiler is used to compile programs that run on the uClinux operating system. It automatically links our program with the uClinux run time libraries, which in turn call the uClinux operating system when required ( for example ) to print to the console. The compiler collection for the Blackfin processor also includes another compiler, bfin-elf-gcc, which is used to compile the actual uClinux operating system and uses a different set of libraries. If you want to try porting other RTOSs to Blackfin, you will have to use bfin-elf-gcc.

Now we need to set up the Ethernet port on the Linux box; we'll set its IP address to 192.168.0.2. After booting the uClinux kernel on the STAMP board, you will have to configure its Ethernet interface as well; at the bash prompt of the STAMP, type the following:

root:~># ifconfig eth0 192.168.0.15 up

At this point, the Ethernet link should be up. We can use FTP to upload the 'hello' program from the host to the board. At the bash shell prompt of the linux workstation type the following:

bash# ftp 192.168.0.15

Give the username and password as root and uClinux. Send the 'hello' executable to the STAMP board.

Now modify the permissions and run:

root:~> chown 777 hello
root:~> ./hello
Hello, Welcome to Blackfin uClinux!!

Segmentation faults

The Blackfin processor does not have an MMU, and does not provide any memory protection for programs. This can be demonstrated with a simple program:

// test.c
int main ()
{
	int *p;
	*p=5;
}

Compile 'test.c' using our native C compiler. Try to run the resulting executable, and you'll end up with a segmentation fault.

Now, compile the program using 'bfin-uclinux-gcc' as I described above. Send the executable to the STAMP board using FTP, change the file permissions, and run it.

This time, it should run without a segmentation fault. Due to this, an unprivileged process can cause an address error, can corrupt kernel code, and result is obvious. This is not a limitation of the Blackfin architecture, but simply demonstrates that uClinux is designed to support devices without an MMU.

End of Part I

This is all I know well about uClinux on Blackfin. I have to look deeper into uClinux and I shall try to include more next time!

References

For more information, please visit https://blackfin.uclinux.org.

Talkback: Discuss this article with The Answer Gang

[BIO]

I'm currently doing my final semester as a Btech in Computer Science and Engineering in MES College of Engineering, Kuttipuram, Kerala, India. I am really fond of everything about Linux operating systems, maybe because I want to know more about it, and I spend most of the time on my PC for the same. My other areas of interest include reading, web surfing, and listening to music.


Copyright © 2006, Jesslyn Abdul Salam. Released under the Open Publication license unless otherwise noted in the body of the article. Linux Gazette is not produced, sponsored, or endorsed by its prior host, SSC, Inc.

Published in Issue 123 of Linux Gazette, February 2006

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