Diminuto Right Out of the Box

Diminuto is my attempt to put together a platform on which to teach real-time software design, systems programming, embedded software development and assembly language programming, all using real commercial hardware and open source software. In Choosing Hardware for Diminuto, I described how I chose the Atmel AT91RM9200-EK evaluation kit has my first cut at a hardware platform. In this article, I'll describe the EK in detail and show you how to get it up and running in about an hour, just with the software it comes with.

(Blogger sometimes truncates the right-hand portion of the photographs. You can always click on a photograph to bring a full-size version up in your browser.)

The EK is a single board computer (SBC) that is about 6.5" x 7.5" in size. It has an AT91RM9200 microprocessor, part of the Atmel AT91 family. The AT91RM9200 is a system-on-a-chip (SoC) that integrates an ARM920T RISC processor core with several special purpose cores that implement controllers for a variety of peripheral devices. The ARM920T is part of the ARM ARM9 family. It incorporates a real-time clock, a system timer, and a memory management unit (MMU). The EK has 8MB of flash, pre-programmed with the U-Boot boot loader, and 32MB of RAM. Peripheral devices include an Secure Digital (SD) card slot, two USB host ports, one USB gadget port, two DB-9 serial ports, one 10/100 Base-T Ethernet RJ-45 port, a video controller (which is the largest chip on the board) with a VGA port, a JTAG debugging connector, and some other goodies.

Here is a photograph of the EK hooked up to my development setup at the Palatial Overclock Estate (a.k.a. the Heavily-Armed Overclock Compound). You can see the beige serial console cable to the left, the blue CAT5 Ethernet cable front center, and the power supply to the right. Other accoutrements include a little fan (not really necessary, the EK runs pretty cool), a static mat and wrist strap (absolutely required), and a magnifying glass (because I'm old).



The EK is available retail in single quantities from any number of distributors for about $1250. (Yes, I know; I bought an absolutely immense Dell 530 quad-core server for less than this; economies of scale are at work here.) I purchased mine from Avnet. Here's what it looks like when the box arrives at your door.



The box contains (left to right, top to bottom): a European electrical cord, a U.S. electrical cord, a 12V power supply, the EK board in a static bag, a DB9S-to-DB9S null modem serial cable, a Type A-to-Type B USB cable, three DataFlash cards with sample systems on them, a CAT5 Ethernet crossover cable, and two CD-ROMs, one with software for the EK, and a Windows CE demo.



You can use the CAT5 Ethernet crossover cable to connect the EK directly to the Ethernet port on another computer. Instead, I connected the EK to a nearby Ethernet switch using a standard CAT5 Ethernet cable. I used the DB9S-to-DB9S serial cable to attach the console port on the EK to an old Windows ThinkPad laptop, and I used PuTTY to talk to the board. For sure, you will need access to the serial console, since much of not all of the work you will be doing on the EK will be in single-user mode. In a teaching laboratory with several stations, you might consider connecting all of the EKs to a reverse telnet server, which is common practice in the field for serial console ports on all kinds of devices.

The EK has a configuration strap, J15, that needs to be set to cause it to boot U-Boot from flash. This is described in the documentation that comes with the EK. (And this is why I needed the magnifying glass.) If you hold the board while you are grounded using a static strap so that the printing is right-side-up, J15 is in the bottom left quadrant on the board. It should be set to the right so that it connects the center of three pins to the right-hand pin labeled EXT. The photograph below is kind of fuzzy, but it shows you the correct orientation of the board.



The software CD-ROM contains a Windows Trivial File Transfer Protocol (TFTP) server utility, U-Bootable Linux 2.4 kernel images for both the EK and the older (and more expensive) AT91RM9200-DK boards, a corresponding initrd RAM disk root file system image, and a tarball of the tool chain. The tool chain includes the C cross-compiler environment which runs under Linux on a i386 platform, and generates code for the ARM9. I used an existing Linux server (a Dell 530) as both my development platform for the tool chain and as a TFTP server.

The U-Boot boot loader is widely used in embedded products. It is preloaded into flash on the EK. It is set up to boot the EK over TFTP. All you have to do is drop the linux-ek kernel image and the initrd ramdisk root file system image from the CD-ROM in appropriate directory on your TFTP server, administer U-Boot with the parameters for your network, and run some predefined U-Boot commands.

Here is the initial U-Boot dialog that you will see on the console when you hit the reset button on the EK. U-Boot is waiting for you to enter a command.



You will need to use a series of U-Boot setenv commands to configure U-Boot by setting parameters in the form of keyword value pairs. Then use the U-Boot saveenv command to save your settings in non-volatile flash so that they persist across resets. Finally, you can use the U-Boot printenv command to see all of your saved settings. This is described in the Getting Started guide on the software CD-ROM. Use the printenv command first to see what parameters might already be set. (There is a typo on the first page of the guide: when setting the bootargs parameter, there is a space instead of a comma between 115200 and mem.)

You will likely want to replace the IP addresses in the guide for the parameters ipaddr, the static IP address of the EK, and serverip, the IP address of your TFTP server. Here are the commands I used.

setenv ipaddr 192.168.1.223
setenv serverip 192.168.1.102

The guide also shows you how to define long U-Boot command sequences, save them as variables, and then run them. I followed the manual and defined the commands kernel to download the kernel image, and ramdisk to download the ramdisk image. This is useful to save time in the long run, although don't hesitate to try using the raw U-Boot commands. (The guide doesn't show using single quotes to enclose strings with embedded spaces as the value portion of the U-Boot setenv command, but I've used that practice on U-Boot on other projects in the past.)

Below you can see how the U-Boot printenv command displays all the settings for my EK.



To boot the board, you need to tell U-Boot to use TFTP to load the kernel image into memory, then load the ramdisk image into memory, then finally to boot the board. U-Boot boots the board by literally calling the kernel image as a subroutine and passing to it the address of the ramdisk image as a parameter.

First, I download the kernel image linux-ek from the TFTP server to memory location 0x21000000 using the command kernel that I defined.



Next, I download the ramdisk image ramdisk from the TFTP server to memory location 0x21100000 using the command ramdisk that I defined.



Finally, I tell U-Boot to boot the kernel image at location 0x21000000 using the bootm command. You can see below that Linux 2.4 boots and I log into the system.



Using the EK right out of the box, I was able to compile "Hello, World!" on my Linux PC using the arm-linux-gcc command in the provided tool chain, download the resulting ARM executable to the board using TFTP from the shell prompt on the EK, and run it on the EK. There is a lot of learning experience in just getting this far.

Of course, I wasn't satisfied. This was the older Linux 2.4. I wanted to run the current Linux 2.6 kernel. I wanted to use Windows and Linux file systems on SD cards and USB drives. I wanted to experiment with tools to reduce the memory footprint of the system. I wanted a richer set of commands on the EK than what was provided out of the box. I wanted to port my Desperado embedded C++ library and run the unit tests. I wanted Linux to configure the Ethernet port automatically during boot.

In upcoming articles, I'll describe how to get Linux 2.6 running on the EK to do all that other fun stuff.