Embedded Linux on the PowerPC

Although Intel provides the greatest user base for Linux, many other architectures are supported. These include ARM, MIPS, PowerPC, Alpha, SPARC and Hitachi. The availability of cheap x86 hardware makes Intel a simple choice, but in cases where performance is important, the PowerPC (and others) can't be ignored.

This article will demonstrate Linux using the MontaVista Hard Hat Linux distribution on an Embedded Planet PowerPC board. We'll also discuss how to flash Linux with a ramdisk root filesystem onto the hardware for self-boot.

With the ubiquity of x86 and available tools, why choose anything else? A quick summary of the architecture may answer that question.

PowerPC is a highly integrated RISC architecture optimized for communication systems. The PowerPC combines a PowerPC processor core with a CPM (communications processor module) that offloads traditional communications tasks, giving the core more cycles for the actual processing requirements. The CPM also permits a variety of serial communication controllers (SCCs) to be programmed with a particular personality chosen from the set available from the core (such as Ethernet, SDLC, IDA or standard serial).

The other driving force behind PowerPC is its power consumption. For applications that require high performance with low-power consumption, PowerPC is a great choice. Tie this altogether with wide Linux and tool support, and you have a very capable solution.

The MontaVista distribution is quite easy to install and configure, and their 100-page CDK document provides details on host configuration, building applications and other topics.

One of the interesting features of the MontaVista distribution is they not only provide configurable kernel source (for a variety of hardware architectures and platforms) but also a tested set of application packages for each architecture. These packages include the Apache web server, standard network tools (including ipchains, ipmasqadm), Perl, SNMP and many others.

Porting apps to the particular architecture can sometimes be nontrivial, but in the Linux Support Package (LSP), MontaVista has already integrated much of what you need.

As far as your development host goes, you must have an Ethernet port and an available serial port to work with the target. More on this later.

For demonstration purposes, I've chosen the RPX Net board from Embedded Planet (see Figure 1). The RPX Net is a PC/104 form-factor PPC855-based board that includes 32MB of DRAM, 8MB of Flash, a Fast Ethernet Controller (FEC), four-port LAN hub, a standard serial port and other interfaces. This particular board can serve as an embedded gateway to provide IP-masquerading (network address translation) and port forwarding between an external WAN and the internal LAN.

Figure 1. RPX Net

While MontaVista supports a variety of Embedded Planet boards directly through their LSPs, the RPX Net is not currently one of them. (MontaVista currently supports the RPX Lite, Classic and Credit Card boards.) Therefore, a number of changes were necessary to support the new Ethernet devices. In most cases, you can pick the LSP that most closely matches your hardware and then modify for the differences.

The RPX Net also has PlanetCore utilities, which include a bootloader, diagnostics software and an external suite of tools for embedded development. PlanetCore can be used for board diagnostics (for any EP board, including I/O modules) and also for OS-neutral boot loading (in the absence of GRUB or LILO), RAM-based software downloads and host-based Flash burning. We'll illustrate RAM-based downloading and flashing of a kernel shortly.

Developing with the RPX Net (and virtually any other embedded network board) requires two connections between the host development system and the target. First, a network connection is required for high-speed download. This can be a Cat-5 crossover cable if the host is connected directly to the board or a standard Cat-5 cable if the host and target are connected through a HUB. A serial connection is required for PlanetCore communication and is provided through a standard DB-9 connector. A terminal emulator, such as minicom, can be used for serial communications with the RPX Net.