Linux-Powered Amateur Rocket Goes USB

After the 2005 crash, it would have been easy for PSAS to rebuild the rocket using this data. We toyed with the idea of rebuilding it exactly like the old rocket, but then “second system syndrome” set in. We just had to make the new rocket better than the old rocket.

The airframe team decided to redesign the airframe and the pyrotechnic parachute deployment system, as PSAS had concluded that was the point of failure for our launch. The avionics team decided to upgrade our flight computer from a 100MHz AMD Elan to a 400MHz Freescale MPC5200 (purchased with a grant from IBM).

The avionics team also wanted to upgrade the various avionics subsystems. We wanted the GPS, inertial measurement unit and all the other avionics sensor “nodes” to get data to the flight computer faster. The old rocket used 8-bit PIC microcontrollers that communicated over the Controller Area Network (CAN) bus. The avionics team wanted faster microcontrollers and a faster bus that was easier to develop software for.

Figure 7. New PSAS Avionics (Credit: Andrew Greenberg)

I was part of the Portland State University senior capstone project that was assigned the task of upgrading the avionics bus and sensor node microcontrollers. After much debate and argument within PSAS, we decided to replace the 1Mb CAN bus with a 12Mb full-speed USB. We chose a 32-bit ARM microcontroller, NXP's LPC2148 (see Resources).

The LPC2148 made the cut above the other 64-pin ARMs with USB because it already had an open-source library (LPCUSB) that would bootstrap the chip and control the USB peripheral. The main LPCUSB developer, Bertrik, was kind enough to let some PSAS members have commit access to the SVN repository, and PSAS has been contributing new features since then.

Choosing the LPC2148 also allowed us to pick from some very inexpensive hardware. An Olimex LPC2148 development board with USB, serial, JTAG and a built-in breakout area can be purchased for about $75. The Olimex JTAG programmers are about $50, and the free and open-source OpenOCD Project can be used to program the LPC2148 over the JTAG port. This makes it easy and cheap to build your own rocket avionics node at home.

You also can program LPC2148 to be whatever kind of USB device you want. The LPC2148 supports all four types of USB transfers and has enough Flash (32KB) and RAM (512KB) to support a moderate amount of code. Hardware hackers also will like the fact that it has I2C, SPI and plenty of GPIO pins. The LPCUSB library already supports several different USB applications, such as a USB COM (serial) device and a mass storage device (Flash drive). These examples easily can be hacked to create custom USB devices.

If you want to start playing around with the LPC2148, you need to set up a development environment with a few different tools: an ARM-ELF cross compiler (for compiling code on a Linux box to ARM machine code), install tools for downloading the binary to the LPC2148, install host-side software to talk to the board and (optionally) the Eclipse IDE to set breakpoints on the LPC2148 and step through the code.

Fortunately, Dave Camarillo and Kay Wilson made a set of scripts to install and download all the necessary software and bundled them into a git repository with the PSAS LPC2148 source code:

$ git clone git://git.psas.pdx.edu/git/lpc-kit.git

The examples in this article assume you cloned the git repository from your $HOME/git/ directory.

Read the installation directions in the Doc/ directory. The psas_lpc_setup.pdf describes the hardware setup and what the scripts are trying to install. The scripts assume you're running on a Debian or Ubuntu Linux box, but they easily can be modified to run on an RPM-based distro.

Once you've run the shell scripts in the Tools/ directory, you can compile and download the simple serial example in the Dev/2148/poke/src/ directory to the LPC2148. The whole process is documented in the “Example Programming” section of psas_lpc_setup.pdf. The document walks you through setting up the cables, making the sample code by using the Makefile in Dev/2148/poke/src/ and using OpenOCD to program the LPC2148 board.

Figure 8. LPC2148 Example Setup (Photo Credit: Sarah Sharp)

When you plug the reprogrammed LPC2148 in to an RS-232 port into your computer, a TTY device is created. If you're using a straight-through serial cable, /dev/ttyS0 is used. If you're using a USB-to-serial adapter, /dev/ttyUSB0 is created. Then, you can use minicom, or any other terminal emulator, to talk to the LPC2148 board. The default minicom settings (115200 baud rate, 8N1) are fine.

The reprogrammed LPC2148 echoes back whatever you type and outputs messages when you press the round black buttons on the board. This simple example should allow you to verify your build environment and ensure that you can talk to your board over the serial port.