Space-Time Processing—Linux Style
Our Linux-powered digital boards normally run complex processing algorithms and communicate over air and Ethernet to other systems. Even under laboratory conditions, it is difficult to know whether all components are operating correctly, so we decided to utilise the power of Linux to implement self-test and monitoring solutions. Like our visualisations, this ties many components together in a flexible way.
Nontechnical users demanded a graphical interface, which we could have created using GTK, Tk, Qt or something similar, but we decided instead on PHP-powered Web scripting. This allows users of all platforms to access the system. The C-like language syntax makes it easy and quick to use for C programmers, and the familiarity of modern users with Web interfaces helps. The source code for most applications is less than 50KB in size, which is a major advantage in debugging and testing and consequently improves our confidence in that code.
Unfortunately, such a user interface has a couple of disadvantages: first, when a background event must be brought to the attention of a user and, second, when the system must interact directly with hardware. The first problem can be solved through the use of ticker-type messages in a separate frame. In our system, we solved the second problem through the use of small low-level C programs with a file-based interface to PHP.
In truth, our system is quite convoluted. The controller in each unit is the ARM processor, but the controller for all controllers in a multi-unit system is the Web server serving the PHP scripts. For self-tests, the system implements a repetitive monitor script running on each ARM every five seconds and interrogates the system for the Web server. Details are written to a file named after the IP address of the Ethernet on each ARM. The monitor script also is responsible for ensuring that each operational system is synchronised in time to zion. We didn't use NTP due to code size and because we require sufficient time resolution only to prevent shared filesystem time consistency errors. We script zion to write the current time and date, using the date command, to the file every five seconds, and each ARM reads that file every five seconds to set its time. This gets around time synchronisation problems and fixes timestamps on files. We also use it as a watchdog to reset boards more than a minute out of date.
Apart from self-tests, the script also queries version numbers of RAM disk and kernel on startup and writes this to a status file. A final use of the script is to execute board-specific instructions. These are written by the PHP control Web page to tell each ARM board what it should be doing. Instructions from PHP are written as a shell script to be executed by a single board, identified by IP address.
All units run identical kernels and RAM disks stored locally in Flash memory and verified against master copies on zion at startup. Any inconsistency causes the correct kernel or RAM disk to be burnt into the local Flash memory. We built custom Flash memory tools to do this. In practice, we have had no storage errors, but we use this to roll out new versions of kernel and RAM disk without user intervention.
The PHP Web pages refresh every five seconds using the autoload HTML tag. Of course, this works only if at least one user has a browser currently viewing the page. If not, the information probably isn't required anyway. A manager's-eye-view Web page hides almost all useful information but looks great. Real users can click down through the levels to get to individual scripts that customise each board.
Linux software on the ARM sets up FPGA firmware to stream small packets of data from the wireless system to a debug buffer in the FPGA. In slower time, the ARM extracts this and stores it on zion. These files can be analysed automatically to look for faults (like all 0s) and if found, the user is alerted through the Web pages.
We recently have started to think about products rather than pure research. It is likely that we will communicate IP packets and some form of embedded Linux will power the product. This will happen not only because we relied upon it during development, but there are few viable options for handling IP packets. WinCE is slow and bloated, and VxWorks is lean but costly and lacks in protocol support.
Tait Electronics is a nonprofit electronics trust run to benefit the employees and society of Christchurch, New Zealand. It was founded by Sir Angus Tait more than 30 years ago. It exports 97% of its mobile radio products to more than 200 countries worldwide.
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