An oscilloscope and logic analyzer for the Raspberry Pi

Tough Measurements

The software programs run at a decent speed on suitable PC hardware; however, practical experience with the Raspberry Pi Model B showed that speeds with a graphical interface tended to be somewhat leisurely. The developers optimized performance by reducing their reliance on the L1 and L2 caches and by changing how they used the X server. The release of the Raspberry Pi 2 with its quad-core ARM Cortex-A7 should also improve performance [5].

BitScope Display is a software diagnostics program that you can use to test whether the graphics performance on a computer is sufficient to display measurement results. According to this program, Raspberry Pi performance is not great, but it does have enough to present a halfway responsive display.

The BitScope programs are identical among operating systems, because they sit on a common software platform. This abstraction appears to be the cause for a fundamental problem with the applications on the Rasp Pi. In the versions used in our lab, startup was very slow, and user interaction with the software interface was sometimes painful. Program operation required dealing with a lot of context menus invoked by a right mouse click. Once clicked, several seconds could go by before the menu appeared. A short software test on the Rasp Pi 2 yielded a much better experience. The user interface always reacted immediately.

In contrast, the dynamic representation of the results is pleasing. The graphics can be somewhat jerky, but they are nonetheless acceptable. The processor workload remains reasonable provided there is no user interaction. The Rasp Pi in our lab displayed a PWM signal for more than an hour in BitScope DSO without the processor becoming noticeably warm.

In addition to the oscilloscope function, I also tried out the logic analyzer to monitor an I2C connection (Figure 5). This program displays not only a signal curve, but also translates the signals into concrete values. I was able to follow the communication process via I2C without a problem. The meager sample memory of the BitScope Micro was the only negative feature.

Figure 5: The logic analyzer, BitScope Logic, performs an analysis of I2C data.

Limited Mobility

Because I was using the older Rasp Pi Model B, I decided not to test the software with an attached touchscreen [6]. Controlling the display requires computing time to the extent that operating BitScope software as well would take the Rasp Pi to its limits; otherwise, it could serve as a small, mobile ensemble.

Those readers who are not daunted by the prospect of programming can write test or logging scripts in C or Python and then execute them via the command line. This should work in combination with a mini-display, but spontaneous measurements and tests are not necessarily always feasible.

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