In purchasing a Mixed Signal Oscilloscope (MSO), users will have to carefully consider its triggering capabilities.
MSOs with more acquisition channels offer more triggering capabilities that will enable users to zero-in on specific digital/analog I/O signal interaction. Although its triggering capabilities still cannot compare with that of high-performance logic analyzer, triggering using MSO goes far beyond the triggering delivered by traditional four- or two-channel oscilloscopes.
Nowadays, most mixed-signal solutions and MSOs can trigger on at least a level of parallel trigger conditions, while some offer up to two levels of pattern sequence triggering with reset conditions. However, even when using relatively simple one-level pattern triggering, users will note significant differences in triggering capabilities among various mixed-signal measurement solutions/MSOs.
In choosing the right MSO, users should select one that can trigger on a combination of digital and analog inputs. Due to signal skew between logic and analog channels, some loosely tethered mixed-signal measurement solutions can only reliably trigger on one side of the acquisition system. This implies that users are allowed to trigger on a single trigger condition only — the traditional analog trigger condition or on the parallel digital condition only, but never on both. Aside from mixed-signal triggering capabilities, MSOs should also provide precise time-alignment triggering among digital channels and analog channels.
In addition, users should also consider whether the pattern triggering of the MSO includes any type of time qualification. The MSO should not only offer entry and/or exit trigger qualification, its pattern trigger conditions should also feature a minimum time-qualification condition.
Users should also keep in mind the importance of minimum time qualification in order to avoid triggering on unstable/transitional conditions. Although switching may be almost simultaneous whenever parallel digital signals change states, it is not exactly simultaneous. Slight delays may still be observed in between signals even in the best-designed systems, in addition to falling edge and limited rising speeds when signals are neither low nor high. Hence, users should not be surprised if transitional/unstable signal conditions may be seen in the system during switching of signals, even if they would want their MSO to be free of these unstable conditions.
Compared to logic analyzers, which use sample-based triggering, oscilloscopes (MSOs included) can accurately trigger at analog trigger level/threshold crossing points.
Sample-based triggering means the device randomly samples the input signal first. Based on the sampled data, it then establishes the trigger reference point. Although this type of triggering may be adequate for typical logic analyzer measurements, it is not acceptable for traditional oscilloscope measurements, much less to an MSO, for viewing repetitive signals.
Finally, when evaluating MSOs, users should consider whether or not the oscilloscope can trigger on a certain address as well as data transmissions of serial I/O, which is very prevalent nowadays in embedded designs, like SPI and I2C.