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I/O sync techniques for complex embedded designs

Posted: 15 Apr 2014     Print Version  Bookmark and Share

Keywords:automation systems  VxWorks  FPGAs  ADC  ENOB 

Measurement and automation systems involving multiple devices often call for accurate timing so as to facilitate event synchronisation and data correlation. For example, an industrial automation application may need to synchronise distributed motion controllers, or a test and measurement application may need to correlate data acquired from sensors distributed across a device under test.

To achieve this synchronisation, devices in the system must either have direct access to timing signals from a common source, or the devices must synchronise their individual clocks to a common timebase. There are advantages and disadvantages to both methods of device synchronisation.

In systems where the devices are located nearby each other, typically a few meters, sharing a common timing signal is generally the easiest and most accurate method of synchronisation. However, a system in one physical location may be composed of several different types of processing units. Systems can be composed of PCs, running a modern operating system like Windows or Linux; embedded devices, sometimes running a Real-Time operating system such as VxWorks and FPGAs. Synchronising these logically distributed systems can be complicated.

This is further complicated when systems are physically distributed over a large area. In these situations, distributed clock synchronisation becomes necessary. Using this approach, devices act on timing signals originating from a local clock which is synchronised to the other clocks distributed throughout the system. Examples of distributed clock synchronisation include devices synchronised to a GPS satellite, a PC's internal clock synchronised to an NTP time server, or a group of networked devices synchronised using the IEEE 1588 protocol. Instead of sharing timing signals directly, these devices periodically exchange information and adjust their local timing sources to match each other.

Another complication to synchronisation is the type of ADC that is used. The most commonly used types of ADC's for measurement and control applications are SAR (Successive Approximation Register) and sigma-delta (also known as a delta-sigma). These two types of ADC's are relatively easy to synchronise if all of the ADC's in the system are of the same type. Synchronisation gets challenging when both types are used due to the different ways that they sample data.

Synchronisation and measurement accuracy
When synchronising a system, it is important to consider the level of synchronisation that is required. For some applications, having correlated data is important to have a snapshot of your entire system. If the parameters you are measuring are changing rapidly, synchronisation within a few milliseconds or even a few nanoseconds may be required. It is also important to know if synchronisation to an absolute time is important, or just a relative time from an event. Jitter, any undesired deviation from a truly periodic event, affects general synchronisation, but may also have an impact on the accuracy of your measurements.

Clock jitter and accuracy. When a system is comprised of multiple clock domains, jitter is introduced whenever a signal crosses between them. For example, say there is a system that pulses convert on a SAR ADC. The system has a trigger input that comes from an external timing source. This could be a pulse generator or a done signal from a sigma-delta ADC. The system that generates the CONVERT pulse to the SAR ADC must sample the trigger on one of its clock edges. This can introduce up to one system clock cycle of jitter into the conversion pulses.

Figure 1 shows a system where the trigger coming into the system must be re-synchronised onto the system clock. There is an uncertainty of one system clock period for when the trigger is seen by the system. This will cause one clock period of jitter on the conversion pulse.

Figure 1: Conversion jitter.

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