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Emission Aware Programming
Additional recommended knowledge
Emission considerations require the evaluation of many details such as the clock frequencies and switching rates which are related to the oscillator. Rise/fall times and signal harmonics are related to the output driver. The data transfer rates can be dependent on hardware or can be controlled by software and often have to meet a certain protocol. Impedances, trace loading and the various circuit components are hardware related and must be considered from the beginning of the design.
Three basic actions for emission reduction can be defined:
In the following all components that can be influenced by the software design will be described.
Microcontrollers require a clock source for the switching of the internal transistors. Almost all controllers require an external crystal or ceramic resonator. Since the ceramic resonator is potentially sensitive to spikes which can shorten clock periods typically a Pierce oscillator configuration is used. Harmonic frequencies of the clock cause unwanted emissions.
The Power Supply
The field strength is proportional to the current as a consequence the power supply, providing the current for the entire system, is a strong source of emissions. Physically, a reduction of the power consumption of the system and the minimising of circuit loops (by the usage of decoupling capacities) emitting the noise, improves the EME performance. A software based solution is to temporary disable peripheral devices when not needed and thereby reduce unnecessary power consumption.
The I/O Pins
If a MC uses external memory space or peripheral devices continuous transitions on several data/address bus lines are implied. The emission depends on the frequency of the transitions, their quantity, rise/fall time and duration. The quantity of transitions, of port traffic can be influenced by the use of interrupts instead of continuous polling of ports. The use of interrupts is software based as well.
Rise and Fall Time
One way of reducing the emissions of a MC System is to extend the rise and fall time (Slew Rate Control). Some controllers like the Motorola HCS08 offer the feature of software controlled slew rate output buffers enabling the user to extend the rise time from 3 ns to 30 ns for each pin separately.
Some microcontrollers offer current limiting, a way of limiting the maximum current driven by the output port (e.g. Motorola MCS12).
Emission Control: Power Saving
The most effective way of reducing emissions is to to temporary shut down unused modules in the MC, thereby saving power. Most controller support several sleep modes.
In order to reduce electromagnetic emissions on software basis the following measures should be considered:
Most software implemented improvements on emission can only be detected using an Average-Detector!
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Emission_Aware_Programming". A list of authors is available in Wikipedia.