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Application notes - HTML

Few notes about handling electronic devices.

The following essay tries to explain all possible ways in which you can damage - or destroy - an electronic device without any visible signs.

For the introduction, an explanation of some terms used in the essay:

Electronic device is any device which electronic part may get damaged by improper handling as a result of an electrostatic discharge (ESD) or by equalizing currents. All our simulators and programmers fall into this group.

Electrostatic charge may be generated and kept on various objects, e.g. clothes made of artificial fibres, on moving machines bypassed by the air, on various types of plastic packaging, on a sheet of paper from a copier and even a person. Charging occurs when two surfaces are mutually rubbed, while at least one of them is electrically not conductive. The amount and polarity of the charge depend on different affinities of electrons towards the mutually rubbed materials, on the rubbing force, as well as on humidity and charge of the surrounding air.

Electrostatic discharge (ESD) is the transmission of an electrostatic charge between objects with different potentials during their mutual contact or by a discharge through an air slot with an electrostatic field. According to the standard, the amount of the electric charge on a person may lead to up to +/- 8 kV discharge by a direct contact and up to +/- 15 kV discharge in the case of a discharge through an air slot, which results in peak currents of 1 – 10 A.

Equalizing current is a current between two electric devices interconnected by a conductor, when both devices try to acquire a common potential. This current may travel for a very short time (split of a second) or continually. In the former case the devices are not galvanically connected, but they are connected, for example, by a capacity coupling or trough a separation transformer, and they have different potentials. In the latter case they may be network devices, supplied from the same mains branch, but from two different sockets with different potentials. The difference occurs as a result of unequal drop in the voltage on the grounding wire as the current passes along it in the mains branch.

And now to the point: ESD

The general rules for handling electronic chips that are sensitive to an electrostatic discharge are mentioned in most electronic chip catalogues, therefore I am going to mention just a few of them, without trying to make the list complete: grounding of the table, of the floor as well as of the handling person through a 500 kOhm resistor; grounding of all instruments; relative humidity at least 50 %; use of an optional air ioniser. Similar rules, of course, also apply to devices whose parts with such sensitive chips are accessible to a direct contact (e.g. by a finger)! It is the case of various "starter kits" without casings, but also the case of all emulators, simulators and programmers that (with the exception of the 'high-end' ones) do not have their external interfaces protected by additional circuits. For example, terminals of an ISP connector of programmer lead directly to the core circuits of the programmer emulator, without protective elements, which, if they were expected to really work, would unacceptably increase the prices of particularly low-cost devices.

For most of us the recommendations for handling chips sensitive to the ESD sound to be out of proportion, and we ask which are the ones whose ignoring will not show. Optimists tend to say that chips will survive it. Pessimists would say that nothing will help. It is up to a person to consider the risks resulting from ignoring recommendations. I recommend to an always hurrying developer to observe at least some of the cult rituals:

• Try to identify a conductive object, grounded through an about 500-kOhm resistor, which you touch before you start working ('touch me first' point). If you touch a metal frame with a photo of your mother-in-law, it will become almost alive.
• Try to use clothes, chair upholstering and a table cloth made of natural materials.
• Suppress your longing to touch a connector or an emulation socket contact at least until after you discharge your electromagnetic (electrostatic) energy into something else. The best is to completely avoid touching connectors with your fingers also because of the possible dirt/sweat contamination of their contacts.
• Put the protective sponge on the simulator's emulation socket /emulator always when you remove it from the device.
• If your pass on chips to others, just throw them. In this way your will prevent the equalisation of potentials through the chips. If you do not suffer from a touch aversion towards the other person, and alternative solution is to touch him/her before you pass on a chip to him/her.

This picture show an ideal ESD protected workplace:

Rules for the interconnection of electronic devices

Each developer knows that the 'ground' is most important. If two devices do not have a common ground, equalizing currents flow through them when they are interconnected. They flow, of course, as they want and particularly as you do not want. The intensity of equalizing currents depends on the difference in the potentials of the two devices (ranging from millivolts to hundreds of volts), on their internal impedances and on the impedance of the connecting path (up to a shortcut).

If we think about devices powered from a mains supply, the differences in the potentials of the 'grounds' may be up to tens of volts at very low impedances of the power supplies. In the case of "good" conditions - particularly the GND/GND connection – wire insulation (or wires itself and copper tracks on PCB inside of devices) may get damaged as a result of excessive currents flowing through the wires. The range of damage also depends on which interface contacts touch each other as the first. With some exceptions, if the grounds touch each other as the first, nothing will happen. If the signal contacts touch each other as the first, damaging of electronic circuit protective elements (blocking diodes) occurs. In general, I recommend to power devices from the same mains wall socket or to check the differential voltage between the grounds of the interconnected devices. It is suitable to interconnect the grounds of the devices by using a separate wire.

Note: The above mentioned interconnection rules do not concern the devices, whose interface contain by the galvanic separation, but to keep common ground is better habit than don't do it..

This picture show a recommended wiring of ISP programming workplace:

Notes on the handing of ELNEC products

• Higher integration chips are the most liable to damage, therefore at the top of an imaginary damage liability chart are the FPGA-based programmers and EPROM simulators (whose simulation socket contacts are connected directly to the target application). Therefore, if you work with T51prog/T51prog2, PIKprog+/PIKprog2, MEMprog/MEMprog2 and SmartProg/SmartProg2 programmers, please, observe ESD handling precautions.
• Our higher-grade programmers (BeeHive8S, BeeHive4+/BeeHive4, BeeProg/BeeProg+, JetProg, LabProg+) and SIMEPROM-02aLV are protected by special circuits in order to withstand an ESD discharge of up to 15 kV.
• In the very conclusion: Somewhere I read a claim that "the designer's experience is proportional to the number of chips he/she destroys". To decrease one's sense of guilt, I recommend identifying with this claim. Even though its author might have meant "inversely proportional".