BEST RULES FOR A GOOD ELECTRONIC BOARD DESIGN
THE 10 COMMANDMENTS IN NO PARTICULAR ORDER
NUMBER 2 “WHAT IS NOT TESTED DOES NOT WORK”
NUMBER 4 “WHAT ABOUT ELECTRIC NOISE”
NUMBER 6 “3V3 VERSUS 5V AND VICEVERSA”
NUMBER 7 “SINGLE LAYER NO THANK YOU”
NUMBER 8 “THE MOST IMPORTANT SOFTWARE”
NUMBER 9“RESOLUTION VERSUS PRECISION”
NUMBER 10“EEPROM AND FLASH MEMORIES”
BEFORE READING
Bernini Design, particularly Bernini Mentore, started designing electronic boards in 1984. Since then over a million boards have been running worldwide. Still, Bernini Design supplies spare parts and fully compatible pin-to-pin replacement electronic boards. We have evidence of electronic boards running for over 30 years. There are no particular secrets to that. It is only about maniacal attachment to the quality and style of the design. For us designing an electronic board is a matter of art. Of course, we made mistakes. But we continuously learn forììrom them. After you are around to plan an electronic board, in expansion to the standard strategies you learned at school and composed clearly within the worldwide guidelines, consider taking after proposals.
it is certainly the most important. It goes for software, it goes for hardware. Avoid unnecessary complexity and component redundancies. Try to limit the microcontroller peripherals. To interface a load cell, for example, it makes no sense to use a dedicated 24-bit integrated circuit. Using 2 AD inputs in 10-bit differential mode, with the oversampling technique, we can certainly have lower resolution but excellent precision. An integrated circuit is eliminated!
“WHAT IS NOT TESTED DOES NOT WORK”
This should be a permanent rule in every project. Keep a notebook in which to write down all the details to be tested. Follow a clear CHECKLIST as aeroplane pilots follow before starting to fly. Never trust your memory. Write and enrich the CHECKLIST progressively as the projects increase.
ON/OFF/ON/OFF DYNAMIC TESTING IS AN EFFECTIVE METHOD TO VERIFY THE RESISTANCE OF THE PRODUCT TO POWER TRANSISTORS
This is the best way to track your progress and experience. Yes, using computer support, but pen and paper is the best method to create new neuronal circuits in your brain. Dynamic testing in which programmable logic activates and deactivates the power supply by providing random events to the inputs is the best methodology to highlight particular hardware and software defects.
Never underestimate a real low-temperature and high-temperature test. In this way, you are sure to deliver a product capable of working in the nominal range of temperature.
It is another fundamental point overlooked by designers. In other words, it is necessary to make a table that contains all the possible causes, even bizarre ones, of failure. Under no circumstances should the card react in such a way as to create 'TROUBLE'. For example, imagine the interruption of a connection. It would be enough to put a pullup or a pulldown; the microcontroller understands that it is a fault. For example, imagine that you unsolder a microcontroller terminal. It is necessary to avoid the driver, being floating, and generating unwanted outputs. Another frequent case, typical of battery-powered systems, is the gradual loss of power. 'IZIOUS' phenomena can be triggered which introduces operating uncertainties. The system must work or not work. There must be no middle ground.
While designing the electronic board by EMI regulations is undoubtedly important, it is insufficient. Particularly critical is the design of electronic boards that must control inductive loads. The fundamental problem is the generation of particular electromagnetic disturbance patterns that cause an unwanted reset. It is therefore not a question of the inability to control the inductive load, but rather of altering the global behaviour of the electronic board.
"BEWARE OF OPTOISOLATORS". The performance of opto-isolators does not take into account their installation on the printed circuit. You can set the PCB routing according to the insulation regulations. Consider that the presence of humidity and contaminants can degrade the insulation. Another parameter to evaluate is the life of the transmitter. Avoid working at the current limit and keep the transmitter permanently switched on.
Firstly, this philosophy allows you to extend your life by over 25 years. Do not underestimate the variation of CTR with temperature and the enormous variations even in the same production batch. Certain low-current photocouplers degrade their characteristics significantly when the temperature increases and decreases. A real test in a climatic chamber, or similar, carried out on some samples is always recommended. Finally, consider the global effects on the board due to a possible malfunction of the photocoupler.
The number one source of trouble is the management of the electronic board's power supplies. Particularly complex is the situation in which there are mixed 3.3V and 5V components. First of all, it is necessary to check that there are no 3.3V inputs in contact with 5V sources. Always ensure series resistance to avoid destroying the internal parasitic diode. Usually, the processor is powered by 3.3V. Check that the 3.3V is the power supply that arrives first and is the last to drop. In this way, the processor can manage all the resources at different voltages. Always use zener diodes, specially sized, in antiparallel to the power supplies. In the event of overvoltages, they will short circuit to protect all components. Do several power tests with very slow-rising edges and very slow-falling edges. Check the global behaviour.
If the aim is a professional board intended to work in the long term, absolutely avoid single-sided circuits. They should only be used for consumer, low-cost products with limited product life. By limited, no offence, we don't mean designed to fail. These are products that have a short life by definition as they will soon be overtaken by new devices involved in technological evolution. The double-sided circuit, through the metallized holes and thanks to the appropriate design of the tracks, better dissipates the heat generated by the components.
The double-sided printed circuit exponentially increases reliability, especially at the connection points of electromechanical parts such as connectors, cables, relays, buttons and various accessories. The micro-vibrations associated with them often cause electrical continuity problems.
Don't immediately dive into the final software. Think of the project as a fruit plant. First, it must be well cultivated and pruned regularly. Of course, never lose sight of the final goal. Get familiar with the hardware by developing test software. This allows you to check whether the hardware platform is compatible with the software.
First of all, keep at least one UART available for TROBLESHOOTING. Directly from the serial port, you can capture the essential information. You can watch the stability of the analogue readings. You can check the input-output, the display interface and keyboard buttons. Thoroughly fine-tune key drivers along with your watchdogs where necessary.
Never confuse resolution and precision. Let's not go into the merits of monotonicity (complex topic). First of all, it is necessary to respect the project specifications. However, it is easy to be fooled, for example, by the exaggerated resolution of the converters (e.g. 22 bit) when the board cannot support the presence of an adequate voltage reference. Worst case also, when the reference of the AD is the power supply of the board itself. A high-resolution converter costs a few euros, but a precision reference cost can rise to hundreds of euros. What are the solutions? Consider the accuracy of the readings. If 0.5% is required, a 10-bit AD is sufficient. Internal references of ADs must be carefully used: they vary greatly from CHIP to CHIP. They are widely used for different purposes. Accuracy can be increased by implementing a software calibration. Constants can be stored in the flash memory. The calibration procedure is activated in the board testing phase. An infinite number of electronic boards can be calibrated with an external reference costing 200 euros. The time required for a good calibration is only a few seconds. In other words 20 euro cents per card.
These components are extremely critical parts. They store store system parameters and flags. The technology itself is extremely reliable. The bottleneck is to control writing when the board is subjected to intense electromagnetic disturbances or supply instability. To prevent this, the processor has to measure its supply voltage before beginning a write. Once writing has started, the circuit must have sufficient energy to complete the writing (calculation of power supply support capacitors).
Certainly, a small supercap helps. Another strategy is to save in at least 3 memory areas and perform consistency checks. Avoid writing partial data consisting of long packets. Upon rereading, everything seems incongruent. Write only if you have the time necessary for the entire string. The software must present the user with safety options in case of memory corruption. Carefully analyze the behaviour during voltage drops, especially if you have an external watchdog.
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