Yep, I´ve tried different power sources, I added additional decoupling caps at the CPU, I also checked the supply voltage with a scope. The 5V at the CPU are rock stable, all other tests are running perfectly but 47uF or 100uF caps can not be tested.
There are a lot of questions about this (and not only) problem with measuring capacitors on AVR ATMEL (purely Chinese MKs are a different story), so let's try to summarize what we know at the moment. There will be a lot of text, so whoever is not interested or too lazy to read, you can skip it.
So, description of the problem:
The tester turns off, "you (are) cut off", the display goes off during the measurement of capacitors, mostly of large capacity. When measuring through the menu, there is usually no such effect.
This problem is typical for software from Karl-Heinz k-firmware (1.12-1.13k).The software from Markus Reschke (m-firmware) also has this effect.
What do you mean it turns off? That is, for some reason the MK resets or the control program fails. The most likely reasons:
1. The power supply voltage of the MK and/or display disappears for a short time;
2. Power supply of the tester (MK) from a pulsed source with large ripples, interference and noise;
3. Triggering of WDT (watchdog timer) - enable/disable is set by the appropriate fuse before compilation or in the software;
4. BOD activation (supply voltage level monitoring system) - enable/disable, the threshold is set by the corresponding fuses before compilation.
5. MK defective (quite possible, given the strong rise in price of original chips and the slag that the Chinese industry is now trying to “sniff”).
The solution:
1. In 99% of cases there is a contact problem (bad or “cold” soldering, “straight-handedness” of the assembler).
2. The effect is eliminated by supplying power to the MK from a high-quality LBP or battery.
3. The authors suggest setting WDTON to the disabled state. In the software of both authors, this timer is reset programmatically before each measurement cycle and is activated if the operation (measurement) is not completed after a certain time. Usually in such cases the message TimeOut appears on the display. For more details, see the manual from Karl-Heinz.
4. The authors recommend turning on BOD and setting the response threshold to 4.3V. For the clock frequency (quartz 8 MHz), high-quality +5V stabilizer and filter capacitors, this threshold does its job perfectly and no problems were noticed with any measurements.
Factory Chinese components in most cases cannot digest this threshold, since under increased load (measuring a capacitor is one of the longest processes in terms of time and load), short-term voltage dips appear, which are sensitively detected by the BOD. Therefore, advice such as set additional fuses and lowering the BOD threshold to 2.7V sometimes really helps solve (mask) the problem, but not always.
With an increase in the clock frequency of the MK to 16-20 MHz, the current consumption and, accordingly, the load on the power bus and stabilizer also increases. The safe operation zone of the MK narrows.The picture that explains this is below.
You can also lower the BOD threshold or turn it off completely; the MC will not reset, but then there is a very high probability that measurements during such voltage dips will be incorrect or distorted. The optimal solution would be to install a QUALITY +5V STABILIZER (for example, MCP1702) and similar quality filter capacitors at its input and output!
5. In most cases, a faulty MK is determined quickly when running self-testing and calibration using hardware tests. If possible, you should try the m-firmware software or replace the MK with another one.
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