Thanks a lot for sharing this. Yet it is a bit strange, because libusb0.dll is supposed to be in the same folder as arduino.exe. It has always been there and on my PC it still is. Compiling for and uploading to the Elektor Uno R4 with Arduino IDE 1.8.5 and the Boards package on Windows 10 works perfectly fine for me. Didn't other boards show the same problem?

Could be that your contrast is set wrong.

hi  i used your PCB to make a power meter and used a blue light LCD 1602 but the characters show in black not white in LCD screen ! what should I do? I know that in your layout your are not connected vss to gnd ! do you think there is a problem with my power supply?

I calibrated the sketch with the LDR I use but the display doesn't dim. I checked the values in serial console and they should be right (going from 255 to 1). Any hints where to search the error?

Answers to oetelaarNG: 1.) Yes, this is the OTP random number sequence generated with the TRNG, to be used for OTP encryption. 2.) This random data is a random meaningless sequence that is to be used for encryption of sensitive data if the box gets delivered to the recepient without tampering along the way.  This data is not an encrypted message - anyone can open the box along the way and read it and eventually decrypt it, so we don't risk that. 3.) The answer is  at 1.) 4.) -You don't actually destroy the data on the SD because zeroising the data in SD card is very slow and difficult. You simply declare it invalid and insecure   by zeroising the the challenge and response in MCU SRAM which is much easier. -If the box has been tampered with, the recepient can use his RNG to generate a new sequence on a new SD card and send it to the original sender. Maybe it won't be tampered with this time... No special secure vault is needed, every pair of peers can generate the random numbers from their RNGs. -Check my answer to Poul Borg Petersen about the challenge-response procedure to check for possible tampering during the transport. As you can see, intercepting it is actually no big deal if the secure key (random seauence) on the SD hasn't been read. The only problem is if somebody manages to actually alter the challenge or response, but this is much harder to do. -Someone else getting the data is not a problem because this is a meningless random sequence. It is enough to simply warn the recepinet about it. -I read "Secrets and lies" from Bruce Schneier and also "Applied Cryptography" from the same author. Basically I like Bruce as an expert, although I don't share his harsh legalistic attitudes - like calling Kevin Mitnick "a convicted criminal hacker". -A good RNG and a good crypto device solve the problem of virtually unbreakable OTP encryption. Tamper-evident box solves a problem of secure distribution of OTP keys. I am not a very good mathematician, so I trust OTP method because its mathematical background is very simple. That's why I chose the OTP.