First Luminardo prototype has been assembled and tested. As it usually happens, first revision is not without flaws. There are two issues with circuit diagram, one issue with component footprint, a few typos in BOM as well. First and the most annoying issue is related to 74AHC125 buffer (U5) which outputs never go to high impedance state causing conflicts on SPI bus and reset line. As a result, it is possible to program ATMega chip via ISP but after it indefinitely remains in reset state even with ISP programmer disconnected as the buffer pulls ‘RESET’ low. Solution is simple, buffers outputs should be active only during programming procedure, in other words, when ‘RES_5V’ is pulled low. As 1, 4 and 10 inputs work as ‘output enable’ lines which active LOW level, it means that they should be driven by ‘RES_5V’ instead of just being connected to GND. Additional pullup resistor should explicitly put the outputs to Hi-Z state when nothing is connected to ISP connector. The required modifications are shown on the picture below.
The second problem is related to LD1117 (U9) voltage regulator, its TAB pin is actually internally connected to Vout and in our case it was the cause of short circuit. The required modification is shown on the picture below.
And finally, there is an error with temperature sensor (U1) footprint, first and third pins are swapped, currently fixed by 180 degrees rotation of the component.
All in all, the first PCB has been successfully soldered with the abovementioned fixes and fully tested (except for 12-5V DC-DC convertor). The assembled PCB with MVFD 16S8D Module attached is shown on the pictures below:
More information will come very soon, stay tuned.