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If the synth module is not even plugged in you still get
text -- "QRP Labs U3S ver 3.12 etc....."until an error message is
On 03/04/2018 21:45, Daniel Ekman
If I'm not mistaken, the lcd display is initialized
before the mcu even speaks to the synth over i2c. So the two
rows should disappear (and probably display text) before the
i2c synth stuff begins. So the 27MHz crystal is irrelevant
during this phase. The mcu has it's own 20MHz crystal to
run, so it does not need anything from the synth, as is
clear when you power up the board without the synth.
Search on the synth board for the problem first. I suspect
there's something wrong with the level shifters (Q1,Q2) or the
i2c pins (4,5) on the Si-chip (measure at R3,R4). They should
both idle at 3.3V at the chip side and 5V at the pinheader to
the QCU board.
I'm not so familiar with the Si-chip as to know what it does
when it's powered up without a controller talking to it, perhaps
in the datasheet. Still this does not help too much if the
problem lies in the i2c pins and level shifters.