What's the precison of the temperature control? The cold junction isn't measured, right? Or do you use the controller internal temperature sensor? Some of the SAMD have terrible tolerances on those as well! 

there is thermocouple in solder tip, measured is via operation amplifier and MCU analog input

But a thermocouple only measures the temperature difference between the hot and cold side (temperature of the pcb). This could shift the measured temperature by quite a few degrees, additionally to the errors already in the system!

If you connect segments a and d of the first digit in parallel (e.g. always switch then on and off together) and add element f, I think you may be able count from 000 to 699 with 20 charlieplexed LEDs on 5 GPIO lines. You would need to choose higher brightness LEDs for the 2 in parallel so they appear as bright when supplied with the same current, which complicates this quite a bit ... some experimental time needed to prove this idea first!

If it does work it could be useful if you want to set the tip above 399, or use this trick more generally in other projects

At the risk of talking to myself ;-) 

Looking at the Wikipedia page on charlieplexing ... the more complex scheme using one resistor per led can be used with 2 leds in parallel - see https://commons.m.wikimedia.org/wiki/File:3-pin_Charlieplexing_matrix_with_individual_resistors.svg for the schematic of 3 gpio lines for 6 leds with individual resistors, scale to 5 gpio and replace one of the leds with a pair in parallel ... obviously making sure the mcus current source limit is adhered to

The 'cost' is 21 resistors rather than 5.

The alternative is to have 19 leds that provide a certain light output (milli candelas, mcd) at (say) 20mA and the 2 in parallel that provide the same (or similar) light output at (say) 10mA ... if they are the same colour they'll have a very similar forward voltage to the less efficient leds, but should share the 20mA current evenly (especially if they are from the same batch). This alternative allows you to keep the 5 resistors, but makes led selection more complex. I'm happy to look for suitable leds if you are able to add the schematic and bom ... if you happen to know the current limit on the Samd11C14 pins it'll save me looking it up.

To clarify the 'trick' for anyone confused by the description - numbers 1 & 4 don't light segments a & d - numbers 0, 2, 3, 5 & 6 light segments a & d together (as does 8, 9 can too, though it can look like a g) - unfortunately 7 needs a on and d off ... so the limit is 699

I am a solder fanatic, I can't count on one hand how much irons i have, but this one is still out of my reach for NOW.

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