@lacuna said:

Fixed point arithmetic can be exact...

You and I are on the same wavelength. Someone on the Arduino forum gave me a link to the Arduino printFloat() routine, so I rewrote it in Q4x60 fixed point (i.e. 4 bits of integer, 60 bits of fraction). It now prints out those excess digits correctly. So the issue with their library routine is that it uses doubles to calculate what to print for doubles, which just happens to print zeroes forever past the 10ths place in the case of 0.1 (but not other values, like 1.1). It's a simplistic algorithm due to resource constraints, i.e. why waste limited resources on correctly displaying digits beyond the precision limit? Someone else on the Arduino forum linked to a 15 page academic paper on how hard the fp print problem is--I'm only up to page 4