How about


shift right 4 times (thus losing bottom 4 bits and shifting 0 into top 4 bits)
if 0-9 then add x30
else add x37

mask off the 4 msb's
if 0-9 then add x30
else add x37

so for example 3A
shifted becomes 3
3 + x30 = x33 ASCII for "3"

masked becomes x0A
x0A + x37= x47 ASCII for "F"