Reorder the resistor ladder sequence
In the hardware, the 2600 key appears above the 2 key. However, when I wired up the prototype on perfboard, I put the 2600 key after the hash key in the resistor ladder. That was an accident. I should have placed the 2600 key at the top of the resistor ladder. However, I'm stuck with the configuration in getkey() while I'm still using the prototype. Once I fix the problems with tones not being recognized, I should be able to make some PCBs and retire the prototype.