Teensy 3.5/3.6 40 I/O + 2

Hi - Why are the number of 3.5/3.6 I/O pins described as 40 + 2 ? I can see the 42 pins on the PCB edge, why are the two pins above pin 39 not labeled ? Are they available for use ? On the topside these two pins are labelled (in the picture) "A21" and "A22", on the underside I think I can read "DM0" and "DM1" (silkscreen).

Some pins do not work with digitalRead, or digitalWrite. These pins don't have conventional digital pin numbers, only analog pin number (i.e. they begin with an A). On the 3.5/3.6, these pins are:

• Pins A10 & A11 (inner through hole pins, next to pins 22 and 23);
• Pins A21 & A22 (outside, between the middle ground pin and pin 39) -- note these pins can do a true analog output (DAC) as well as analog input (they can be used to emit stereo sound using amplification);

In addition, the 3.5 has two additional analog input pins (A25 and A26) that are in the second inner through hole row by pins 2 and 3. The 3.6 puts the USB host pins there, but the 3.5 doesn't have support for the USB host, but instead has 2 more analog input pins.

Except for the two DAC pins (A21 and A22 on the 3.5/3.6, A14 on the 3.1/3.2, A12 on the LC), analogWrite is mis-named, and it should be called something like PwmWrite. The analog output is done by turning the pin on/off rapidly, so that the sum of the voltage will be between 0 and 3.3v. This works for LEDs, but it isn't as useful for sound output where you want the voltage to be actually changing instead of turning the pin on/off. The DAC pins can vary the voltage (between 0 and 3.3v or 0 and 1.2v IIRC, depending on the settings).

Ok. Thanks.