Hi,
Just got my Teensy 4 and audio shield up and running, it's wonderful. To explore these wonders i'm making a control panel, i'd like it to have at least 6 analog pots and 8 digital buttons.
From what i see in the documentation when the teensy 4 is connected to the audio shield only digital pins 0 to 5 and 9 and analog pins 14 to 18 on the front of the teensy are available.
On the back of the board i see analog pins 24 to 27 and digital pins 28 to 33.
Can all these pins be used for programing?
Pin 18 is NOT available. It is used along with pin 19 for the I2C bus. Pin 15 has some resistors and capacitors that can make using it for things other than analogRead harder. Pin 22 is available. All of the pads underneath the Teensy can be used. Note, if you are trying to use the bottom pads, you probably want to mount the audio shield above the Teensy.
I posted this some time ago that listed all of the pins 0..23:
This page out of the Teensy wiki lists many of the boards to breakout the bottom pads (scroll to the bottom):
An alternative approach is to use I2C cards like the MCP23017 that can provide 16 digital inputs or outputs. There is the ADS1015 that can off-load 4 analog inputs. You can put multiple MCP23017's and ADS1015's on an I2C bus using different address bits for each card:
Note, Adafruit is now closed for direct sales due to the virus. You can order it from distributors that are still open (like digikey.com at the time I wrote this response). Also the parts are fairly generic, you don't have to get the Adafruit branded version of the board.
Now, I2c is a somewhat slow bus when you are looking at computer speeds, but for doing things with human interaction speeds, it is reasonable. Note, there are some techniques to use to help mitigate some of the potential performance issues when you are reading many, many inputs.
Another thing that moving this stuff off to outboard processors, is it can simplify the wiring, in that each i2c chip only has 16 or 4 inputs on it. If things go south, it can be easier to replace one chip and 16 inputs, rather than having to replace them on the Teensy.
