Just added the 128gpio setup to teensy 4.0 in my car project and in its test stages of wiring everything was fine. When i started adding consistant SPI polling in the loop controlling the MCP23S17s, within several seconds I would lose control over the chips. At first I thought it was the thread, so I put everything in the loop(), same issue. I then went into the MCP23S17 library and added a 5ms delay() after each endTransaction, and now everything is responding properly. Is there something hindering back-to-back SPI transactions? I am using very short jumper cables between the MCP23S17s, and tested them with the car off so noise is not an issue. if I remove the delay, I cant control the gpios after the back-to-back SPI writes, until i reboot or reprogram again