Have been working on projects with GPIO to unusual sensors. Intermediate idea:
Let any more GPIO peripherals be SPI for now?
I looked around for sensors prepackaged SPI sensors in the SD form factor.
ala Horwoitz and Hill Chapter 15 with no luck...
See Chapter 15 of latest Horowitz and Hill - Art of Electronics.
https://artofelectronics.net/
https://artofelectronics.net/
Look particularly at the tables/diagrams of SPI and I2C peripherals.
Table 15.20 p1080 All IO
Table 15.21 p1083 SPI
Table 15.22 P1085 I2C
Table 15.20 p1080 All IO
Table 15.21 p1083 SPI
Table 15.22 P1085 I2C
One could use the 6x1 pin header Digilent PMOD Standard for SPI.
There are many PMOD sensors available COTS.
Put in a secondary SPI header/socket on the existing SPI bus. Use the GPIO for CS.
The CS would also control power to the sensor in the socket.
One could not talk to sensor and SPI memory at exactly the same time. But maybe no big drawback?
One could also do TTL RS485 or RS232 to a UART to SPI bridge ala FT9xx. (or similar) on a bigger daughter board. Again a GPIO would be needed for CS and power ON/OFF for the target sensor on the bus. In fact one would power up and down the whole FT9xx as needed to conserve power.