To support custom input thresholds, the devide needs SIO pins. Unfortunately, PSoC 4 family devices supports maximum of 2 SIOs. Since your application is SPI you need muinimum 4 SIO pins.
You can use PSoC 58Lp family devices which supports maximum of 8 SIO pins. In the PSoC Creator, you have to select 4mA, 25mA sink and drive level as VDDIO for output pins and threshold as VDDIO for input pins. You can find the attached project for reference.
Please note that you have to give 1.8 volts as power supply to VDDIO pins. Then only the input low and high thresholds of VDDIO pins will be 0.3VDDIO and 0.7*VDDIO respectively. Also, please note that VDDA should be greater than VDDD and VDDIO.
>>"Would the internal resources required to use SIO pins for SPI limit the speeds SPI can operate?"
>>"Could analog inputs on PSoC 4 be used as Adjustable Input Level SIO pins? If so, are there any examples?"
--> No. Analog pins should only be used for analog applications.
Can you please let us know why you want to operate PSoC only. Is it possible for you to operate PSoC 4 at 1.8 volt VDDD so that you need not to go for PSoC 4?
Design02.cyprj.Archive01.zip 1.5 MB