If you noticed the SPI data is never write and read at the same time, it's more of a SDIO system, so share those
You would never read SPI_IRQ once you already have set nCS active, so share those.
A IRQ signal would also activate nCS, but you planned to do that anyway just after the irq notification.
CLK is 50/50 duty, so a R/C filter would make it 1.6V at the most.(make sure duty is not above 51% high and if you can adjust it make it 45% high)
As CLK spend rest of the time low, the P-Mosfet (-0.8Vgs) is always on, so end result is that Enable is also on.
To Shut down, just put CLK at a steady high (250uS). It was hard to figure out a circuit that does not short a pull-up resistor while at deep sleep,
as battery operated devices spend most of the time with Enable low.
If you use less than 250KHz CLK increase Cap, above 1Mhz clk I would change R1= 1K
As SPI-CLK have Pull-down you may have to R3=2K, C1=1uF (or weak pull-up resistor after R3) as you want -0.2V or less VGs when you want OFF.
I have not tested this circuit yet and may not be practical but I will create a board for the Parallax Propeller as it does not have SPI in hardware anyway.
