Using SPI devices on Propeller C3

The Serial Peripheral Interface Bus or SPI bus is a synchronous serial data link standard named by Motorola 

that operates in full duplex mode. Devices communicate in master/slave mode where the master device initiates the data frame

weblink Wikipedia
 
Propeller C3 board has a SPI bus system embedded with several onboard devices such as SRAM, Flash Memory, and others. Since the SPI bus has only one Slave Select  ( SS ) line, this board has two chips that let user select the right device within the 5 available onboard and 2 on external I/O.

So whenever you need to select a device you need to activate its device number by clocking it to the first chip ( the counter ), and the system will do the rest in driving the second one ( the multiplexer ).

You can consider this system in this way:
SPI devices
The selection os SS# lines is made with an ipotetical "SPI initializazion function"

C3 has two 32Kbytes SRAM  chips on channel 1 and 2. Whenever you need to access to these banks you need to select channel 1 or 2 according to the address you need to access to.