CYFI is Cypress Propieratary RF protocol. CYFI protocol is available in the form of CYFISNP user module in some of the PSoC1 family devices. CYFI protocol provides APIs to the developer thereby reducing the efforts in low level details.
For better understanding of CYFI protocol it is strongly reccomended to go through following contents.
1. CYFI kit contents may be downloaded from here
2. CYFI Video lectures are available in the following page.
3. CYFI Kit Application demo is shown here
Note: The CYFISNP user module data sheet gives a very detailed explaination of the protocol