First of all i want to thank everyone for his answers.
Writing in asm will be difficult for me : i don't have lot of time (the project must be finished in last week' beginning) and i don't remember well how we write in asm. So i'll try to make it in C.
A question for motion: where and how can i realise all the steps you gave me to use TMR2 interrupt. In fact to create my file .c i use PIC C W and i supposed that when i configured my project all the steps were realised automatically, but apparently they weren't! So must i write them in c in the main part of my program? What are the codes?
To motion again: i do use the RC3 pin to generate the clock signal. In fact, from what i understood reading the datasheet concerning the spi module, the clock signal is generated when we write something in the buffer SSPBUF. That's what i'm doing and apparently it works, i do have the clock signal with the frequency i programmed. The issue is the time between two successive clock signal (7 us is what i need).
Apparently you all lack explanations of what i'm doing, so i try to be more explicit.
So, to be simple, for my project i need to make an acquisition and conversion of different types of signals (sinus, square, "bits signal" (i don't know the english word),...) with a sufficient sampling frequency in order to have sufficient samples to rebuild the analog signal later. So i use an ADC with good features, the AD7827, which needs a serial communication with a microcontroler. This one is the 18F452. To understand my issue it would be good to look at the datasheet of the AD7827 and the chronogrammes, but i can't post them here! Anyway the fact is that i need to generate a clock signal and send it to the ADC so that this one serialize the data and send them to me (to the 18F452). To have a stable and fast sampling frequency i thought i 'd better use an interrupt. Maybe i'm wrong!
I hope you understand what i say. It's quite hard for me to explain a concrete technical issue in english. I'm french and i try to do my best!
Really really thanks to everyone.
I really want the project to work well!