SPI problem

[harshanahnd]

Hi,

I am using SPI communication to send data from my main pic (say pic B)to another secondary pic which works perfectly fine. My main pic gets data from another pic (say pic A) through UART which also works perfectly.

I used an encryption function to encrypt data on pic A and send to my main pic(B). After I decrypt data in pic B, I used printf function to check whether the decryption works. And it also works fine. But then my SPI communication to the secondary pic wont work.

The only thing I did was adding up encryption and decryption functions and it also works fine.

What could be the problem here? Is it because the pic (B) has more computations to do now? Is SPI gets halt or something?

PIC(A and B) - 16F877A
Secondary pic - 16F873
compiler version - 4.057.

[Ttelmah]

Almost certainly.
Key to remember, is that with SPI, unless you add a separate handshake, the master is controlling the timing, and the slave _must_ receive and handle it's byte, in the time the master is allowing. If the decryption in the slave, slows it enough that it can't service the reception, then data will be lost.
You don't say whether your reception is interrupt based or not, or where you have 'put' the decryption, but in general, you will need to buffer the reception using an interrupt driven approach, and do the decryption 'outside' the interrupt code, ensuring that the buffer is large enough to cover the worst case packet.
You will also find that there will be a time difference between encryption and decryption. If you look at compression algorithms (like MP3), these are designed to be fast to decode, while being quite machine intensive to encode. This is part of their design, since for 'on the fly' decompression, the least possible computing time is required in the receiving device. Most encryption algorithms, are deliberately the 'other way round', since the longer it takes to decode a packet, the less likely it is that a 'brute force' crack can be applied, using a lrge number of keys.

Best Wishes

[maamu]

HI,

Iam tiring to use SPI communication to interface external memory. when i use " read_spi() " function i am not getting clock from MCU 16F727.

1.) I configured MUC is master
2.) I am using fosc/4 speed.
3.) i am not passing parameter to read function.
4.) Even slave not connected, there is no signal in clock pin.

can any one give me, what to do get clock in read operation.
T.Q
maamu

[PCM programmer]

Download the CCS manual:
http://www.ccsinfo.com/downloads/ccs_c_manual.pdf
In the spi_read() section, it says:

[Guest]

Thanks its working PCM