Hi!

Well, you asked the question I have done some days ago exactly. My new project requires SPI flash memory, here: Atmel`s DataFlash^(R). For some reasons I may not use a part with built-in SPI controller (like RD2,ID2 etc). And so what I have started from:
- MCU: T89C51CC01 (clocked with 18.432MHz; x2 mode is utilized);
- SPI: AT45DB041B (4Mbit SPI flash memory; 20MHz clock frequency max.);
- CPLD: Altera EPM7128STC100.

First of all, I have drop the idea of "software SPI" because I need very fast access while read the flash and so if software emulation be used, it decreases MCU productivity with all these bits` manipulations.

So, the firsts story is started.
I have created SPI module inside CPLD. It is really easy because needs only one 8-bit shift register and a little framework about its maintenance:

To access SPI by such way, I just do MOVX with 15-clock /RD/WR stretch. It produces ~800ns duration of /WR and /RD signals. Inside this time, shift register is loaded (if I write to memory via MOVX), shifted 8 times to left (~450ns) and read back (if I read from memory via MOVX). This works perfect.
The end of the first story.

But... Looking on the timings of the SPI mode 3 (which is default mode for that flash memory), I remembered that I have already seen similar figure somewhere. So I set down and thought. And I got it! It is the shift register of UART in the mode 0 !!!
Okay, then I reconfigured CPLD and utilized just UART: TX as SPI clock and RX as bi-directiolal SI+SO line. Simple test program, power-on... it works! I do MOV SBUF,#xx and it sends byte to flash memory! I do CLR RI then SETB REN and it receives a byte from flash memory!
Well, I did it for test purposes, because I will use MOVX (first story). Anyway, the simplest SPI is just UART mode 0. If you plane to use such way, just some more notes:
- generally, SPI uses format "MSB first" while UART uses "LSB first". So you must rearrange bits (for send you may use just a macro; for receive - simple subroutine);
- UART in mode 0 uses fixed clock ratio (OSC/12 or OSC/6 in x2 mode). So you need to check SPI and UART timings for that they are compatible. If they are not then there is no way to fix this problem :(

Regards,
Oleg