Hi Amforthers, I was learning a bit about the internals of the SPI driver and finally found the "core" word c!@spi
I'm comparing side by side the assembler source with the 328P datasheet (=Arduino) and I can't understand completely the purpose of some of this code, which I list here for convenicence do_spirw: 1. out_ SPDR, tosl do_spirw1: 2. in_ temp0, SPSR 3. cbr temp0,7 4. out_ SPSR, temp0 5. in_ temp0, SPSR 6. sbrs temp0, 7 7. rjmp do_spirw1 ; wait until complete 8. in_ tosl, SPDR 9. ret In 1) we transmit a byte (and activate sthe SPI clock). Ok. In 2) we read the status register, so far so good In 3) we clear thr 3 least significant bits of ths status just read Why ? In 4) we write the status register with the just modified byte. Why ? In 5) we again read the stats register. Why ? &) to 9) is usual stuff I don't really understand the purpose of the sequence given by steps 2 to 5. - Bit 0 is the SPI2X: Double SPI Speed Bit in SPSR, if we clear it and write again to etatus register we are possibly changin the SPI clock rate. - Bits 1 to 5 in SPSR are marked as reserved in 328P datasheet, so why setting these as 0 ? why the mask cover only three bits and not six bits ? - Why we write the status register ? I have not seen any other effect of writting SPSR in the datasheet other than doubling the clock rate. The only thing I an think of is that this is needed for other AVR models, so if someone can comment on this I would appreciate it. Another issue This code does a busy poll. I was wondering if it should instead call PAUSE instead Two reasons that I can think of: 1) SPI is a high speed interface. Using PAUSE would degrade performance for the highest clock rates. 2) We can't invoke PAUSE from inside this assembler word. Which is the correct assumption ? Best regards Rafael ------------------------------------------------------------------------------ Slashdot TV. Video for Nerds. Stuff that matters. http://tv.slashdot.org/ _______________________________________________ Amforth-devel mailing list for http://amforth.sf.net/ Amforth-devel@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/amforth-devel
