- Thu Apr 09, 2015 5:21 pm
#14037
Thanks for doing all this. I'm interested in reading your findings on the SPI controller. I am currently starting a project that will use it and more detail is always good.
- Thu Aug 25, 2016 7:30 pm
#53818
Just to clarify, there is a single 32-bit clock configuration word, and the references above to "SPI_CLKCNT_N", "SPI_CLKCNT_L_S" and so on describe how the different bits of the word work.
The constants that start SPI_CLKDIV/SPI_CLKCNT but don't end in _S indicate the maximum value for that part of the configuration word, while those ending in _S indicate the shift, in bits, of that part of the configuration word.
So for example you can do this in the Arduino IDE:
which would be interpreted like so:
H < L so difference (4) is number of clock periods clock is low. So the clock will be low 4 cycles, high 4 cycles (i.e. 50% duty cycle)
And looking at the output on an oscilloscope, that's exactly what I see! Thanks, metalphreak!
The constants that start SPI_CLKDIV/SPI_CLKCNT but don't end in _S indicate the maximum value for that part of the configuration word, while those ending in _S indicate the shift, in bits, of that part of the configuration word.
So for example you can do this in the Arduino IDE:
Code: Select all
SPI0CLK = 0xC7C7004;
which would be interpreted like so:
Code: Select all
0xC7C7004 = 0000 1100 0111 1100 0111 0000 0000 0100
EPPP PPPP PPPP PPNN NNNN HHHH HHLL LLLL
P = Predivider = 80MHz / (0b0001100011111 + 1) = 80MHz/800 = 100kHz
N = Divided clocks per SPI clock = (0b000111 + 1) = 8 = 25kHz SPI clock rate
H = 0b000000 = 0
L = 0b000100 = 4
H < L so difference (4) is number of clock periods clock is low. So the clock will be low 4 cycles, high 4 cycles (i.e. 50% duty cycle)
And looking at the output on an oscilloscope, that's exactly what I see! Thanks, metalphreak!
