Electronics > Microcontrollers

32F417 SPI running at one third the speed it should

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AndyC_772:
It depends on the device. Usually there's a register you can program to set the word length.

One interesting feature on some of the STM32 devices is that the SPI transmit register supports 'packing'.

What this means, is that if your code performs a 16-bit write to it, then 16 bits get clocked out. If, however, your code performs an 8-bit write, then only 8 bits are sent.

The register is usually defined in the header as 16 bits wide, because it is. This leads to an infuriating symptom, where you write:


--- Code: ---uint8_t d;
SPI_DR = d;

--- End code ---
... and wonder why the SPI port sends twice as many bits as it should, and every other byte is zero. Instead you must write:


--- Code: ---uint8_t *spi_dr_8bit = (uint8_t*) &SPI_DR;
uint8_t d;
*spi_dr_8bit = d;

--- End code ---
...since this explicitly performs an 8 bit write, as opposed to a write of an 8 bit value to a 16 bit register.

peter-h:
Yes the 32F417 SPI can do 8 bits or 16 bits.

The byte order is not configurable for the 16 bit mode however and in some applications one has to swap the two bytes in software.

The 16 bit mode does no more than sending 2 bytes in the 8 bit mode, but at least you know the 16 bits will come out without any gaps if there are interrupts etc. It also supports some code optimisation if one is polling SPI and running at high bit rates. The 32F4 peripherals are not as fast as you might expect from a 168MHz CPU; they run at say 42MHz and a lot of stuff takes several clocks, so polled implementations run surprisingly slowly, with gaps. See this thread further back.

DMA is the best way and then the 16 bit mode is generally not needed.

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