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MANISH
MANISH
Associate III
February 22, 2018
Question

STM32F7 MCU SPI interface issue..!

  • February 22, 2018
  • 1 reply
  • 968 views
Posted on February 22, 2018 at 12:12

Hello.,

I am working on STM32F7 Micro controller , In that I need SPI interface with more then 16 bit data size . As per SPI device requirement the SPI Chip select needs to be active low for more then 16 bit data size. Presently I am operating CS pin by using GPIO pin. But in this case I can't use DMA .

Is there any possible ways to get more then 16 bit data size.?

I have attached the SPI bus cycle requirement of the device.

0690X00000609kcQAA.png

#armc7 #spi #stm32f7
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1 reply

Tesla DeLorean
Tesla DeLorean
Guru
February 22, 2018
Posted on February 22, 2018 at 17:44

Not sure why DMA would preclude you from using CS as a GPIO pin. The point of DMA for a 34-bit sequence seems questionable.

34 isn't divisible into units the the SPI uses. You could drive the GPIO via a pattern buffer, using DMA+TIM to drive outputs.

Are you reading back data here? What are the speed requirements here?

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MANISH
MANISHAuthor
Associate III
February 24, 2018
Posted on February 24, 2018 at 07:14

Hello Clive.,.,

I have more then 8 SPI interfaced devices , so if I use GPIO line as CS it takes more time to access each devices (including GPIO oscillation time ), that's why to avoid this and to increase the access speed I should have to use SPI_NSS , So is there any possible ways to get SPI data size as more then 16bit . 

Tesla DeLorean
Tesla DeLorean
Guru
February 24, 2018
Posted on February 24, 2018 at 16:07

The STM32 SPI implementation historically has been a bit brain-damaged, for it to drive SPI_NSS automatically you have to disable the SPI at the end of a transaction, it is not particularly 'automatic' in a sense that most peripherals need it to be.

If your device (unspecified) is tolerant of clocks/data beyond 34-bits (also a brain-damage choice) you could send a multiple of 8-bits (say 40-bits).

The other alternative is to make a more custom CPLD/FPGA and attach that as an FSMC/FMC peripheral that can generate more specialized patterns.

With the DMA pattern buffer to GPIO you can drive up to 16 pins at once. If you used a common CS and CLK for the SPI devices you could blast 8-bit wide data (1 bit to each device) with 10 pins.

If you have GPIO oscillation issues consider dropping the slew-rate to some more appropriate pin speed vs trace length setting.

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