needing low speed SPI communication

abbas · ‎2015-04-12

Posted on April 12, 2015 at 16:50

Hello everyone!

I am using STM32F103RET for my project. I need to communicate by SPI with a low speed digital device with the speed of at most 1KHz clock. At the same time, I need to have my own cpu clock as high as possible.

As I saw at the reference manual, I just can decrease SPI clock to APB2/256 which is too high ( at the best situation, CPU_clock/16/256 -> 72MHz/16/256 = 17KHz - by loosing APB2 speed to CPU_clock/16 )

have I hopefully, missed a point? Is there any way to have lower speed SPI with keeping system speed high?

#stm32f10x

Tesla DeLorean · ‎2015-04-12

Posted on April 12, 2015 at 22:57

Is there any way to have lower speed SPI with keeping system speed high?

Pretend to be a Slave on the bus, an supply the clock? I2S mode might afford more rates.

Other than that perhaps consider bit-banging the bus via GPIO pins in the 1ms ticker?

SPI is one of the areas where the STM32 is unduly inflexible.

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Chris1 · ‎2015-04-13

Posted on April 13, 2015 at 15:44

You may be able to use a USART, which can be set to low baud rates. For example, we have used USART3 on the F103 for SPI as a master driving MOSI.

Depending on the needs of the attached device, you may have to reorder the bits sent.

abbas · ‎2015-04-15

Posted on April 15, 2015 at 12:24

Thanks clive1

I reached to this same solution.

Here, the device was a shift register which the digital lines were passed through opto-couplers ( that limit the rate ) and I was about to clock the register by mcu.

By the way, it was weird to here this limitation in my favorite STM MCUs!

abbas · ‎2015-04-15

Posted on April 15, 2015 at 12:30

Thanks Chris T

Your solution looks really smart! I may test it if the latter solution were in trouble. If I knew it, I would consider it in my PCB