CODEWITHSUNDEEP
linuxbeagleboneblackspi
Wiingaard
Wiingaard

Reputation: 4302

Beaglebone Black; Wrong SPI Frequency

I'm new at programming the Beaglebone Black and to Linux in general, so I'm trying to figure out what's happening when I'm setting up a SPI-connection. I'm running Linux beaglebone 3.8.13-bone47.

I have set up a SPI-connection, using a Device Tree Overlay, and I'm now running spidev_test.c to test the connection. For the application I'm making, I need a quite specific frequency. So when I run spidev_test and measure the frequency of the bits shiftet out, I don't get the expected frequency.

enter image description here

I'm sending a SPI-packet containing 0xAA, and in spidev_test I've modified the "spi_ioc_transfer.speed_hz" to 4000000 (4MHz). But I'm measuring a data transfer frequency of 2,98MHz. I'm seeing the same result with other speeds as well, deviations are usually around 25-33%.

How come the measured speed doesn't match the assigned speed? How is the speed assigned in "speed_hz" defined? How precise should I expect the frequency to be?

Thank you :)

Upvotes: 1

Views: 1578

Answers (1)

Sorcrer
Sorcrer

Reputation: 1644

Actually If you look closely on the DSO you can see that each clock cycles takes approx 312.5 ns , which makes the clock frequency to be 3.2Mhz,. May be the channel you're monitoring i

Then, the variation between the expected & actual speed, In microncontrollers I've worked the all the peripherlas including the SPI derives ots clock from the Master clock which is supplied to the MCU(in your case MPU), the master frequency divided by some prescalar gives the frequency for periperal opearations, where as peripherals use this frequency and uses its prescalar for controlling the baud rate,

So in your case suppose if the master frequency is not proper this could lead to the behavior mentioned above.

So you have two options 1. Correct the MPU core frequency 2. Do a trial & error method to find the value which has to be given is spi test program to get the desired frequency

Upvotes: 0

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