Changing SPI0 Clock Rate

[George Rung]

I am able to get SPI0 working on the BeagleBone Black by using only config-pin commands:

config-pin P9_17 spi
config-pin P9_18 spi
config-pin P9_21 spi
config-pin P9_22 spi

Then writing to /dev/spidev1.0 causes data to come out the pins P9_18 (data) and P9_22 (clock).

Note that searching for how to enable SPI on the BeagleBone returns tutorials that all involve "device trees" or a "cape manager".  None of this was necessary for me to get SPI working.  I assume they used to be required at some point in the past.  I simply set the pins to SPI and it works.

Now I'd like to change some SPI settings, like the clock rate.  It seems the hardware has a frequency divider that can be set to change the SPI clock rate.  What is the clock rate by default?  How can I change it?  Ideally I'd like to change it by command with the BeagleBone running, hopefully no recompiling or rebooting required every time I make a change.

How should I go about this?

Note that I'm running the image "bone-debian-8.6-iot-armhf-2016-10-16-4gb.img.xz" found here:

https://rcn-ee.com/rootfs/bb.org/testing/2016-10-16/iot/

[William Hermans]

On Thu, Oct 20, 2016 at 8:56 PM, George Rung <rick...@gmail.com> wrote:

I am able to get SPI0 working on the BeagleBone Black by using only config-pin commands:

config-pin P9_17 spi
config-pin P9_18 spi
config-pin P9_21 spi
config-pin P9_22 spi

Then writing to /dev/spidev1.0 causes data to come out the pins P9_18 (data) and P9_22 (clock).

Note that searching for how to enable SPI on the BeagleBone returns tutorials that all involve "device trees" or a "cape manager".  None of this was necessary for me to get SPI working.  I assume they used to be required at some point in the past.  I simply set the pins to SPI and it works.

Actually, device tree file(s) were involved, you just did not realize it. By using config-pin, you're using one of the universal IO device tree overlays.

Now I'd like to change some SPI settings, like the clock rate.  It seems the hardware has a frequency divider that can be set to change the SPI clock rate.  What is the clock rate by default?  How can I change it?  Ideally I'd like to change it by command with the BeagleBone running, hopefully no recompiling or rebooting required every time I make a change.

How should I go about this?

Note that I'm running the image "bone-debian-8.6-iot-armhf-2016-10-16-4gb.img.xz" found here:

https://rcn-ee.com/rootfs/bb.org/testing/2016-10-16/iot/

So, I do not really know much about the SPI hardware per se. However, I do know that if you check an overlay file for SPI, say : https://github.com/beagleboard/bb.org-overlays/blob/master/src/arm/BB-SPIDEV0-00A0.dts#L63 Look at the highlighted line. If you're wanting to raise the frequency, there is is. But except if you're wanting to use a universal IO overlay, which I completely understand. You'll have to edit the file that is preloaded at boot, *or* the overlay file that is loaded when you invoke the config-pin script.

As far as the clock, and divider . . . Well as I recall the clock is 48Mhz, which means if the frequency is 16Mhz, the divider is 3x. Anyway, as I said I really do not know the SPI hardware well. SO take what I say with a grain of salt. As what I'm saying concerning the clock and divider is going by memory. Where I've never used the hardware, and my memory is sometimes less than perfect.

[Mark A. Yoder]

RIcky:

  Why are you needing to change the clock rate?

--Prof. Yoder