missing documentation and example code: how to quickly and permanently change SPI clock divider

[shiftleftplusone]

missing documentation and example code:
how to quickly and simply change SPI clock divider ?
About setClockDivider():
"This function should not be used in new projects. Use SPISettings with SPI.beginTransaction() to configure SPI parameters."

But I don't understand how to use SPI.beginTransaction() or either function for AVR, and ARM, and ARM extended mode. More source code examples would be needed for this.

[cmaglie]

Is the following useful for you?

https://www.arduino.cc/en/Tutorial/SPITransaction

[shiftleftplusone]

thank you, I already checked this.
What I am actually looking for is a command like

setup() {
  setSPIClockDivider(2); // SPI on AVR = 16MHz/2=8MHz
  //..
}

to setup SPI on AVRs to 8MHz and make it work for all attached devices (e.g., SPI TFTs and SD cards) at this clock speed, at once and for all.

The same for either Due mode (standard or extended):

setSPIClockDivider(7); // SPI on ARM = 84MHz/7=12MHz

[cmaglie]

the closest equivalent for AVR to:

SPI.setClockDivider(2);

is:

SPI.beginTransaction(SPISettings(8000000, MSBFIRST, SPI_MODE0));
SPI.endTransaction();

By the way you should re-think your software design because using the same clock speed for all the SPI devices means that they all go at the speed of the slowest, even if they can go much faster.

[shiftleftplusone]

most of all my devices seem to work at 4MHz, especially TFTs which often are far too slow.
I just want to make them work as fast as possible, surely I have to test the limits then and re-define it.
I don't write own SPI code.

But what about Due and Due extended mode?

[cmaglie]

Bur what about Due and Due extended mode?

It's the same code. Moreover you don't need to worry about the "divisor" to obtain the desired frequency, because SPISettings does the math for you.

[shiftleftplusone]

aaahh, thanks, that's fine.
This explanation would be appreciated if added to the function reference!
Thank you for your input!

[shiftleftplusone]

ps,
and then

SPI.beginTransaction(SPISettings(8000000, MSBFIRST, SPI_MODE0));
SPI.endTransaction();

would work for all attached devices at once, no matter how many there are currently, like TFT(s) and SD slots?

[PaulStoffregen]

This explanation would be appreciated if added to the function reference!

The reference page does have this text:
https://www.arduino.cc/en/Reference/SPI

"What is the maximum SPI speed your device can use? This is controlled by the first parameter in SPISettings. If you are using a chip rated at 15 MHz, use 15000000. Arduino will automatically use the best speed that is equal to or less than the number you use with SPISettings."

[shiftleftplusone]

but not this reference page:
https://www.arduino.cc/en/Tutorial/SPITransaction

[shiftleftplusone]

now what confuses me is:
" Your settings remain in effect for the duration of your "transaction". "

But I don't want to limit the clock speed to the duration of any "transaction", I want to set it once and for all transactions, for the whole program, the same way I'm setting Serial.begin(BAUDRATE) once and for all

[cmaglie]

Your settings remain in effect for the duration of your "transaction".

The wording is a bit misleading it should be:
the SPI settings are applied at the begin of the transaction and endTransaction() doesn't change SPI settings.

Unless you, or some library, calls beginTransaction a second time, the setting are maintained.

[shiftleftplusone]

thank you, now it's clear to me (I hope) ;)

[shiftleftplusone]

...not quite though, I'm afraid..:

let's say, I #include SD.h and Adafruit_ILI9340.h and Adafruit_GFX.h and Pixycam.h and whatever.
Now I actually have no control over anything what happens in these libs, e.g. if they call beginTransaction once or twice or repeatedly. So it might happen (CMIIW) that a device lib overwrites my setting at any time.
But I want to set 8MHz once and for all libs and all devices and block all individual device settings - what is the command to maintain my personal settings all over the time regardles what's written in either lib?