SPI: restore 12MHz as maximum frequency

[facchinm]

Fixes #593

[CLAassistant]

All committers have signed the CLA.

[matthijskooijman]

Is 12Mhz really the limit of the SPI hardware? Or is this just a limit of the slave devices used in the bug report and/or the wiring used? Also, this says "restore", so it would be good if the commit message would refer to the commit that lifted this limitation previously and comments on why that was wrong (if it is)?

[facchinm]

The "previous behaviour" I was referring to is the one documented here https://github.com/arduino/ArduinoCore-samd/blob/master/libraries/SPI/SPI.h#L37-L43 .
It's very likely that the flash under test doesn't support 24Mhz speed, but the library doesn't allow changing the frequency https://github.com/PaulStoffregen/SerialFlash/blob/master/SerialFlashChip.cpp#L33

[cmaglie]

I remember writing the comment in SPI.h. Also I remember that someone experimentally determined that the maximum clock is 12Mhz in SPI Master mode.

Looking to an updated datasheet this seems confirmed by the following:

84ns translates to roughly 1.000.000.000/84=11.904.761 Hz

[ArduinoBot]

✅ Build completed.

⬇️ Build URL: http://downloads.arduino.cc/PR/samd/package_samd-b212_index.json

ℹ️ To test this build:

1. Open the Preferences of the Arduino IDE.
2. Add the Build URL above in the Additional Boards Manager URLs field, and click OK.
3. Open the Boards Manager (menu Tools->Board->Board Manager...)
4. Install Arduino SAMD core - Pull Request SPI: restore 12MHz as maximum frequency #622
5. Select one of the boards under SAMD Pull Request SPI: restore 12MHz as maximum frequency #622 in Tools->Board menu
6. Compile/Upload as usual

[sambartle]

Do I need to turn off some kind of signature verification to test this?
The "http://downloads.arduino.cc/PR/samd/package_samd-b212_index.json.sig" file doesn't seem to exist.

[sambartle]

Any progress on fixing the signature verification for this so it can be tested?
It's a really annoying issue causing many problems with other libraries due to their reliance on working SPI.