Enabling SPI transaction features about sdfat

[greiman]

SPI transactions are only required if you access the SPI bus in an interrupt routine for another device. The Arduino implementation of SPI transactions is in the SPI.h library so you must include this file.

I believe SPI transactions is a very bad idea but added it as an option to SdFat because several people thought is would allow adequate access to to SPI devices in interrupt routines. Since you can't interrupt a 512 byte block transfer on an SD, there are still major interrupt latency problems.

If you enable SPI transaction, do not set ENABLE_SPI_YIELD nonzero. This will cause problems for read with some cards.

The best solution would be multiple SPI buses which is common on modern ARM chips like the STM32.

Often, an adequate solution is to use software SPI to avoid the sharing of the hardware SPI bus between interrupt and non-interrupt code. Software SPI can be enabled in SdFat for the SD or you can use my DigitalIO library for software SPI with other devices.

https://github.com/greiman/DigitalIO-beta