Am I missing something or is there no documentation on implementing SPI with a Talon SRX? It’s clearly mentioned on the spec page: http://www.vexrobotics.com/217-8080.html
However, none of the documentation covers it at all. One of the reasons we bought the SRX over the Victor SP was because of the capability of using SPI in the case of low voltage conditions.
SPI was not a legal speed controller interface for FRC in 2015. The roborio cuts control signal on low voltage in any case. No reason to think the SPI would any different. For none FRC you would need different firmware. Suggest contacting CTRE to ask the status of that.
http://i.imgur.com/9UrLcrr.png
SPI continues to operate to 4.5 V. SPI on motor controllers wasn’t really necessary this year with no defense. But even if it isn’t FRC legal, if it’s an advertised capability of the hardware, there should be documentation on how to use it.
The motor controllers are disabled by the program logic to control the brownout. Why do you think the SPI contol would be any different? CAN continues to run until 4.5 V despite the CAN motor controllers being disabled by logic.
Not all features were completed in the firmware for 2015. Current PID for example was not available. And that would have been legal & useful.