Velocity PID Programming: Terms dependent on time

[kenfox]

Originally Posted by Ether

There's a much more accurate system timer that is available

There are 3 ways to get a timestamp in the 2013 WPILib C++ code. All of them are black box calls into lower level code, so the information here comes from the comments. If I have time tomorrow I will test timing on the cRIO and cRIO-II.

Timer::GetFPGATimestamp() and ::GetClock() -- double precision seconds since FPGA was reset. Uses FPGA clock so is synchronized with other FPGA timers. FPGA clock counts time in microseconds with an unsigned 32-bit integer. Rolls over every 71.6 minutes. (Timer::Reset does NOT reset the rollover.)

Timer::GetPPCTimestamp() -- double precision seconds since cRIO was powered on. Uses PPC system clock so is not synchronized with FPGA timers. Hardcoded assumption of 33MHz in WPILib. The counter is fetched with niTimestamp64() so it's probably 64-bit and rolls over every 9 billion minutes.

::GetTime() -- double precision seconds since January 1, 1970. This uses the vxWorks real-time clock which I can't find documentation on. The API allows nanosecond precision which seems optimistic.