[kenfox]

Here's far more than anyone probably wanted to know about the clocks available with WPILib C++. I used 10 runs of 1000 clock accesses for each of the 3 clocks. I did this with the default task (thread) priority, high task priority, and high priority locked (which prevents the task from being scheduled off the cpu unless it explicitly blocks).

The clock of choice is definitely the PPC clock: Timer::GetPPCTimestamp.

I experimented more with external timing with a signal analyzer and the external timing matched the PPC clock to +/- 4 microseconds. I only measured durations between 0.5 msec and 1.5 sec. It's kind of fun to see code profiled on a signal analyzer.

Here's the inner loop of the test:

#define RUN_TIMER_TEST(F) \
elapsed = last = Timer::GetPPCTimestamp(); \
for (j = 0; j < NumInnerIters; ++j) { \
	now = F(); \
	dt = now - last; \
	if (j > 0) { \
		if (dt > max_dt) max_dt = dt; \
		if (dt < min_dt) min_dt = dt; \
	} \
	else { \
		max_dt = 0.0; \
		min_dt = 1.0; \
	} \
	last = now; \
} \
elapsed = Timer::GetPPCTimestamp() - elapsed; \
avg_dt = elapsed / NumInnerIters;

Here's the raw data from a cRIO (older 8 module hardware):

Clock name,
Avg time per call (measured by PPC clock),
Min delta time,
Max delta time (as returned by clock under test)

TIMING TEST - Normal FRC priority, task unlocked
FPGA,0.00002835,0.00002300,0.00088700
FPGA,0.00002785,0.00002300,0.00080900
FPGA,0.00002644,0.00002300,0.00065500
FPGA,0.00002736,0.00002300,0.00087600
FPGA,0.00002860,0.00002300,0.00213800
FPGA,0.00002668,0.00002300,0.00062900
FPGA,0.00002698,0.00002300,0.00109000
FPGA,0.00002833,0.00002300,0.00179700
FPGA,0.00002706,0.00002300,0.00065600
FPGA,0.00002622,0.00002300,0.00064400
GetTime,0.00000171,0.00000143,0.00013781
GetTime,0.00000154,0.00000143,0.00002217
GetTime,0.00000186,0.00000143,0.00031662
GetTime,0.00000300,0.00000143,0.00085568
GetTime,0.00000155,0.00000143,0.00002766
GetTime,0.00000171,0.00000143,0.00014114
GetTime,0.00000154,0.00000143,0.00001907
GetTime,0.00000347,0.00000143,0.00190639
GetTime,0.00000153,0.00000143,0.00001216
GetTime,0.00000154,0.00000143,0.00001144
PPC,0.00000043,0.00000042,0.00000076
PPC,0.00000094,0.00000039,0.00051412
PPC,0.00000049,0.00000042,0.00006042
PPC,0.00000043,0.00000042,0.00000052
PPC,0.00000044,0.00000039,0.00001458
PPC,0.00000043,0.00000042,0.00000055
PPC,0.00000044,0.00000042,0.00000924
PPC,0.00000043,0.00000042,0.00000052
PPC,0.00000043,0.00000039,0.00000045
PPC,0.00000043,0.00000039,0.00000736

TIMING TEST - Highest priority, task unlocked
FPGA,0.00002249,0.00002100,0.00005700
FPGA,0.00002248,0.00002100,0.00005100
FPGA,0.00002251,0.00002100,0.00006300
FPGA,0.00002252,0.00002100,0.00005500
FPGA,0.00002249,0.00002100,0.00005100
FPGA,0.00002248,0.00002100,0.00004900
FPGA,0.00002249,0.00002100,0.00005000
FPGA,0.00002248,0.00002100,0.00005100
FPGA,0.00002250,0.00002100,0.00004600
FPGA,0.00002248,0.00002100,0.00006000
GetTime,0.00000156,0.00000143,0.00002623
GetTime,0.00000155,0.00000143,0.00002289
GetTime,0.00000157,0.00000143,0.00002337
GetTime,0.00000155,0.00000143,0.00001884
GetTime,0.00000159,0.00000143,0.00002933
GetTime,0.00000156,0.00000143,0.00002670
GetTime,0.00000157,0.00000143,0.00002551
GetTime,0.00000157,0.00000143,0.00002885
GetTime,0.00000155,0.00000143,0.00002384
GetTime,0.00000155,0.00000143,0.00001431
PPC,0.00000043,0.00000042,0.00000094
PPC,0.00000045,0.00000039,0.00002179
PPC,0.00000046,0.00000042,0.00002609
PPC,0.00000043,0.00000042,0.00000094
PPC,0.00000045,0.00000039,0.00002306
PPC,0.00000043,0.00000039,0.00000045
PPC,0.00000043,0.00000042,0.00000045
PPC,0.00000044,0.00000039,0.00001088
PPC,0.00000043,0.00000042,0.00000045
PPC,0.00000044,0.00000042,0.00000988

TIMING TEST - Highest priority, task locked
FPGA,0.00002047,0.00001900,0.00005600
FPGA,0.00002048,0.00001900,0.00006400
FPGA,0.00002048,0.00001900,0.00006900
FPGA,0.00002047,0.00001900,0.00006300
FPGA,0.00002046,0.00001900,0.00006900
FPGA,0.00002047,0.00001900,0.00005000
FPGA,0.00002048,0.00001900,0.00004700
FPGA,0.00002046,0.00001900,0.00006400
FPGA,0.00002049,0.00001900,0.00006800
FPGA,0.00002046,0.00001900,0.00005100
GetTime,0.00000155,0.00000143,0.00002122
GetTime,0.00000156,0.00000143,0.00002432
GetTime,0.00000158,0.00000143,0.00002551
GetTime,0.00000160,0.00000143,0.00002837
GetTime,0.00000157,0.00000143,0.00002694
GetTime,0.00000156,0.00000143,0.00002766
GetTime,0.00000155,0.00000143,0.00002146
GetTime,0.00000155,0.00000143,0.00002623
GetTime,0.00000157,0.00000143,0.00002432
GetTime,0.00000156,0.00000143,0.00002766
PPC,0.00000043,0.00000042,0.00000085
PPC,0.00000046,0.00000039,0.00002967
PPC,0.00000043,0.00000042,0.00000064
PPC,0.00000045,0.00000042,0.00001970
PPC,0.00000043,0.00000042,0.00000058
PPC,0.00000044,0.00000039,0.00000985
PPC,0.00000043,0.00000042,0.00000055
PPC,0.00000044,0.00000042,0.00001009
PPC,0.00000043,0.00000042,0.00000055
PPC,0.00000043,0.00000039,0.00000045