DT-Ints latency and other interrupt conciderations

[DDDvvv]

my two cents worth,

im getting 55us on the doservo code. is that possible? im using an 18f4431 @ 20mhz (couldnt get 40mhz going)

the scope is a 2211 tektronix. i thought there was something wrong with it, but its working perfect, and upto calibration.

i have the I_term function every 4th call, hence the long 73us on-time after every three loops on the main int code

here's an image of the scope screen.

all interupts are dt_interupts, in pbc.

thanks for all the ideas.

[HenrikOlsson]

Thats nice, I'm glad you got it working! Are you running the DoServo code as part of an Interrupt service routine? What are you using as "input"?

The long execution time (up to 400uS) I mentioned earlier was when the code was compiled with PBPL, when compiled with PBP it runs a lot faster. With PBP I've timed the PID code alone to as low as 40uS so your 55uS for the complete doServo seems plausible. The fact that it works points in that direction as well ;-)

The problem starts to appear when you:
1) Need 32 bit math for the position and setpoint variables (or anything else). Switching to PBPL makes the execution time of almost all arithemtic operations go WAY up. Without switching to PBPL you have to resort to ASM for the 32bit variables and math.

2) Using step- and direction as the input and DT-Ints with PBP interrupt handler for counting the step pulses. This pretty much kills it above a few kHz due the latency caused by saving and restoring the PBP system variables.

At least that's what I've found this past week. If you have yours running with step and direction input I'd love to hear what kind of steprate you can achieve.

Thanks!
/Henrik.

EDIT: Wait a minute, the "period time" is 55uS (ie. the servo code is run at 18kHz), the execution time is probably closer to 40uS or so. Are you really trying to run the servocode at 18kHz?

[DDDvvv]

thanks for the prompt reply.
right now im fighting with the step/dir int routine, which is not showing any sign of life. so im assuming that step/dir is not working.

whole routine is in the interrupt, (just as you had it)except the lcdout, which is the only command in the main routine.

yes, that is 18khz according to the scope. is that not good?

the only motor connection is the encoder and im using leds to show hpwm output. saves a lot on h-bridges and smoke from explosions.
will eventually move to the power control module once everything is figured out.

[HenrikOlsson]

Running the PID-loop at 18kHz is probably overkill, for motorcontrol somewhere between 1000 and 2500Hz seems to be common depending on inertia of the motor etc but YMMV.