Go to Post If duct tape isn't the answer, you're not asking the right question. - Morgoth [more]
Home
Go Back   Chief Delphi > Technical > Programming
CD-Media   CD-Spy  
portal register members calendar search Today's Posts Mark Forums Read FAQ rules

 
 
 
Thread Tools Rate Thread Display Modes
Prev Previous Post   Next Post Next
  #11   Spotlight this post!  
Unread 15-09-2008, 12:50
EricVanWyk EricVanWyk is offline
Registered User
no team
 
Join Date: Jan 2007
Rookie Year: 2000
Location: Boston
Posts: 1,597
EricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond reputeEricVanWyk has a reputation beyond repute
Send a message via AIM to EricVanWyk
Re: Servo 'smoothing'

Quote:
Originally Posted by Kevin Sevcik View Post
Bomber,

Also to Erik and viewers at home,

The domain of tau is obviously changeable with the #define SCL. To move to a 0..255 range, you'd state #define SCL 8. It's important to note that at that point, you can't have an equivalent of tau=1.0, which basically removes the effect of the filter. Also, fixed point math may be painful at times, but it is and always* will be stupendously faster than floating point. If our audience members are at all interested in programming embedded controllers and/or high speed DSPs in the future, it can't hurt to look into fixed point math and play around with it.

*For the foreseeable future, until FPUs are fast enough that you couldn't possibly want to do things any faster than they can be done with floating point.
I don't want to belabor the point too heavily, but that future is currently in boxes on its way to the beta test teams. I'm going to follow with a lot of boring technical blah blah blah, but the conclusion is pretty simple to follow:

DON'T OVER OPTIMIZE

A fixed point multiply takes two or three clock cycles to execute.
A single precision floating point multiply takes three clock cycles to execute.
A double precision floating point multiply takes four clock cycles to execute.

In addition to the "execute" phase, each instruction must also go through fetch, decode/dispatch, and complete, which adds three more clock cycles.

Simply put, the difference between single fixed or floating instructions just don't matter any more. What will matter is how many instructions. By converting to fixed point math (and adding the shift instructions), you actually slowed down the filter.

To make things even more interesting, the processor is capable of dispatching up to 2 instructions per clock, but only if they are different types of instructions: You can dispatch a fixed point and a floating point instruction simultaneously, but you can't do the same with two fixed point instructions.

This means that you are *much* better off allowing a compiler to order your instructions. Write what you mean, and trust in the compiler.


- Eric (with a C)

PS:
Eliminating divides are almost worth your time.
Fixed point divides take 20 cycles to execute.
Floating point divides take 18 (single precision) or 33 (double precision) cycles to execute. For reference sake, a double precision divide on the cRIO takes about the same amount of time as an 8 bit fixed point instruction on the PIC. However, it will also be executing fixed point math while the FPU is working on the divide.
 


Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Data Smoothing Lafleur Programming 3 14-01-2008 22:06
Servo behavior question / advanced servo/PIC programming question DanL Electrical 12 18-10-2005 18:33
Servo Values DanDon Motors 8 14-02-2005 15:49
Buying Servo Gamer930 Motors 4 13-02-2005 20:44
Servo MASherry General Forum 6 04-10-2004 22:46


All times are GMT -5. The time now is 01:13.

The Chief Delphi Forums are sponsored by Innovation First International, Inc.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2017, Jelsoft Enterprises Ltd.
Copyright © Chief Delphi