RE: Timing Functionality on MPC555

> I want to measure how long certain functions are taking in my code, so
> I want a way to track time rather accurately (precision on the order
> of micro-seconds, hopefully).  I was wondering what I should be using
> just to get two values - a starting time, and an ending time.  I was
> hoping to read directly from the real-time clock, but that seems to
> give garbage as output.
>
> Anyone know what I should be looking at?

I'd suggest using the time base registers.  Takes (2) 32 bit reads to get
the whole thing, so the code goes something like :

    mftbu   r5          # load upper word of time base
    mftb    r4          # load lower word of time base
    mftbu   r3          # re-read upper word
    cmpw    r3,r5       # check that upper word didnt change
    beq     got_time    # If not, branch
    mftb    r4          # It changed, have to get new LSW to match
got_time:               # (result in r3:r4)

As I recall, the resolution depends on your CPU clock and some prescaler
options on the MPC555 ... should be microseconds or better.  You'll also
have to enable the time base, which as I recall also enables the decrementer
and it's exception.

Randall Young
Senior Software Engineer
NavCom Technology, Inc.
A John Deere Company



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