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Olin Lathrop wrote...

>Dave Dilatush wrote:

>> My vague, uninformed, totally off-the-wall WAG impression is that
>> the current consumption of the two approaches will probably be
>> approximately equal
>
>I haven't looked at the number with that in mind either, but my WAG is that
>they are about equal at the same clock rate.  In other words, they are
>probably close in joules/instruction.  If you can use the wider word and
>special math capabilities of the dsPIC at that clock rate, then I expect
>(but again, you should sit down and do the math) that the dsPIC takes less
>energy to crank the same computation.  In other words less joules/equation.

I set up a test with a PIC18F242 and a dsPIC30F3013, operating in
various oscillator modes and frequencies.  Full results are in
the attached text file, but overall it looks like the
dsPIC30F3013 can be expected to draw roughly two and a half to
three times the operating current as a PIC18F242 operated at the
same clock frequency.

However, that disadvantage was more than offset by the dsPIC's
ability to get more done per instruction: the PIC18F version of
the time-critical code took 546 instruction cycles to do the job,
versus only 64 instruction cycles for the dsPIC-- more than an
8:1 improvement.  The bottom line is that if I do this
application with a slow dsPIC rather than a fast PIC18F, I can
actually reduce operating current.

This is my first dsPIC project, and my initial impression is that
while the learning curve is big (not difficult, just a lot of
work), so is the payback: these things are REALLY easy to work
with.

Dave D.

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Current Consumption vs. Clock Rate
for PIC18F242 and dsPIC30F3013

              MHz         mA       mA/MHz
            ------      -----      ------
PIC18F242:

HS:           4.00       1.80       0.45
              8.00       2.95       0.37
             10.00       3.50       0.35
             20.00       6.72       0.34

HS/PLL:      16.00       5.19       0.32
             32.00       9.64       0.30
             40.00      11.78       0.29

dsPIC30F3013:

LPRC:         0.512      1.10       2.15

XTL:          4.00       4.06       1.02

XT:           4.00       4.42       1.11
              8.00       7.70       0.96
             10.00       9.26       0.93
             20.00      17.63       0.88

XT/PLL4:     16.00      14.33       0.90
             32.00      27.76       0.87
             40.00      34.05       0.85

XT/PLL8:     32.00      27.76       0.87
             64.00      52.80       0.83
             80.00      64.60       0.81

XT/PLL16:    64.00      52.40       0.82

FRC:          7.37       6.57       0.89
FRC/PLL4:    29.48      24.38       0.83
FRC/PLL8:    58.96      47.30       0.80
FRC/PLL16:  117.92      87.60       0.74

HS:          10.00       9.54       0.95
             20.00      18.07       0.90

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