[sdiy] Small MCU MIPS, DMIPS?

[Eric Brombaugh]

On 02/28/2011 04:02 PM, Scott Gravenhorst wrote:
> karl dalen<dalenkarl at yahoo.se>  wrote:
>> Now...?...huumm.... i have forgot what i was supposed to ask?
>> It was something about dsPIC STM32 and real figures and
>> something..else...?! Ah!!, yes interrupt....something!?
>> wait states, flash not able to run at 72Mhz....or what,
>> else, if! Ah, yes dsPIC being faster then STM32 due to
>> dual port RAM for routines using DMA....or so when...if?!
>
> I disagree with the premise that it's useless, meaningless or just
> marketing blather.
>
> Why don't YOU tell us exactly how we should measure it?
>
> The actual application is most important since it may or may not be able to
> use the special features provided.  Application X on device A may run
> faster than on device B, and application Y might run faster on device B
> than on A.  So which device is faster?  That question is of little value.
> So do your research, read the datasheets of the devices you are considering
> and understand how the features will affect the application you want to
> implement.  That will tell you more than MIPS - for YOUR application.
> However, generally, MIPS is a basic starting point for a very general
> comparison of the devices you would consider.  I don't believe that any
> engineer would consider MIPS rating the ultimate answer, when they need a
> device, they will get/buy samples and benchmark themselves.  But MIPS will
> help them choose a few from the many.

MIPS tells only part of the story - raw CPU execution. In any specific 
application, the required combination of CPU speed, bus bandwidth, IRQ 
latency, peripheral performance, etc will only be known after you've got 
the system up and running. OTOH, knowing some of these things in advance 
may give you a sufficient knowledge to weed out the prospects that are 
clearly insufficient.

I've tried a number of different MCUs over the years - various flavors 
of ARM, dsPIC, PIC, etc. ARMs are nice, but the older ones (ARM7, ARM9) 
tend to have hidden performance snags that don't come through in the 
marketing material - wait states on flash access, long IRQ response 
times, slow peripherals, etc. That's why I tend to use dsPIC most often 
these days - the CPU may max out at 40MHz, but the other aspects are 
top-notch: dual port RAM for DMA keeps bandwidth high, peripherals are 
fast and tightly coupled to the CPU, IRQ response times are only a few 
cycles (vs over 100 for ARM in some cases).

Recently I've been trying some STM32 / ARM Cortex M3 stuff. It's pretty 
nice so far, but I haven't pushed it very hard yet. Time will tell...

Eric