steve said:
I don't know, but I suppose many of us have applications that very low
sleep current, which ARM's don't have, or very fast wake up times,
which ARM's don't have, or wide operating voltages (e.g., 1.8 to 5
volts for AVR's) which ARM's don't have, or require integrated LCD
controllers to drive simple LCD glass, which ARM's don't have. PICs,
AVR's,MSP430s, H8's ,8051s, or Elans all have the above features in a
wide variety of flavors and tiny sizes.
The ARM is really a different class of embedded processor, higher
power, higher throughput, higher speeds, less integrated, requires
external regulators etc.
There is some overlap, with certain ARM's under certain conditions, if
I remember correctly, the SAM was the only ARM that beat the AVR in
active mode but that was only under certain strict conditions (no PLL),
the other ARM's had vastly greater power consumption (ADUC7000's for
example) making high active power another difference between ARM and
the gang I mentioned above.
Thus, although ARM's are nice, they don't cover the same range of
applications as the others, it's nice to have one processor that you
can use when the power is provided by a coin cell or power mains.
Some of what you say is true, but the apps where the wide voltage range
of the AVR is useful is only a subset of embedded apps. It may be that
*all* of *your* apps fall in that category, but that still does not
mean that that is a large market compared to the rest.
When you talk about the SAM7 not beating the ARM other than under
"strict conditions", you may not understand the SAM7. With the PLL on,
it is still lower power than the ATmega128 at the same clock speed.
The information I have from Atmel shows the current for the "clock
divisor" with the PLL on at under 300 uA or about three times the
current for the rest of the circuit running at just under 1 MHz. With
the PLL off, the chip current can get below 40 uA at very low clock
speeds. Meanwhile, the spread sheet shows the chip drawing 6 mA at
12.5 MHz with PIO, I2C and SPI all running.
The sleep current of the AVR will be lower than the SAM7, but that will
only be useful if your sleep ratio is higher than 99% of the time.
Also what you say about the "wake up" time of the ARM is not
universally valid. For most apps you never have to put the CPU into
"sleep", you just slow down the clock with the divisor. It all depends
on how deep you need to "sleep".
No, the ARMs of any flavor do not cover the same range as the low power
8 bit processors. But there is a lot of overlap and the ARM is clearly
the better in those cases. Part of the selection needs to consider
what your last design used (for reuse) and next design will require
(again for reuse). For many of us, the ARM is the right answer based
on the need to provide more processing power in a small package with
low power. Some still need the wide Vin range and ultra low sleep
current.
The parts I think will really kick some 8 bit butt is the next
generation of CM3 devices from LM. I don't have any hard data yet, but
once they target the low power modes I think the CM3 has a lot of
potential to do low power "right"! I just hope they don't give up 5
volt tolerance.
