[sdiy] Contemplating DIP microcontrollers

[Neil Johnson]

Hi Tom,

> Are these Atmel AVR XMegas 8-bit or 16-bit?

8-bit registers, which can also be used as pairs of 16-bit registers,  
or in triples as 24-bit pointers.

> I've read the Atmel pages, and they claim that the XMega are "8/16- 
> bit" whatever that means.

8-bit registers that can be paired up for some 16-bit operations  
(mov,add, etc).  The Z80 could also do this, while the 6502  
couldn't.  Its a bit of marketing hype and a bit of truth.  The  
larger XMEGA devices also carry some peripheral blocks that you don't  
tend to see on plain vanilla 8-bit machines, such as DMA controllers,  
funky interrupt and peripheral controls, cryptography support, etc.

> Elsewhere they say that the AVR series are 8-bit or 32-bit chips,  
> which sort of rules out 16-bit.

The original AVR is 8-bit with some 16-bit modes as described above  
(hence "8/16").  AVR32 is 32-bit, for example the AVR UC3.  These  
larger ones can run Linux or BSD.

> It's pretty hopeless if I can spend 5 minutes reading their website  
> and not manage to find that out.

Well its not that bad.  This page seems quite a good overview:

http://www.atmel.com/products/avr/default.asp?family_id=607

> I'm guessing from the comments below that although we might get 16- 
> bit multiply, we don't get an extra-large accumulator? That'd be a  
> DSP feature, right?

Indeed, so you need think a bit more about the maths that you're  
doing rather than relying on extra accumulator bits to carry the  
carries, as it were.

> Also, are there good reasons to use these aside from Arduino? Some  
> of us are more interested in speed than ease of use or a learning  
> curve designed for non-programmers.

That depends.  They offer price, performance, good tool set support  
(AVR GCC is pretty good these days), a good choice of devices, and  
many other attributes.  If they do something for you, then use them.   
If not, move on and try something else.  If you're after DSP  
performance then either get friendly with Terry, or go look at the  
Analog Devices Blackfin DSPs.  For example, 400MHz kit:

http://cgi.ebay.co.uk/P1-Blackfin-Kit-Breakout-Adapters-and-Flash- 
memory-/310258298707

How about XMOS?  Lots of cores there, and they're damn fast, and  
funky to program in "I-can't-believe-its-not-occam" XC.

Cheers,
Neil
--
http://www.njohnson.co.uk