Lpc2000

Hi,

In the general case, you're pretty much guaranteed overflow: 
multiplying two numbers of a fixed precision (e.g. 16-bits) gives a 
result twice that precision (in that case 32-bits). As another reply 
has pointed out, you can simply declare the result to have twice the 
precision of the two arguments. However, for 32-bit arguments this is 
likely to be slow, as there's no native processing of 64-bit values: 
the compiler will call an internal library function to do the math.

It might be worth your while looking at the "Enhanced DSP 
instructions" (see ARM documentation from ARM): these provide various 
forms of multiply and add, including some that saturate. The LPC2000 
series as ARM7TDMI devices support these instructions.

A final thought that strikes me is that do you really need 32-bit 
arguments? In general 16-bit is fine for most purposes as an input 
resolution. You can still accumulate at 32- or 64-bits (scaling back 
to 16-bits if further processing is required).

Hope this is of some help.

Regards
Brendan Murphy

--- In lpc2000@yahoogroups.com, "Aalt Lokhorst" <lokhorst@s...> wrote:
> Hello All,
> 
> I have concerns about a PID routine which is doing some 
calculations 
> with 32-bit values. It uses multiply and add instructions. My 
concern 
> about this is, how to detect overflow.
> 
> A overflow can result in a process correction with a wrong sign and 
this 
> is not acceptable for the process. What I need in a overflow 
situation 
> is a saturated value.
> 
> In the past I was using a 8052, multiplying 32-bit values was 
possible 
> but it was done in a lot of 8-bit portions.
> With the ARM I can do the same in one instruction but I am missing 
the 
> overflow flag. Is there a option to detect an overflow after a 
multiply?
> 
> I am using the LPC2129 (ARM7)
> 
> Greetings
> Aalt
> 
> -- 
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