AVR-Chat

--- In AVR-Chat@yahoogroups.com, "James Wagner" <jim_d_wagner@a...> 
wrote:
> You really DON'T need to think of "constants".
> 
> There are only two issues. 
> 
> 1. Where is the value stored? It can be stored as part of a
> table in program code and that is a real constant, for most
> practical purposes unchangeable. It can be stored in SRAM.
> In this case, it is highy changeable and goes away every
> time power is turned off. It can be stored in EEPROM. It is
> still changeable, but you don't want to do it really often.
> And, it stays when the power goes off. The operations used
> to read each is similar but slightly different.
> 
> 2. What determines the value? A constant is determined at
> the time that code is written. While it can be changed
> later, it makes little sense in this kind of application.
> All of the other kinds of data have to be determined some
> other way; it might be by receiving data over the serial
> port. It might by by reading a constant at startup and copy
> to the memory location (ie, a "default" value). 
> 
> My point is that thinking about "constants" is not really
> very useful or productive in this kind of application. A
> number is a number.
> 
> Jim
> 
> 
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Hi Jim,

I am just on low end of the learnig  curve.  To me, at this point, a 
variable is something that changes all the time, like soil moisture 
or air temperature.  A constant is a more fixed number that I am 
thinking is a set value that does not change unless the chip is re-
programmed.

As far as the place it is stored, I would not have a problem if I 
needed to use a button battery to keep an a memory chip from losing 
any values.

But if the internal eeprom is similar to eeprom chips the limit is 
something on the order of 100,000 writes ?  That would allow three 
changes every day for the next --- 100 years ?   I would expect a 
change once or twice a week.

I guess simplicity would be a key factor.  what is the easiest way to 
send the AVR the data and what is the easiest way to keep it thru a 
power loss.

The button battery would keep a memory chip alive for some 10 or more 
years so that would eleminate any power issues if sram was much 
easier.

Eventually, this will be connected to a memory card like a MMC or SD 
or CF, but I think one could not put program variables on that ?

Dave
(realizing the trees have branches......)