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ADUC812: Battery lifetime

1)For our needs we found that the total power dissipation of the ADUC821 is
typically 100mW. How can we know how long(how many minutes or hours) the micro
controller can work before the battery becomes insufficient?
2)In the data sheets we found that there is the possibility to address an
external 16MBytes data memory. this memory is referred to as an SRam memory. Is
it possible to use a different kind of memory (such as a Flash Memory)?
Moreover, is there any possibility to address a larger external data memory?


1) You need to know the load characteristics of your battery (ideally a curve
showing terminal voltage vs charge). When the terminal voltage of the battery
drops below the operating voltage of the components in the system this is the
point when the system will no longer function reliably. The curve will give you
the total value of CHARGE that you can draw from the battery before the
terminal voltage drops below e.g. 4.75V (for a 5V+/-5% system). Work out the
current that your system draws. Work out how much time it will take for the
system to draw this charge t= CHARGE/CURRENT. This is your battery life. A less
accurate estimate can be obtained if you know the capacity in Amp/hours of your
battery. Again, calculate the average current drawn by the system . This will
directly give the time that your system can run for.

2) The 24-bit addressing feature allows the ADuC812 to address 16M of external
SRAM, mapped into the same memory space as the on board programme and data
memory. You can use any memory type with parallel address and data bus and
which has compatible read and write timing with the ADuC812. Write times for
flash memory are much longer so you probably won't be able to find one which is
suitable. You can address a larger memory space by using additional i/o pins to
decode the address of further memories. This is more awkard to manage (in
software) becuase you need to ensure that the I/O pins are correctly set before
accessign a particular memory location. 16M is the maximum that can be accessed
"automatically" using the conventional addressing of the device.