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PCD
The compiler makes every effort to optimize usage of RAM. Understanding the RAM allocation
can be a help in designing the program structure. The best re-use of RAM is accomplished
when local variables are used with lots of functions. RAM is re-used between functions not
active at the same time. See the NOT ENOUGH RAM error message in this manual for a more
detailed example.
RAM is also used for expression evaluation when the expression is complex. The more complex
the expression, the more scratch RAM locations the compiler will need to allocate to that
expression. The RAM allocated is reserved during the execution of the entire function but may
be re-used between expressions within the function. The total RAM required for a function is the
sum of the parameters, the local variables and the largest number of scratch locations required
for any expression within the function. The RAM required for a function is shown in the call tree
after the RAM=. The RAM stays used when the function calls another function and new RAM is
allocated for the new function. However when a function RETURNS the RAM may be re-used
by another function called by the parent. Sequential calls to functions each with their own local
variables is very efficient use of RAM as opposed to a large function with local variables
declared for the entire process at once.
Be sure to use SHORT INT (1 bit) variables whenever possible for flags and other boolean
variables. The compiler can pack eight such variables into one byte location. The compiler does
this automatically whenever you use SHORT INT. The code size and ROM size will be smaller.
Finally, consider an external memory device to hold data not required frequently. An external 8
pin EEPROM or SRAM can be connected to the PIC? with just 2 wires and provide a great deal
of additional storage capability. The compiler package includes example drivers for these
devices. The primary drawback is a slower access time to read and write the data. The SRAM
will have fast read and write with memory being lost when power fails. The EEPROM will have a
very long write cycle, but can retain the data when power is lost.
What is an easy way for two or more PICs? to communicate?
There are two example programs (EX_PBUSM.C and EX_PBUSR.C) that show how to use a
simple one-wire interface to transfer data between PICs?. Slower data can use pin B0 and the
EXT interrupt. The built-in UART may be used for high speed transfers. An RS232 driver chip
may be used for long distance operations. The RS485 as well as the high speed UART require
2 pins and minor software changes. The following are some hardware configurations.
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