Four
Functions and Beyond
Tips
for Designing an RPN Calculator
FIRMWARE
As
I wrote the firmware for the HP-45 RPN-based calculator,
I didn’t attempt a complete emulation—that would have
required extra hardware and development. Instead, I
opted to emulate the basic features and leave out the
most complex ones. As such, the calculator is useful
as a demo or learning tool; however, it is not ideal
for tasks that require additional precision and accuracy.
The
stack manipulation functions that handle the pushing,
popping, and exchanging are straightforward. You may
download the functions from the Circuit Cellar ftp site.
Note that it didn’t require complete stack-handling
capabilities because of the small number of elements
in the stack. The evaluating function I used is the
same as the one for the four-function calculator (calc1.c).
The only difference is how the HP-45 application calls
the function. You may download the firmware for the
RPN calculator (hp45.c), as well as the firmware for
the trigonometric and scientific functions (mathtst.c),
from the Circuit Cellar ftp site.
I
employed various C compiler flags, which allowed me
to include drivers for the different hardware configurations
used during the development process. This includes flags
for the LED drivers, LCD drivers, and keypad drivers
(if used).
In
addition, I have flags that support the serial port
and debugging (using MPSIM). I used the flags to save
space on the PIC. By enabling or disabling each flag,
I was able to maximize the PIC’s storage capacity depending
on the hardware configuration (e.g., PICDEMO2, ICD2,
or my own RPN calculator hardware). Again, I would like
to integrate all of these features in one application,
but to accomplish the task, I need to optimize the software
or use a microcontroller with more capacity.
The
values entered in the HP-45 display register, X, are
sent to the LCD, LED, or serial port depending on which
driver flags have been defined. If the keypad driver
flag hasn’t been defined, input is redirected from the
keypad to the serial port. Otherwise, input comes directly
from the keypad, which means the application may hang
if the keypad isn’t connected to the four-function calculator
hardware.
The
software contained in the myprintf.c file implements
a floating-point format I/O routine similar to the Visual
C++ printf function
for displaying floating-point numbers on the LCD and
LED displays. It only provides one kind of output format
that is similar to the %f
format used by printf.
SOFTWARE
ISSUES
The
application can be programmed in a PIC18F452 (flash
memory) or PIC18C452 (EEPROM). Use the hp45.hex file
from the Circuit Cellar ftp site to program your
own.
If
you are interested in customizing the software, I divided
the firmware for various hardware functions into separate
C files to facilitate development, debugging, and maintenance.
The code for the hp45.c application reuses code from
modules I developed for the four-function calculator
and other applications (see Figure 3). In my opinion,
this is the best way to organize a large application,
because it may be successfully tested and integrated
in the hardware one module at a time.
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(Click
here to enlarge)
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Figure
3—Certain C software modules make up the current
RPN calculator’s firmware configuration. |
There
are better methods for organizing the firmware (e.g.,
the object-oriented programming methodology with a language
such as C++ or Java). Although I would like to see a
C++ or Java compiler for the PIC, it would require a
high-end PIC with lots of on-chip RAM and flash memory.
The
source lines of code (SLOC) metric is used by most software
developers as a productivity metric to estimate how
long it will take to develop software and to determine
its cost. The numbers range from 200 to 1000 SLOC per
man month (SLOC/MM), depending on the kind of development
system and computer language. For instance, if assembly
code is used for an embedded real-time microcontroller,
the SLOC could be around 250 SLOC/MM. On the other hand,
if the application is a GUI written in Visual C++, the
SLOC count could be as high as 1000 SLOC/MM. Each line
of C code converted to assembly could take from five
to 10 additional assembly instructions to do the same
thing, depending on your skill level and the optimization
efficiency of a high-level compiler.
The
SLOC count for the RPN calculator application is shown
in Table 1. Even though the SLOC count for the hp45.c
application is modest, the reused drivers and utilities
from other modules increase the SLOC count. Although
I wasn’t keeping track of the time required for each
module (I developed the code in my spare time), the
figure shows the complexity of the application. Actually,
according to the map file generated by the PIC18C linker,
I used approximately 87% of the PIC’s memory resources
for the four-function calculator’s firmware (calc1.c).
Table
1—The calculator’s firmware is complicated.
Listed is a count of all the C source statements
in each file (excluding comments). I didn’t use
all of the code from each file in this application.
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