June
20065, Issue 191
Earth
Field Magnetometer
Cypress
PSoC High Integration Challenge 2004 Winner
SOFTWARE
DETAILS
I
wrote the application code in C language with the Cypress
PSoC Designer 4.1 Service Pack 1. The C compiler doesn’t
perform any optimization during compilation, but it
uses a post-processor that carries out code compression
and eliminates unused code. I ran into some difficulty
with this approach, so I had to rewrite some sections
of the code in assembly to save flash memory space.
When
I first generated the module application files, the
module code size was 2,024 bytes. That left only 24
bytes of the CY8C22213’s 2 KB of flash memory for user
code! Clearly, something had to change. I looked at
the linker map and saw that just over half the code
was from the LCD and I2Cm modules. I ended up rewriting
both of them and saving nearly 600 bytes.
Now
that I had room for user code, it was time to start
writing it. The API documentation was thorough enough
to make the code development process easy. To simplify
development, I started with the larger CY8C27443 microcontroller
on the PSoC Invention Board. I then used it to download
to the CY8C22213 after debugging and testing the code.
There
still wasn’t enough room after everything was written,
so I ended up rewriting the initialization code in boot.asm
and the LoadConfig routines. I also noticed that almost
half of the registers were being initialized to zero,
but they were already reset to zero by a system reset.
Commenting these out saved quite a bit of space. Unfortunately,
there was no way to do this automatically; it had to
be redone manually every time the Generate Application
function was used. I had to avoid commenting out the
code for the I/O port DriveMode registers because they
weren’t set to zero at reset.
The
final code weighs in with a whopping 6 free bytes of
flash memory. It uses about 80% of the RAM. I used stackwalk.exe,
which analyzes stack usage from the listing file. It
showed a maximum stack size of 25 bytes. This still
left enough space for comfort, making a stack overflow
very unlikely.
