newb assistance - arrays

Guest · Posted: Fri May 19, 2006 4:00 am

I'm having something of a problem using an array.

Basically I've got a 7*5 LED matrix and am using a 16f628 to drive it.

I've written out a charector map to display some numbers/letter/symbols and put then in a large array.

the trouble is that the array seems to be giving me some problems, I have about 90 numbers/letters/symbols
but I only seem to be able to make an array that has 51 spaces in it.

Ttelmah · Guest

The problem is the total size. Constant array accesses on the 16 chips, are done by adding an offset to the program counter, to 'jump' to a RETLW instruction containing the byte required. The maximum offset possible, is 255. 51*5, just fits into this, but 96*5=480, which can't be handled.
This is a hardware limit of the 16 architecture, not something that is fixed in a latter version. It would be possible for the compiler to do multiple arrays, and 'hide' the problem, but it doesn't.
May I suggest a macro like:

#define c32(x,y) (c32##y[x])

Then if you define five 96 element arrays, c320[n]..c324[n], you can access these as 'c32(n,0)'.

The only 'caveat' with the macro version, is that the second digit has to be a constant text character, rather than a variable. Otherwise just have a function to access the five arrays.

Best Wishes