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Poking around in the firmware and I'm stuck.


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Posted · Poking around in the firmware and I'm stuck.

Tried StackOverflow (good luck getting noticed in that avalanche of questions...) and another forum without luck so now I'm here.

Basically I'm trying to modify lcd_lib_draw_stringP and since I've never touched Arduino code before I'm flailing around in the dark even more than usual. The problem is that a variable doesn't seem to hold the value I'm expecting even though printing it out on the screen suggests it does... :)

The array I'm trying to get data from:


static const uint16_t f_04b036ptDescriptors[95][3] PROGMEM =
{2, 7, 0}, //
{1, 7, 2}, // !
{3, 7, 3}, // "

Snippet of the function:


void lcd_lib_draw_small_stringP(uint8_t x, uint8_t y, const char* pstr)
uint16_t offset;
uint8_t index;
for(char c = pgm_read_byte(pstr); c; c = pgm_read_byte(++pstr))
index = c - ' ';
offset = f_04b036ptDescriptors[index][2];

How I'm calling it:


lcd_lib_draw_small_stringP(15, 10, PSTR("M"));

Now, the trouble I'm having is with the c variable. If I print it out on the screen using the helper function int_to_string I get the expected result (77 for 'M'). index also shows the expected value after subtracting ' ' if I print it out (45). However, I can't seem to use index to get the correct item from the array into offset. I get back "weird" values like -8467 instead of the expected 1-300 range.

If I set index = 45 and/or set c = 'M' manually inside the function everything works just fine.

What am I missing?


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    Posted · Poking around in the firmware and I'm stuck.


    offset = f_04b036ptDescriptors[index][2];

    That's not valid for accessing program memory. You have to use the pgm_read_word() function like this:


    offset = (uint16_t)pgm_read_word(&f_04b036ptDescriptors[index][2]);

    Or alternatively you should be able to do (but above syntax better and I may have the ptr math all wrong here):


    offset = (uint16_t)pgm_read_word(f_04b036ptDescriptors+index*3+2);


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