send an entire structure thru i2c

Ed Arnold · Joined: 02 Nov 2003 Posts: 18

I first searched for this subject on this forum, but got a bunch of similar related i2c or stucture topics. But still not the answers I an looking for. Well here goes. The following code should explain all, but what I want to do is send an entire structure to a slave PIC to print out a receipt. Can anyone help to fill in the blanks.

Ttelmah · Guest

SteveS · Joined: 27 Oct 2003 Posts: 126

First, to be clear, when I say a 'member' I mean a single structure out of the array of structures. When I say 'element' I mean one value out that structure (like UID):

A couple of things:

I2C_write takes an 8 bit int as an argument. You are sending it - well I'm not quite sure - you are almost sending it a member from an array of strucures, but you don't have an index number in the brackets..

I think you really are trying to send all the elements from a single member of the structure array, but you have 8 ints in each member of the structure array and your loop will go 9 times (0,1,2,3,4,5,6,7,8).

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You need a a bit more code to get there. First you need to choose which of the 16 members of the structure of arrays you want to access. Then walk thru that structure member int by int, sending the 8 elements of that structure.

Try starting with a simple structure (not an array of structures) and figure out how to send the elements of that structure out. Once that is working, add in the complexity of an array of structures.

SteveS

Ed Arnold · Joined: 02 Nov 2003 Posts: 18

Thanks for all of the replies. First, Ttelmah, this stucture is part of pre-existing code which works fine. When I tried to use a name or tag and pointers with this stucture, I got a compiler error, so I removed them and now the old code works. Second, SteveS, I do wish to send only one member from an array of structures. I made a mistake with my for loop. It should be set up to send 8 bytes and as for the member index[], it will be some index variable which I haven't created yet. It looks like I may have to use pointers after all, but I would like to avoid it if possible.

Thanks for the input so far,

Ed Arnold

SteveS · Joined: 27 Oct 2003 Posts: 126

I think it may be hard to not use pointers. Why don't you want to?

Ttelmah's code example points in the right direction. I took your code, modified it a bit (similiar to what Ttelmah suggests) and it works fine:

1. Modify your print_receipt to take an index into your array of structures: void print_receipt(int8 idx) - where idx is 0-15

2. Add a pointer to an int8 in the routine: int8 * sp;

3. Point to the first member of the indexed structure:
sp = &TRANS_QUE[idx].UID;

4. Use that pointer in your call to I2C_write: i2c_write(*sp++);
Each time you call it, sp points to the next value in that structure.

- Now, I used putc instead of I2C_write to test it, but it should work the same.

- SteveS