Using EEMEM inside a struct

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dpaulsen
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Posted by dpaulsen: Tue. Aug 3, 2010 - 09:03 PM
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Eventually what I am trying to do is to have my MCU keep an error log in eeprom,

In the past I have accomplished this by defining an array in eeprom. I kept track of the location of the next free space by writing 0xff to the next available spot. Every time my mcu would reset, one of the first thing it would do in the initialization is to poll every element in the eeprom until it got to a 0xff. Then it would store the element number of this location into a SRAM variable.

I thought it might be a little cleaner if I tried to put the eeprom array and the element number in a struct.

Here is my declaration:

struct ee_log_t {
uint8_t index;
uint8_t EEMEM list[256];
} reset_log, error_log;

I get a compiler error:

Quote:

Terminal.h:4: error: section attribute not allowed for 'list'

Did I get the syntax wrong or is this simply not allowed?

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Tools, Compilers and General Programming
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Gold Markclawson
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Posted by clawson: Tue. Aug 3, 2010 - 09:39 PM
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Surely you want the whole thing in EEMEM?

typedef struct { 
uint8_t index; 
uint8_t list[256]; 
} ee_log_t;

ee_log_t EEMEM reset_log, error_log;

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dpaulsen
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Posted by dpaulsen: Tue. Aug 3, 2010 - 09:54 PM
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clawson wrote:
Surely you want the whole thing in EEMEM?

typedef struct { 
uint8_t index; 
uint8_t list[256]; 
} ee_log_t;

ee_log_t EEMEM reset_log, error_log;

The only reason I can think of for not doing it that way is if you loose power while writing to index, you will loose your place.

Of course if my MCU gets hit by a meteorite, it might loose it place also, and I didn't account for that :).

Your way might work fine for what I am using it for.

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curtvm
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Posted by curtvm: Wed. Aug 4, 2010 - 03:42 AM
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If you are simply checking for the first 0xFF in eeprom, you don't need to keep an index. Just 'find' the next location by searching for it in your log routines. I can't imagine speed is important since you are not going to be logging that often (this is eeprom after all).

log_routine(){
x=get_next_avail_ee();
//log data
}
get_next_avail_ee(){
//find first 0xFF following !0xFF
//make sure 'next' ee is erased
return address;
}

Quote:
Did I get the syntax wrong or is this simply not allowed?
Think about what you are trying to do. You want part of the struct in ram, part in eeprom. You no longer have a consecutive 'group' of addresses, and it is no longer a struct, which is why the compiler complains.

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dpaulsen
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Posted by dpaulsen: Wed. Aug 4, 2010 - 09:15 PM
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curtvm wrote:
If you are simply checking for the first 0xFF in eeprom, you don't need to keep an index. Just 'find' the next location by searching for it in your log routines. I can't imagine speed is important since you are not going to be logging that often (this is eeprom after all).

log_routine(){
x=get_next_avail_ee();
//log data
}
get_next_avail_ee(){
//find first 0xFF following !0xFF
//make sure 'next' ee is erased
return address;
}

Quote:
Did I get the syntax wrong or is this simply not allowed?
Think about what you are trying to do. You want part of the struct in ram, part in eeprom. You no longer have a consecutive 'group' of addresses, and it is no longer a struct, which is why the compiler complains.

Thank you for helping me understand. I guess I never really thought of a struct as a fancy array, but thats all it really is.

Also for some reason I was thinking that reading from eeprom was slow, but now that I think about it, the time involved to read 250 bytes of eeprom is 250 x 0.5 us = 125 us (6 cycles at 12Mhz), where as writing twice makes my program stall in a wait loop for 3300 us. Good call.

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