AT24C64-data logging in circular buffer type

pad · Joined: 29 Nov 2007 Posts: 15

hi all,
I am working on a energy meter project with MAXQ3180, PIC18F6722 and 4.020 CCS compiler.
I want to store the data from energy meter to 24C64 EEPROM (5 no.s)which is connected through i2c. I want data to be stored in circular fashion so that at any time I can access last 100 entry's.
I can write and read eeprom with easy and can log 72 bytes of data on predefined time. But cannot make it in circular buffer as I don't have the last address as there are number of ICs.
After ICs become full, my data get overwritten on old values.
How to deal with it? Is there any simple method to do that.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Store the data in a format, that allows to identify the last write position without an explicite write pointer. E.g. the last block before an erased block is assumed as last write position. The only prerequisite is to erase the next block before writing a new one. On application startup, you can search the stored data, until you find the first erased block. It's the next write position.

languer · Joined: 09 Jan 2004 Posts: 144 Location: USA

You could use the first two EEP locations to store the next free address. In your program you have to keep track of which address you're writing to (even if you're using the auto-increment feature - read PAGE WRITE). You have to make sure that when you're back at the first page, the first two EEP locations are "skipped".

After I wrote this, I did a quick search on the forum and there was a similar post: http://www.ccsinfo.com/forum/viewtopic.php?t=41312&highlight=eeprom+address+pointer

pad · Joined: 29 Nov 2007 Posts: 15

Thanks FvM & languer for your quick & valuable response.
I have 5 EEPROMS(A0,A2,A4,A6,A8). Of which I start data logging from 2nd A2 to 5th A8. I check the address before write the data. If eeprom is full then jump to next IC. Here I have only last address so if I want to retrieve the data I need to start from currently logged one. Of which starting address I can found. But how to get other starting addresses as they may be on other ICs.
Hope you people understand what I want to say. Suppose my latest data is on A4 and its previous data is on A2 then A8, A6 and A4. So how I get their starting addresses. Can I subtract 72 byte locations from each last address to get starting address of each block, as I know I am writing 72 bytes each time, or need to save each blocks start and end address in eeprom & access them to read again.

FvM · Joined: 27 Aug 2008 Posts: 2337 Location: Germany

Both suggested methods can be extended to multiple memory devices, if you treat them logically as one large memory space. Some more detail programming is involved, of course.

The main difference betwen using a dedicated pointer location or an implicite marking of the next write location, as I suggested, is in the achieved wear leveling. If wear leveling is a concern at all, depends on your application requirements, particularly the average write frequency. In any case, you should review the respective memory endurance specifications and check, if they are sufficient for your intended usage.