External EEPROM Basics For Newbies

[zogbog]

I have been teaching myself C and PIC programming for the last twelve months and thanks to this forum i have got to grips with many of the basics including LCDs Keypads A/Ds etc.

My next task is to learn how to read and write to an external I2C EEPROM using a 16F877 and a 24L256 32K X 8 (SDA RC4 and SCL RC3).

I have read many of the posts and the examples from CCS. What i apear to be missing is a basic understanding of memory addresses.

QUESTIONS AND INFO REQUIRED

1) I need and explination of the page system.

2) I need to understand how to organise and manage my memory.

3) Example problem:
If i have a routine that reads an A/D port and returns a value called "volts" where volts is = 1023 how would this be written in to an external memory?

would i write and read "volts" or would i have to write and read 1023 to/from a defined address?

If i have "volts" and "amps" and they both need to be written to memory how do i define where one finishes and the other starts?

I dont have a problem with code examples as the are many in the posts. I have a lack of knowlage on memory. I need to understand what the code examples do if i am to make any progress

If any one can explain how it works in dummies talk or know of a dummies type link i would appreciate it

[newguy]

The page system is not that hard to grasp.

Imagine a book with 500 pages. Imagine that you are dictating to a person who is actually typing the book for you. This rather gifted person can listen and retain one page worth of text before they start typing. Let me also add that no words get lost either. However, if you try to dictate more than one page worth of text, the extra "spills over" or "loops back" onto the beginning of the page.

That's sort of like the page system with external EEPROMs. For instance, I'm presently using a 24LC32, a 32k serial eeprom. This particular one can accept 32 bytes for storage, at most, before actually writing the data all at once. In this case, 32 bytes is the page size. Note that you can't overflow one page when doing a page write. In the case of the 24LC32, I can do a page write to address 0x0000, then again at address 0x0020 (32), and so on. I can start a page write at address 0x0014, but the data being written will start at address 0x14, then "roll over" at address 0x1f back to address 0x00.

I should also mention that the 24LC32 is a 32 kbit EEPROM, which means you can store 32/8 = 4 kbytes. You can store one byte/address, in addresses from 0x0000 - 0x0fff.

The datasheet for your EEPROM will tell you how big a page is.

As for storing data, that's up to you how you'd like to handle it. Since it sounds like you're storing only two things, V & I, I'll use that as a basis for the explanation.

First you should note that serial EEPROMs are usually 8 bit. That is, they are capable of storing 8 bits in every address. If you have a variable, such as voltage, which goes up to 1023, you'll need a 16 bit variable to store it. That means that you'll need to break up this variable into a low and a high byte, and store those individually in the EEPROM. When you read voltages from the EEPROM, you'll need to reassemble the 16 bit variable yourself based on the low and high bytes you read from memory.

Note that you should always put the low & high bytes of 16 bit variables right next to each other in memory (i.e. address = high byte, address + 1 = low byte, or vice-versa).

For instance,

[zogbog]

Good clear explanation just what i have been looking for. Now all the example code that i have been reading is crystal clear. If only more subjects could be explained in this way. Maybe a series of books?......

Thanks for the help.