- Published: Wednesday, 11 March 2009
- Written by Digital DIY
- Hits: 12837
EEPROMS are great for storing data, and to interface with them is not all that hard. Most use the I2C protocol. In this example the 24LC256 is used. I2C communication is great as it can allow serial data communication between many devices on a single 2 wire bus. These two lines are referred too as SDA and SCL. Basically SDA is the serial data and SCL maintains the clock/frequency in which data is transferred. Don't be too concerned if you can't understand the I2C examples for the 24LC256 below, we'll break it down.
Looks like a bunch of 1's and 0's.. lets break it down;
Control Byte: contains 8 bits.
"Device" is comprised of bits 4-7 of the control byte. Each device (eg; EEPROMS/Digital Thermistors/other I2C devices) have a device code. This is how the PIC communicates with the correct device. The device code is specified in the datasheet of your component. There will be a whole section on how to interface with it via I2C. You may ask, what if 2 devices are on the bus with the same device code (eg; your using 2 of the same EEPROM's on the same bus), answer, assign the address for which device you want to talk to.
"Address" is comprised within bits 3-1 of the control byte. Every I2C device has 3 pins on it labeled "A0, A1, A2". Tying them all to ground will give the address of the device "000" or tying A1 too +5V will give it the address of "010" on the I2C bus (while A0 and A2 are tied to earth). This allows up to 8 different devices with the same "Device" code to be on the same bus. Handy. An example using 4 EEPROMS:
Pin 4 on 24LC256 = Vss (Earth)
Pin 8 on 24LC256 = Vdd (5V)
All of the above devices share the same device code (unchangeable), but to define each ones address, simply set the address pins (A2, A1, A0) to the address that you would like to refer to them over the I2C bus. The upper 2 resistors are connected to the bus and tie it to +5V, these are called 'pull-up' resistors, and must be used on any I2C data bus, you now have 128K bytes of storable memory.
Finally R/W: This single bit is used to control whether a read or write is going to be performed.
Address High/Low Byte contains the data for the address within the EEPROM you want to read/write. As each device contains 32K x 8 (256 Kbit) of memory, the address of each value can't be stored in a single byte. To access them all, the address is broken up into 2 bytes, address high byte and address low byte (eg 10000000 00000000 = byte 32768, 00000000 00000001 = byte 1 and 00000000 10000000 = byte 128 etc..). With that in mind, its good practice to use a word (16 bit variable) to store the address.
Now to communicate with the 24LC256 via I2C. Before each read or write, a 4 bit device code along with a 3 bit address code and a read or write bit must be sent (control byte). The 4 bit device code for the 24LC256 is %1010 (found in the datasheet), and we will just talk to the 1st EEPROM in the diagram, who's address 001 (set via A2, A1, A0). With all that in mind, the control bit we will send is 1010001x where x is a 1/0 depending if its a read/write.
This example shows how to interface to the EEPROM with address 001 and fill 20 bytes of data, then read it back;
Note: After writing the data, the data is then read from the EEPROM, it is all stored in the array DATA_IN. Also take note that the register Address is actually a Word, this is so that the High Address byte and Low Address byte are sent at the same time (a simple way to combine the two bytes).
Finally, you must think of the PIC as the MASTER on the I2C bus. It tells everything on the bus what to do. Nothing talks unless the PIC asks it to. You control the bus, and to do so there are some commands like BSTART/BRESTART/BSTOP to tell everything on the bus when to listen up, and when to stop. These are shown above in the example. You should read the datasheet on every device to see how to communicate with it via I2C!! Some devices require unusual setting up (eg, to initialize it on the I2C bus, you must send a BSTOP, BSTART, BSTOP..), this is only on some devices, but its definitely worth your time to check it out.