Eeprom is a separate memory segment, it is not part of program memory (ROM), nor data memory (RAM). Even though EEPROM memory locations are not easily and quickly accessed as other registers, they are of great importance as the EEPROM data is permanently saved even after the loss of power and can be changed at any moment. These exceptional features make each byte of EEPROM valuable.

The PIC16F887 microcontroller has 256 locations of data EEPROM controlled by the bits of the following registers:

• EECON1 (control register);
• EECON2 (control register);
• EEDAT (stores data ready to write and read); and
• EEADR (stores address of EEPROM location to be accessed).

The EECON2 register is not true one, it does not physically exist. It is solely used in data write program sequence. In addition to the registers mentioned above, the EEDATH and EEADRH registers are also used for EEPROM write and read. They are also used for program (FLASH) memory write and read. These registers are considered a risk zone and are rarely used as any error can be fatal to your program (you surely don’t want your microcontroller to accidentally delete or change it's own program). There will be no further discussions here, just be careful.

EECON1 Register

EEPGD - Program/Data EEPROM Select bit

• 1 - Access program memory.
• 0 - Access EEPROM memory.

WRERR - EEPROM Error Flag bit

• 1 - Write is prematurely terminated and error has occurred.
• 0 - Write completed.

WREN - EEPROM Write Enable bit.

• 1 - Write to EEPROM enabled.
• 0 - Write to EEPROM disabled.

WR - Write Control bit

• 1 - Initiates write to EEPROM.
• 0 - Write to EEPROM completed.

RD - Read Control bit

• 1 - Initiates read from EEPROM.
• 0 - Read from EEPROM disabled.

READ DATA FROM EEPROM

In order to enable data to be read from EEPROM, follow the procedure below:

• Step 1: Write the address (00h - FFh) to the EEADR register.
• Step 2: Select EEPROM memory block by clearing the EEPGD bit of the EECON1 register.
• Step 3: Set the RD bit of the same register to read the appropriate memory location.
• Step 4: Data is stored in the EEDAT register and is ready to use.

The following example illustrates the above procedure when the program is written in assembly language:

BSF STATUS,RP1 ;
BCF STATUS,RP0 ; Access bank 2
MOVF ADDRESS,W ; Move address to the W register
MOVWF EEADR ; Write address
BSF STATUS,RP0 ; Access bank 3
BCF EECON1,EEPGD ; Select EEPROM memory block
BSF EECON1,RD ; Read data
BCF STATUS,RP0 ; Access bank 2
MOVF EEDATA,W ; Data is stored in the W register

The same program sequence written in Basic language looks as follows:

W = EEPROM_Read (ADDRESS)

Now you realize the advantages of Basic language, don’t you?

WRITE DATA TO EEPROM

Prior to writing data to EEPROM memory it is necessary to write data to the EEDAT register and the appropriate address to the EEADR register. All that’s left is to execute a special program sequence to initiate write for each byte. Interrupts must be disabled as long as the proces of writing data into EEPROM is in progress. The example below illustrates the above procedure when the program is written in assembly language:

BSF STATUS,RP1
BSF STATUS,RP0
BTFSC EECON,WR1 ; Wait for the previous write to complete
GOTO $-1 ;
BCF STATUS,RP0 ; Bank 2
MOVF ADDRESS,W ; Move address to W
MOVWF EEADR ; Write address
MOVF DATA,W ; Move data to W
MOVWF EEDATA ; Write data
BSF STATUS,RP0 ; Bank 3
BCF EECON1,EEPGD ; Select EEPROM memory block
BSF EECON1,WREN ; Write to EEPROM enabled
BCF INCON,GIE ; All interrupts disabled
BSF INTCON,GIE ; Interrupts enabled
BCF EECON1,WREN ; Write to EEPROM disabled
MOVLW 55h
MOVWF EECON2
MOVLW AAh
MOVWF EECON2
BSF EECON1,WR

The same program sequence written in Basic language looks as follows:

W = EEPROM_Write(ADDRESS, W)

Need a comment? Let's do it in mikroBasic...

' This example demonstrates the use of EEPROM Library in mikroBasic PRO for PIC.
program eeprom_test
dim ii as byte ' ii variable used in the loop
main:
ANSEL = 0 ' Configure AN pins as digital I/O
ANSELH = 0
PORTB = 0
PORTC = 0
PORTD = 0
TRISB = 0
TRISC = 0
TRISD = 0

for ii = 0 to 32 step 1 ' Fill data buffer
 EEPROM_Write(0x80+ii, ii) ' Write data to address 0x80+ii
next ii

EEPROM_Write(0x02,0xAA) ' Write data (number 0xAA) to EEPROM address 2
EEPROM_Write(0x50,0x55) ' Write data (number 0x55) to EEPROM address 0x50
Delay_ms(1000) ' Blink PORTB and PORTC diodes
PORTB = 0xFF ' to indicate start of reading
PORTC = 0xFF
Delay_ms(1000)
PORTB = 0x00
PORTC = 0x00
Delay_ms(1000)
PORTB = EEPROM_Read(0x02) ' Read data from EEPROM address 2 and display it on PORTB
PORTC = EEPROM_Read(0x50) ' Read data from EEPROM address 0x50 and display it on PORTC
Delay_ms(1000)

for ii = 0 to 32 step 1 ' Read 32 bytes block from address 0x80
 PORTD = EEPROM_Read(0x80+ii) ' and display data on PORTD
 Delay_ms(250)
next ii

end.