In this tutorial we will discuss how to access the PIC16F877A internal EEPROM memory to store and retrieve the data. Eeprom is basically used to store the non volatile data which is required to be stored even if there is power loss or controller resets.

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PIC16F877A Memories

PIC16F877A comes with three memories Flash,RAM and EEPROM. Below table shows the memory capacity of PIC16F877A:

Memory	Size	Description
FLASH	8k-bytes	Used to store the programs
RAM	368-bytes	Temporary/ScratchPad memory used during program execution.
EEPROM	256-bytes	Used to store the non-volatile data across power cycles

EEPROM Registers

The below table shows the registers associated with PIC16F877A UART.

Register	Description
EECON1	Eeprom read/Write Control register
EECON2	Used to execute special instruction sequence(0x55-0xAA) during write
EEDATA	Holds the data to be Written/Read to/from Eeprom.
EEADR	Hold the Eeprom memory address from where the data needs to be read/written.

EEPCON1
7	6	5	4	3	2	1	0
EEPGD	-	-	-	WRERR	WREN	WR	RD

EEPGD: Program/Data EEPROM Select bit
1 = Accesses program memory
0 = Accesses data memory

WRERR: EEPROM Error Flag bit
1 = A write operation is prematurely terminated
0 = The write operation completed

WREN: EEPROM Write Enable bit
1 = Allows write cycles
0 = Inhibits write to the EEPROM

WR: Write Control bit
1 = Initiates a write cycle. The bit is cleared by hardware once write is complete. The WR bit can only be set (not cleared) in software.
0 = Write cycle to the EEPROM is complete

RD: Read Control bit
1 = Initiates an EEPROM read; RD is cleared in hardware. The RD bit can only be set (not cleared) in software.
0 = Does not initiate an EEPROM read

Steps For Eeprom Write

Check the WR bit to see if a write is in progress and wait till it becomes zero.
Write the address to EEADR. Make sure that the address is not larger than the memory size of the device.
Write the 8-bit data value to be programmed in the EEDATA register.
Clear the EEPGD bit to point to EEPROM data memory.
Set the WREN bit to enable program operations.
Disable interrupts (if enabled).
Write 55h to EECON2
Write AAh to EECON2
Set the WR bit
Restore the Interrupts.
Clear the WREN bit to disable program operations.

	void EEPROM_WriteByte(unsigned char eepromAddress, unsigned char eepromData)
	{
	unsigned char gie_Status;

	while(WR); // check the WR bit to see if a previous Write operation is in progress
	EEADR=eepromAddress; // Write the address to EEADR.
	EEDATA=eepromData; // load the 8-bit data value to be written in the EEDATA register.
	WREN=1; // Set the WREN bit to enable eeprom operation.
	gie_Status = GIE; // Copy the current Interrupt state
	GIE = 0; // Disable the interrupts
	EECON2=0x55; // Execute the special instruction sequence
	EECON2=0xaa; // Refer the datasheet for more info
	WR=1; // Set the WR bit to trigger the eeprom write operation.
	GIE = gie_Status; // Restore the interrupts
	WREN=0; // Disable the EepromWrite
	}

view raw pic16f877a_eepromWrite.c hosted with ❤ by GitHub

Steps For Eeprom Read

Check the WR bit to see if a write is in progress and wait till it becomes zero.
Write the address to EEADR from where the data needs to be read. Make sure that the address is not larger than the memory size of the device.
Set the RD bit to start the read operation
Read the data from the EEDATA register.

	unsigned char EEPROM_ReadByte(unsigned char eepromAddress)
	{
	while(RD \|\| WR); // check the WR&RD bit to see if a RD/WR is in progress
	EEADR=eepromAddress; // Write the address to EEADR.
	RD = 1; // Set the RD bit to trigger the eeprom read operation.
	return(EEDATA); // Return the data read form eeprom.
	}

view raw pic16f877a_eepromRead.c hosted with ❤ by GitHub

Code

Lets put all together whatever we have discussed and write a simple program to write the data(A-Z) to eeprom and then read it back.

	#include <pic16f877a.h>
	#include "uart.h"
	#include "eeprom.h"


	/* start the main program */
	int main()
	{
	unsigned char eeprom_address = 0, write_char, read_char;

	UART_Init(9600);

	for(write_char='A';write_char<='Z';write_char++)
	{
	UART_Printf("\n\rEeprom Write: %c ",write_char); //Print the message on UART
	EEPROM_WriteByte(eeprom_address, write_char); // Write the data at memoryLocation 0x00

	read_char = EEPROM_ReadByte(eeprom_address); // Read the data from memoryLocation 0x00
	UART_Printf("Eeprom Read: %c",read_char); //transmit the data read from Eeprom
	}

	while (1);
	return 0;
	}