Embedded Systems Lab Lab 8 EEPROM

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1 Islamic University of Gaza College of Engineering Computer Department Embedded Systems Lab EEPROM Prepared By: Eng.Ola M. Abd El-Latif Apr. /2010 :D 0

2 Objectives To get familiar with using the EEPROM of PIC for permanent storage Theory EEPROM An EEPROM (also called an E 2 PROM) or Electrically Erasable Programmable Read-Only Memory is a non-volatile storage chip used in computers and other devices to store small amounts of volatile (configuration) data. Some microcontrollers use built-in (internal) EEPROM, while other microcontrollers don t have built in ones. Otherwise, they use external ones connected via parallel or serial communication. PIC18F4550 has 256 bytes of EEPROM memory locations on addresses from 00h to FFh those can be written to or read from. The most important characteristic of this memory is that it does not loose its contents during power supply turned off. That practically means that what was written to it will be remaining even if microcontroller is turned off. Data can be retained in EEPROM without power supply for up to 40 years (as manufacturer of PIC18F4550 microcontroller states), and up to cycles of writing can be executed. EEPROM memory is placed in a special memory space and can be accessed through special registers. These registers are: EEDATA: which holds read data or that to be written. EEADR: which contains an address of EEPROM location being accessed. EECON1: which contains control bits. EECON2: The dummy register does not exist physically and serves to protect EEPROM from accidental writing. 1

3 EECON1 Register: is a control register with seven implemented bits, Bit 5 is not used. Interpretation of EECON1 register bits follows. EECON1 Register CFGS bit 0 RD (Read Control bit) Setting this bit initializes transfer of data from address defined in EEADR to EEDATA register. Since time is not as essential in reading data as in writing, data from EEDATA can already be used further in the next instruction. 1=initializes reading 0=does not initialize reading bit 1 WR (Write Control bit) Setting of this bit initializes writing data from EEDATA register to the address specified trough EEADR register. 1=initializes writing 0=does not initialize writing bit 2 WREN (EEPROM Write Enable bit) Enables writing to EEPROM If this bit was not set, microcontroller would not allow writing to EEPROM. 1=writing allowed 0=writing disallowed bit 3 WRERR (Write EEPROM Error Flag ) Error during writing to EEPROM This bit was set only in cases when writing to EEPROM had been interrupted by a reset signal or by running out of time in watchdog timer (if it's activated). 1=error occurred 0=error did not occur 2

4 Bit 6 CFGS (Flash program/data EE or Configuration Select bit). 1= Access Configuration Registers 0=Access Program Flash or Data EEPROM memory Bit 7 EEPGD (Flash program or Data EEPROM Memory Select bit). 1= Access Program Flash memory 0=Access Data EEPROM memory INTCON Register: Interrupt Control Register Bit 7 GIE (Global Interrupt Enable). 0= Disable All Interrupts, no Interrupt is acknowledged, even if they are Enabled individually. 1=Interrupts are allowed to happen. PIR2 Register: (Peripheral Interrupt Flag Register 2) bit 4 EEIF (EEPROM Write Operation Interrupt Flag bit) Bit used to inform that writing data to EEPROM has ended. When writing has terminated, this bit would be set automatically. Programmer must clear EEIF bit in his program in order to detect new termination of writing. 1=writing terminated 0=writing not terminated yet, or has not started Writing to EEPROM Memory In order to write data to EEPROM location, programmer must first write address to EEADR register and data to EEDATA register. Only then is it useful to set WR bit which sets the whole action in motion. WR bit will be reset, and EEIF bit set following a writing what may be used in processing interrupts. Values 55h and AAh are the first and the second key whose disallow for accidental writing to EEPROM to occur. These two values are written to EECON2 which serves only that purpose, to receive these two values and thus prevent any accidental writing to EEPROM memory. Therefore, it is very important to turn off interrupts which could change the timing needed for executing instructions. After writing, interrupts can be enabled again. 3

Set the EECON1 register for the EEPROM write by making (a) EEPGD=0, (b) CFGS=0, and (c) WREN=1. 4. Disable all interrupts globally using BCF INTCON, GIE. 5. Write 55H to the EECON2 dummy register. 6.

5 Steps in writing to EEPROM To write a byte of data to a location in the EEPROM memory, we go through the following steps: 1. Load the EEADR registers with the address of the EEPROM location we want to write the data byte to. 2. Load the EEDATA registers with the data byte we want to write to EEPROM. 3. Set the EECON1 register for the EEPROM write by making (a) EEPGD=0, (b) CFGS=0, and (c) WREN=1. 4. Disable all interrupts globally using BCF INTCON, GIE. 5. Write 55H to the EECON2 dummy register. 6. Write AAH to the EECON2 dummy register. 7. Set WR to 1 with the instruction BSF EECON1, WR, with the WR=1, the write cycle begins. 8. Upon completion of the write cycle, the WR bit will be cleared automatically to indicate that the write cycle is finished. 9. Re-enable the interrupts globally using BSF INTCON, GIE. 10. The WREN bit should be cleared to prevent an accidental write to the EEPROM by some runaway program. In the above steps notice the last one. It is recommended that WREN be turned off the whole time except when writing data to EEPROM, so that possibility of accidental writing would be minimal. Reading from EEPROM Memory Steps in Reading from EEPROM: Reading a byte from the EEPROM memory is simple and straightforward as shown in the following steps: 1. Load the EEADR register with the address of the EEPROM location we want to read from. 2. Set the EECON1 register for the EEPROM write by making (a) EEPGD=0, (b) CFGS=0, and (c) RD=1. 3. Within the next instruction cycle, the PIC18 will automatically fetch the data from the EEPROM location and place it in the EEDATE register. The only thing we have to do is to move data from the EEDATA register to a safe place before we do another read. Setting the RD bit initializes transfer of data from address found in EEADR register to EEDATA register. As in reading data we don't need so much time as in writing, data taken over from EEDATA register can already be used further in the next instruction. 4

6 EEPROM Library EEPROM data memory is available with a number of PICmicros. mikrobasic includes library for comfortable work with EEPROM. Library Routines Eeprom_Read Eeprom_Write 5

Lab Exercises Part 1 Write assembly program to write 38 H to Location 12 H in EEPROM then output it to PORTD. Simulate the circuit using Proteus ISIS program.

Now, write a basic program that do the same thing. Part 2: Write assembly program to send data from PORTC to EEPROM then output it to PORTD.

7 Lab Exercises Part 1 Write assembly program to write 38 H to Location 12 H in EEPROM then output it to PORTD. Simulate the circuit using Proteus ISIS program. Use the Debug tool in Proteus ISIS program to see the values written to the EEPROM & the values written to EECON1, EECON2, EEADR, EEDATA, PIR2, and PORTD Registers. Now, write a basic program that do the same thing. Part 2: Write assembly program to send data from PORTC to EEPROM then output it to PORTD. Simulate the circuit using Proteus ISIS program. Use the Debug tool in Proteus ISIS program to see the values written to the EEPROM & the values of EECON1, EECON2, PIR2, and PORTD Registers. Now, write a basic program that do the same thing. 6

8 Part 3: Write a basic program that modify the EEPORM locations from 5 to 9 with numbers 1 to 5 then display the contents of these locations on PORTD. Simulate the circuit using Proteus ISIS program. Use the Debug tool in Proteus ISIS program to see the values written to the EEPROM & the values of EECON1, EECON2, PIR2, and PORTD Registers. Now, write a basic program that do the same thing. 7

Chapter 2 Sections 1 8 Dr. Iyad Jafar

Introducing the PIC 16 Series and the 16F84A Chapter 2 Sections 1 8 Dr. Iyad Jafar Outline Overview of the PIC 16 Series An Architecture Overview of the 16F84A The 16F84A Memory Organization Memory Addressing