Файл: Microcontroller Programming. Thi Micro Chip PIC (Julio Sanchez, 2007).pdf

;Routine to handler received data. Overrun and framing

;errors are detected and remembered in the variable

;errorFlags, as follows:

;Test for overrun and framing errors.

;Bit 1 (OERR) of the RCSTA register detects overrun

;Bit 2 (FERR) of the RCSTA register detects framing error

;At this point no error was detected

;Received data is in the USART RCREG register

;==========================

;error handlers ;==========================

;Errors are returned as bits in the errorFlags register

; Error responses to be made by main code

is available as separate ICs that can be placed on the circuit board and accessed through PIC ports. For example, the Microchip 24LC04B EEPROM IC is a 4K electrically erasable PROM with a 2-wire serial interface that follows the I2C convention. Programming serial EEPROM ICs is also covered in this chapter.

15.0 PIC Internal EEPROM Memory

Some PICs contain internal EEPROM data memory that is accessible to code. The amount of memory and the access mechanism varies from PIC to PIC. In fact, the mapping and access mechanisms varies even in devices belonging to the same family. In the sections that follow, we describe EEPROM data memory in the context of two different PICs of the mid-range family: the 16F84 and the 16F877.

15.0.1 EEPROM Programming on the 16F84

The 16F84 and 16F84A contain 64 bytes of EEPROM data memory. This memory is both readable and writable during normal operation. It is not mapped in the register file space, but is indirectly addressed through the Special Function Registers EECON1, EECON2, EEDATA, and EEADR. The address of EEPROM memory starts at location 0x00 and extends to the maximum contained in the PIC, in this case, 0x3f. The following registers relate to EEPROM operations:

1.EEDATA holds the data byte to be read or written.

2.EEADR contains the EEPROM address to be accessed by the read or write operation.

3.EECON1 contains the control bits for EEPROM operations.

4.EECON2 protects EEPROM memory from accidental access. This is not a physical register.

Figure 15-1 shows the bitmap of the EECON1 register in the 16F84.

The CPU continues to access EEPROM memory even if the device is code protected, but in this case the device programmer can not access EEPROM memory.

Reading EEPROM Data Memory on the 16F84

Reading an EEPROM data memory location in the 16F84 requires the following operations:

1.Bank 0 is selected and the address of the memory to be read is stored in the EEADR register.

2.Bank 1 is selected and the RD bit is set in the EECON1 register.

3.Bank 0 is selected and data is read from the EEDATA register.

The following procedure returns in the w register the data stored at the specified EEPROM memory address.

;==============================

;read EEPROM 16F84 ;==============================

;Procedure to read EEPROM memory. Address of memory

;location to read is stored in local register EEMemAdd

Figure 15-1 16F84 EECON1 Register Bit Map

; On exit: read data in w EERead:

16F84 EEPROM Data Memory Write

Writing to 16F84 EEPROM data memory consists of the following operations:

1.Bank 0 is selected and the address of the desired memory location is stored in the EEADR register.

2.The value to be written is stored in the EEDATA register.

3.Bank 1 is selected, interrupts are disabled, and the write enable bit (WREN) is set in the EECON1 register.

4.The special values 0x55 and 0xaa are written consecutively to the EECON2 register.

5.The WR bit is set in the EECON1 register. The EEPROM write takes place automatically after the WR bit is set.

6.Interrupts are re-enabled and bank 0 is selected.

The following procedure shows the processing for the EEPROM write.

;==============================

;write EEPROM ;==============================

;Procedure to write asc1 byte to EEPROM memory

;Address to write stored in local register EEMemAdd

;Data byte to write is in local register EEByte EEWrite:

;Load byte to write into EE data register

Microchip documentation recommends that critical applications should verify the write operation by reading EEPROM memory after the write operation has taken place in order to make sure that the correct value was stored.

16F84 EEPROM Demonstration Program

The program EECounter, in the book’s online software, is a demonstration of EEPROM memory access on the 16F84 PIC. The program keeps track of the number of times that the code has executed by storing each iteration in EEPROM data memory. The program uses the circuit shown in Figure 15-2 (see following page).

The EECounter program increments the value stored at EEPROM address 0x00 at every iteration and displays the result on the first LCD line. The following procedure is used to convert the binary value in EEPROM to 3 ASCII digits for display.