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PIC18F4520-I/P 参数 Datasheet PDF下载

PIC18F4520-I/P图片预览
型号: PIC18F4520-I/P
PDF下载: 下载PDF文件 查看货源
内容描述: 28 /40/ 44引脚增强型闪存微控制器与10位A / D和纳瓦技术 [28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology]
分类和应用: 闪存微控制器和处理器外围集成电路光电二极管时钟
文件页数/大小: 412 页 / 6898 K
品牌: MICROCHIP [ MICROCHIP ]
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PIC18F2420/2520/4420/4520  
7.6  
Operation During Code-Protect  
7.8  
Using the Data EEPROM  
Data EEPROM memory has its own code-protect bits in  
Configuration Words. External read and write  
operations are disabled if code protection is enabled.  
The data EEPROM is a high-endurance, byte  
addressable array that has been optimized for the  
storage of frequently changing information (e.g.,  
program variables or other data that are updated  
often). Frequently changing values will typically be  
updated more often than specification D124. If this is  
not the case, an array refresh must be performed. For  
this reason, variables that change infrequently (such as  
constants, IDs, calibration, etc.) should be stored in  
Flash program memory.  
The microcontroller itself can both read and write to the  
internal data EEPROM, regardless of the state of the  
code-protect Configuration bit. Refer to Section 23.0  
“Special Features of the CPU” for additional  
information.  
7.7  
Protection Against Spurious Write  
A simple data EEPROM refresh routine is shown in  
Example 7-3.  
There are conditions when the user may not want to write  
to the data EEPROM memory. To protect against spuri-  
ous EEPROM writes, various mechanisms have been  
implemented. On power-up, the WREN bit is cleared. In  
addition, writes to the EEPROM are blocked during the  
Power-up Timer period (TPWRT, parameter 33).  
Note:  
If data EEPROM is only used to store  
constants and/or data that changes rarely,  
an array refresh is likely not required. See  
specification D124.  
The write initiate sequence and the WREN bit together  
help prevent an accidental write during brown-out,  
power glitch or software malfunction.  
EXAMPLE 7-3:  
DATA EEPROM REFRESH ROUTINE  
CLRF  
BCF  
BCF  
BCF  
BSF  
EEADR  
; Start at address 0  
; Set for memory  
EECON1, CFGS  
EECON1, EEPGD  
INTCON, GIE  
EECON1, WREN  
; Set for Data EEPROM  
; Disable interrupts  
; Enable writes  
; Loop to refresh array  
; Read current address  
;
; Write 55h  
;
; Write 0AAh  
; Set WR bit to begin write  
; Wait for write to complete  
Loop  
BSF  
EECON1, RD  
55h  
EECON2  
0AAh  
EECON2  
EECON1, WR  
EECON1, WR  
$-2  
MOVLW  
MOVWF  
MOVLW  
MOVWF  
BSF  
BTFSC  
BRA  
INCFSZ EEADR, F  
; Increment address  
BRA  
LOOP  
; Not zero, do it again  
BCF  
BSF  
EECON1, WREN  
INTCON, GIE  
; Disable writes  
; Enable interrupts  
DS39631E-page 86  
© 2008 Microchip Technology Inc.  
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