The code memory can be protected by programming 3 lock bit's in the controller.
This three lock bit's can be left unprogramed(u) or can be programed(p) to obtain additional feature.
This lock bit's are labeled as LB1,LB2,LB3.
After a chip erace cycle .these lock bit's will be logic "one".to make the lock activate set .the appropriate bit to "zero".
i will explain something.
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MODE 1
lock bits : LB 1=U(unprogrammed),LB 2=U(unprogrammed),LB 3= U.
protection type:no programmed lock features
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MODE 2:
lock bits: LB 1 = P (programmed), LB 2:U(unprogrammed) LB 3: U(unprogrammed)
protection type: MOVC instruction executed from external program memory are disabled from fetching code bytes from internal memory. EA bar sampled and latching on RESET and further programming of the FLASH is disabled.
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MODE 3:
lock bits : LB 1= P ( programmed), LB 2= P (programmed ), LB 3 = U ( unprogrammed )
protection type : same as MODE 2 also verify is disabled.
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MODE 4 :
lock bits : LB 1 = P (programmed ), LB 2 = P ( programmed), LB 3= P ( programmed)
protection type : same as MODE 3 , also external execution is disabled.
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ONCE MODE ( 5 th MODE ):on chip emulation mode facilitate the testing and debugging of system using the line device without the device being removed from the circuit.
the "ones"mode invoked by
step 1 : pull ALE low while the device is in RESET and PSEN is high.
step 2 : hold the ALE low as the RESET is deactivated.
while the device is " ones" mode the PORT 0 pin go into A float state and the other PORT pins and PSEN are weakly pulled high the oscillator remain active while the device is this mode . test CPU can be used to drive the CKT . normal operation is restored after a normal RESET is applied.
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thank youLabels: ATMEL 89C51