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8XC51FXHARDWAREDESCRIPTION
one of the key bytes. Therefore,to read the
ROM/EPROM code, the user has to know the encryp-
tion key bytes in their proper sequence.
Unprogrammed bytes have the value OFFH.If the En-
cwtion Array is left unprogrsrmn
edj all the key bytes
have the value OPPH.Since any code byte XNOR’ed
with OFFH leavesthe byte unchanged, leaving the En-
cryption Array unprogrammed in efkt bypasseathe
encryption feature.
When using the encryption array feature, one import-
ant factor should be considered. If a code byte has the
value OFFH,verifyingthe byte will produce the encryp-
tionbytevsdue.If a large block (>64 bytes) of code is
left rmprograrmned,a verification routine will display
the encryption arraycontents. For this reason all un-
used code bytes should be progrsmm
ed with some val-
ue other than OFFH,and not all of them the same val-
ue. This will ensure maximum programprotection.
Program Lack Bits: Alao included in the Program
Lock scheme are Lock Bits which can be enabled to
provide varyingdegr- of protection.Table 25 lists the
Lock Bita and theircorrespondinginfluence on the mi-
crocontroller. Referto Table24 for the Lack Bits avail-
able on the variousproducts.The user is responsiblefor
pro-g the Lock Bits on EpROM devi~. on
ROM devices, LB1 is automatically set by the factory
when the encryption arrayis submitted. The LmckBit
is not available without the encryption arrayon ROM
devices.
Erasing the EPROM also erases
the Encryption Array
and the Lock Bits, returningthe part to full functionali-
ty.
Table 24. C51FX Program Protection
Device
Lock Bite
Encrypt Array
I 83C51FA I None I None I
I 87C51FC I LB1,LB2,LB3 I 64Bytes I
13.0 ONCETM MODE
The ONCE (ON-Circuit Emulation) mode facilitstea
testing and debugging of systems using the C51FX
without having to remove the device from the circuit.
The ONCE mode is invoked by:
1. Pulling ALE low while the device is in reset and
PSEN is higiu
2. Holding ALE low as RST is deactivated.
While the device is in ONCE mode, the Port Opins go
into a float state, and the other port pins, ALE, and
PSEN are weakly pulkd high. The oscillator circuit
remains active. While the device is in this mode, an
emulator or test CPU can be used to drive the circuit.
Normrdoperation is restoredafter a valid reset is ap-
plied.
Table 25. Lock Bita
Program Lock Bite
LB1
Protection Type
LB2 I LB3
II
Iuuu
I
No programlockfeaturesenabled.(Codeverifywillstillbeencryptedbythe
encryption
array if programmed.)
I
II
2PUU
I
MOVCinstructionsexecutedfromexternalprogrammemory are disabled from
fetchina code bvtes from internal rnemow. EA is samDled and latched on
I
reset ~ndfutlherprogrammingoftheEPROMisdi=bled.
31 P I P I
u
]
3ameas2, alsoverifyisdisabled.
41 P I P ] P
Sameas3,alsoexternalexecutionisdisabled.
I
P = Programmed
U = Unprogrammed
Any other combinationof the Lock Bitais notdefined.
5-41
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