80C31BH Datasheet, PDF (7/16 Pages) Intel Corporation – CHMOS SINGLE-CHIP 8-BIT MICROCONTROLLER

AUTOMOTIVE 80C31BH 80C51BH 87C51
PROGRAM MEMORY LOCK
(EPROM Only)
The 87C51 contains two program memory lock
schemes Encrypted Verify and Lock Bits
Encrypted Verify The 87C51 implements a 32-
byte EPROM array that can be programmed by the
customer and which can then be used to encrypt
the program code bytes during EPROM verification
The EPROM verification procedure is performed as
usual except that each code byte comes out logical-
ly X-NORed with one of the 32 key bytes The key
bytes are gone through in sequence Therefore to
read the ROM code one has to know the 32 key
bytes in their proper sequence
Lock Bits Also on the chip are two Lock Bits which
can be left unprogrammed (U) or can be pro-
grammed (P) to obtain the following additional fea-
tures
Bit 1 Bit 2
Additional Features
U
U
none
P
U  Externally fetched code can not
access internal Program Memory
 Further programming disabled
U
P (Reserved for Future definition )
P
P  Externally fetched code can not
access internal Program Memory
 Further programming disabled
 Program verification is disabled
When Lock Bit 1 is programmed the logic level at
the EA pin is sampled and latched during reset If
the device is powered up without a reset the latch
initializes to a random value and holds that value
until reset is activated It is necessary that the
latched value of EA be in agreement with the current
logic level at that pin in order for the device to func-
tion properly
ONCE MODE
The ONCE (‘‘on-circuit emulation’’) mode facilitates
testing and debugging of systems using the 87C51
without the 87C51 having to be removed from the
circuit The ONCE mode is invoked by
1 Pull ALE low while the device is in reset and
PSEN is high
2 Hold ALE low as RST is deactivated
While the device is in ONCE mode the Port 0 pins
go into a float state and the other port pins and ALE
and PSEN are weakly pulled high The oscillator cir-
cuit remains active While the 87C51 is in this mode
an emulator or test CPU can be used to drive the
circuit Normal operation is restored when a normal
reset is applied
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