80C851 datasheet(9/22 Pages) PHILIPS

Part #	80C851

Description	CMOS single-chip 8-bit microcontroller with on-chip EEPROM

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Manufacturer	PHILIPS [NXP Semiconductors]

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Philips Semiconductors

Product specification

80C851/83C851

CMOS single-chip 8-bit microcontroller

with on-chip EEPROM

1998 Jul 03

9

Security Facilities

EEPROM Protection

The EEPROM is protected using four security

bits which are contained in an extra

EEPROM byte at address 8000H

(EADRH/EADRL). They can be set or cleared

by software. To activate the EEPROM

protection, the program sequence in byte

mode must be as follows:

MOV

EADRH, #80H

MOV

EADRL, #00H

MOV

EDAT, #FFH

If two or more of these bits are reset, SB = 0,

the security mode is disabled and the

EEPROM is not protected. If three or four bits

are set, SB = 1 and the EA mode differs from

the internal access mode.

In this case, access to the EEPROM is only

possible in one mode regardless of how the

external access mode is reached (by pulling

the EA pin low or by passing the 4K

boundary). For SB = 1 and “external access”

only, the “block erase” mode is enabled. The

program sequence has to be as follows:

MOV EADRH, #80H (security byte address)

MOV EADRL, #00H (security byte address)

MOV ECNTRL, #0AH (block erase mode)

MOV EDAT, #xxH

(start block erase)

All 256 data bytes, the security bits, and SB

will be cleared after completing this mode

(EWP = 0). SB will also be affected in byte

mode when writing to the security byte (not

for SB = 1 and “external access”). Figure 3

illustrates the access to SB.

ROM Code Protection

Since the external access mode can only be

selected by pulling the EA pin low during

reset, it is not possible to read the internal

program memory using the MOVC instruction

while executing external program memory.

Furthermore, it is not possible to change this

mode to internal access within the MOVC

cycle.

Additionally, a mask-programmable ROM

code protection facility is available. When the

program memory passes the 4K boundary

using both the internal and external ROMs, it

is not possible to access the internal ROM

from the external program memory if the

mask-programmable ROM security bit is set.

An access to the lower 4K bytes of program

memory using the MOVC instruction is only

possible while executing internal program

memory.

Also the verification mode (test-mode which

writes the ROM contents to a port for

comparison with a reference code) is not

implemented for security reasons. A different

test-mode is implemented for test purposes.

This mode allows every bit to be tested.

However, the internal code cannot be

accessed via a port.

RESET

EADRH

EADRL

EEPROM

EA

EDATA

L

n

EAQ

RESET

REGISTERS EADRH AND

EADRL CONTAIN THE

ADDRESS OF THE

SECURITY BYTE

SECURITY BYTE ADDRESS

AND BLOCK ERASE FINISHED

SECURITY BYTE ADDRESS

AND BYTE MODE FINISHED

RESET

SB

SB = 1

EXTERNAL

ACCESS

INHIBIT ‘READ DATA REGISTER’

INHIBIT ‘WRITE DATA REGISTER’

EXCEPT (ECNTRL) = BLOCK ERASE

NO

YES

Figure 3. EEPROM Protection (Functional and Flowchart)

8

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