ST24/25C16, ST24/25W16
Write Protection.
Data in the upper four blocks of
256 bytes of the memory may be write protected.
The memory is write protected between a boundary
address and the top of memory (address
7FFh).The boundary address is user defined by
writing it in the Block Address Pointer (location
7FFh).
The Block Address Pointer is an 8 bit EEPROM
register located at the address 7FFh. It is com-
posed by 4 MSBs Address Pointer, which defines
the bottom boundary address, and 4 LSBs which
must be programmed at ’0’. This Address Pointer
can therefore address a boundary by page of 16
bytes.
The block in which the Block Address Pointer de-
fines the boundary of the write protected memory
is defined by the logic level applied on the PB1 and
PB0 input pins:
– PB1 =’0’and PB0 =’0’ select block 4
– PB1 =’0’and PB0 =’1’ select block 5
– PB1 =’1’and PB0 =’0’ select block 6
– PB1 =’1’and PB0 =’1’ select block 7
The following sequence should be used to set the
Write Protection:
– write the data to be protected into the top of
the memory, up to, but not including, location
7FFh;
– select the block by hardwiring the signals PB0
& PB1;
– set the protection by writing the correct bottom
boundary address in the Address Pointer (4
MSBs of location 7FFh) with bit b2 (Protect
Flag) set to ’0’.
Note that for a correct fonctionality of the memory,
all the 4 LSBs of the Block Address Pointer must
also be programmed at ’0’. The area will be pro-
tected when the PRE input is taken High.
Remark:
The Write Protection is active if and only
if the PRE input pin is driven High and the bit 2 of
location 7FFh is set to ’0’. In all the other cases, the
memory Block will not be protected. While the PRE
input pin is read at ’0’ by the memory, the location
7FFh can be used as a normal EEPROM byte.
Caution:
Special attention must be used when
using the protect mode together with the Multibyte
Write mode (MODE input pin High). If the Multibyte
Write starts at the location right below the first byte
of the Write Protected area, then the instruction will
write over the first 7 bytes of the Write Protected
area. The area protected is therefore smaller than
the content defined in the location 7FFh, by 7 bytes.
This does not apply to the Page Write mode as the
address counter ’roll-over’ and thus cannot go
above the 16 bytes lower boundary of the protected
area.
Figure 9. Write Modes Sequence (ST24/25C16)
ACK
BYTE WRITE
DEV SEL
ACK
DATA IN
ACK
BYTE ADDR
R/W
ACK
ACK
START
ACK
DATA IN 1
DATA IN 2
MULTIBYTE
AND
PAGE WRITE
DEV SEL
BYTE ADDR
R/W
START
ACK
DATA IN N
ACK
STOP
STOP
AI00793
11/17
STMICROELECTRONICS [ STMICROELECTRONICS ]
