6
FN8123.0
March 29, 2005
The state of the Control Register can be read at any
time by performing a random read at address FFFFh.
Only one byte is read by each register read operation.
The X4643/5 resets itself after the first byte is read.
The master should supply a stop condition to be con-
sistent with the bus protocol, but a stop is not required
to end this operation.
BP2, BP1, BP0: Block Protect Bits (Nonvolatile)
The Block Protect Bits, BP2, BP1 and BP0, determine
which blocks of the array are write protected. A write to
a protected block of memory is ignored. The block pro-
tect bits will prevent write operations to the following
segments of the array.
RWEL: Register Write Enable Latch (Volatile)
The RWEL bit must be set to “1” prior to a write to the
Control Register.
WEL: Write Enable Latch (Volatile)
The WEL bit controls the access to the memory and to
the Register during a write operation. This bit is a vola-
tile latch that powers up in the LOW (disabled) state.
While the WEL bit is LOW, writes to any address,
including any control registers will be ignored (no
acknowledge will be issued after the Data Byte). The
WEL bit is set by writing a “1” to the WEL bit and
zeroes to the other bits of the control register. Once
set, WEL remains set until either it is reset to 0 (by
writing a “0” to the WEL bit and zeroes to the other bits
of the control register) or until the part powers up
again. Writes to the WEL bit do not cause a nonvolatile
write cycle, so the device is ready for the next opera-
tion immediately after the stop condition.
WD1, WD0: Watchdog Timer Bits
The bits WD1 and WD0 control the period of the
Watchdog Timer. The options are shown below.
Write Protect Enable
These devices have an advanced block lock scheme
that protects one of five blocks of the array when
enabled. It provides hardware write protection through
the use of a WP pin and a nonvolatile Write Protect
Enable (WPEN) bit.
The Write Protect (WP) pin and the Write Protect
Enable (WPEN) bit in the Control Register control the
programmable Hardware Write Protect feature. Hard-
ware Write Protection is enabled when the WP pin and
the WPEN bit are HIGH and disabled when either the
WP pin or the WPEN bit is LOW. When the chip is
Hardware Write Protected, nonvolatile writes to the
block protected sections in the memory array cannot be
written and the block protect bits cannot be changed.
Only the sections of the memory array that are not
block protected can be written. Note that since the
WPEN bit is write protected, it cannot be changed
back to a LOW state; so write protection is enabled as
long as the WP pin is held HIGH.
Table 1. Write Protect Enable Bit and WP Pin Function
76543210
WPEN WD1 WD0 BP1 BP0 RWEL WEL BP2
BP2
BP1
BP0
Protected Addresses
(Size) Array Lock
0 0 0 None (factory setting) None
0 0 1 None None
0 1 0 None None
0 1 1 0000h - 1FFFh
(8K bytes) Full Array (All)
1 0 0 000h - 03Fh
(64 bytes) First Page (P1)
1 0 1 000h - 07Fh
(128 bytes) First 2 pgs (P2)
1 1 0 000h - 0FFh
(256 bytes) First 4 pgs (P4)
1 1 1 000h - 1FFh
(512 bytes) First 8 pgs (P8)
WD1 WD0 Watchdog Time Out Period
0 0 1.4 seconds
0 1 600 milliseconds
1 0 200 milliseconds
1 1 disabled (factory setting)
WP WPEN
Memory Array not
Block Protected
Memory Array
Block Protected
Block Protect
Bits WPEN Bit Protection
LOW X Writes OK Writes Blocked Writes OK Writes OK Software
HIGH 0 Writes OK Writes Blocked Writes OK Writes OK Software
HIGH 1 Writes OK Writes Blocked Writes Blocked Writes Blocked Hardware
X4643, X4645
