FM25L04B - 4Kb 3V SPI F-RAM
RDSR - Read Status Register
The RDSR command allows the bus master to verify
the contents of the Status Register. Reading status
provides information about the current state of the
write protection features. Following the RDSR op-
code, the FM25L04B will return one byte with the
contents of the Status Register. The Status Register is
described in detail in a later section.
WRSR – Write Status Register
The WRSR command allows the user to select
certain write protection features by writing a byte to
the Status Register. Prior to issuing a WRSR
command, the /WP pin must be high or inactive.
Prior to sending the WRSR command, the user must
send a WREN command to enable writes. Note that
executing a WRSR command is a write operation
and therefore clears the Write Enable Latch.
Figure 7. RDSR Bus Configuration
Figure 8. WRSR Bus Configuration (WREN not shown)
Status Register & Write Protection
The write protection features of the FM25L04B are
multi-tiered. Taking the /WP pin to a logic low state
is the hardware write protect function. All write
operations are blocked when /WP is low. To write the
memory with /WP high, a WREN op-code must first
be issued. Assuming that writes are enabled using
WREN and by /WP, writes to memory are controlled
by the Status Register. As described above, writes to
the status register are performed using the WRSR
command and subject to the /WP pin. The Status
Register is organized as follows.
Table 2. Status Register
Bit
Name
7
0
6
0
5
0
4
0
3
BP1
2
BP0
1
WEL
0
0
Write Enable Latch. Attempting to directly write the
WEL bit in the Status Register has no effect on its
state. This bit is internally set and cleared via the
WREN and WRDI commands, respectively.
BP1 and BP0 are memory block write protection
bits. They specify portions of memory that are
write-protected as shown in the following table.
Table 3. Block Memory Write Protection
BP1
BP0 Protected Address Range
0
0
None
0
1
180h to 1FFh (upper ¼)
1
0
100h to 1FFh (upper �½)
1
1
000h to 1FFh (all)
The BP1 and BP0 bits allow software to selectively
write-protect the array. These settings are only used
when the /WP pin is inactive and the WREN
command has been issued. The following table
summarizes the write protection conditions.
Bits 0 and 7-4 are fixed at 0 and cannot be modified.
Note that bit 0 (“Ready” in EEPROMs) is
unnecessary as the F-RAM writes in real-time and is
never busy. The BP1 and BP0 control write
protection features. They are nonvolatile (shaded
yellow). The WEL flag indicates the state of the
Rev. 1.3
Feb. 2011
Page 6 of 14
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
