FM25V20 – 2Mb SPI F-RAM
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. The bus
configuration of RDSR and WRSR are shown
below.
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 /W pin must be high or inactive. Prior
S
C
D
Q
Figure 7. RDSR Bus Configuration
S
C
D
Q
Figure 8. WRSR Bus Configuration
Register has no effect on its state. This bit is
internally set and cleared via the WREN and WRDI
commands, respectively.
Status Register & Write Protection
The write protection features of the FM25V20 are
multi-tiered. Taking the /W pin to a logic low state is
the hardware write-protect function. Status Register
write operations are blocked when /W is low. To
write the memory with /W high, a WREN op-code
must first be issued. Assuming that writes are enabled
using WREN and by /W, 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 /W pin.
The Status Register is organized as follows.
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
1
1
0
1
0
1
None
30000h to 3FFFFh (upper ¼)
20000h to 3FFFFh (upper ½)
00000h to 3FFFFh (all)
Table 2. Status Register
7
6
1
5
0
4
0
3
2
1
0
0
Bit
WPEN
BP1
BP0
WEL
Name
The BP1 and BP0 bits and the Write Enable Latch
are the only mechanisms that protect the memory
from writes. The remaining write protection features
protect inadvertent changes to the block protect bits.
Bits 0, 4, 5 are fixed at 0 and bit 6 is fixed at 1, and
none of these bits can be modified. Note that bit 0
(“Ready” in Serial Flash) is unnecessary as the F-
RAM writes in real-time and is never busy, so it
reads out as a ‘0’. There is an exception to this when
the device is waking up from Sleep Mode, which is
described in the Sleep Mode section. The BP1 and
BP0 control software write protection features. They
are nonvolatile (shaded yellow). The WEL flag
indicates the state of the Write Enable Latch.
Attempting to directly write the WEL bit in the Status
The WPEN bit controls the effect of the hardware /W
pin. When WPEN is low, the /W pin is ignored.
When WPEN is high, the /W pin controls write
access to the Status Register. Thus the Status Register
is write protected if WPEN=1 and /W=0.
This scheme provides a write protection mechanism,
which can prevent software from writing the memory
Rev. 3.0
August 2012
Page 6 of 17
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
