FM25040A
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.
Table 4. Write Protection
WEL
/WP
Protected Blocks
0
X
Protected
1
0
Protected
1
1
Protected
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.
Status Register
Protected
Protected
Unprotected
Unprotected Blocks
Protected
Protected
Unprotected
Memory Operation
The SPI interface, with its relatively high maximum
clock frequency, highlights the fast write capability
of the FRAM technology. Unlike SPI-bus
EEPROMs, the FM25040A can perform sequential
writes at bus speed. No page register is needed and
any number of sequential writes may be performed.
Write Operation
All writes to the memory array begin with a WREN
op-code. The bus master then issues a WRITE op-
code. Part of this op-code includes the upper bit of
the memory address. Bit 3 in the op-code corresponds
to A8. The next byte is the lower 8-bits of the address
A7-A0. In total, the 9-bits specify the address of the
first byte of the write operation. Subsequent bytes are
data and they are written sequentially. Addresses are
incremented internally as long as the bus master
continues to issue clocks. If the last address of 1FFh
is reached, the counter will roll over to 000h. Data is
written MSB first.
Unlike EEPROMs, any number of bytes can be
written sequentially and each byte is written to
memory immediately after it is clocked in (after the
8
th
clock). The rising edge of /CS terminates a
WRITE op-code operation.
Read Operation
After the falling edge of /CS, the bus master can issue
a READ op-code. Part of this op-code includes the
upper bit of the memory address. The next byte is the
lower 8-bits of the address. In total, the 9-bits specify
the address of the first byte of the read operation.
After the op-code is complete, the SI pin is ignored.
The bus master then issues 8 clocks, with one bit read
out for each. Addresses are incremented internally as
long as the bus master continues to issue clocks. If
the last address of 1FFh is reached, the counter will
roll over to 000h. Data is read MSB first. The rising
edge of /CS terminates a READ op-code operation..
The bus configuration for read and write operations is
shown below.
Hold
The /HOLD pin can be used to interrupt a serial
operation without aborting it. If the bus master takes
the /HOLD pin low while SCK is low, the current
operation will pause. Taking the /HOLD pin high
while SCK is low will resume an operation. The
transitions of /HOLD must occur while SCK is low,
but the SCK pin can toggle during a hold state.
Rev. 3.0
May 2006
Page 7 of
13
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
