FM25640
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.
The WPEN bit controls the effect of the hardware
/WP pin. When WPEN is low, the /WP pin is
ignored. When WPEN is high, the /WP pin controls
write access to the status register. Thus the Status
register is write protected if WPEN=1 and /WP=0.
This scheme provides a write protection mechanism,
which can prevent software from writing the memory
under any circumstances. This occurs if the BP1 and
BP0 are set to 1, the WPEN bit is set to 1, and /WP is
set to 0. This occurs because the block protect bits
prevent writing memory and the /WP signal in
hardware prevents altering the block protect bits (if
WPEN is high). Therefore in this condition, hardware
must be involved in allowing a write operation. The
following table summarizes the write protection
conditions.
Table 4. Write Protection
WEL
WPEN
/WP
0
X
X
1
0
X
1
1
0
1
1
1
Protected Blocks
Protected
Protected
Protected
Protected
Unprotected Blocks
Protected
Unprotected
Unprotected
Unprotected
Status Register
Protected
Unprotected
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 FM25640 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 next op-code is the WRITE instruction.
This op-code is followed by a two-byte address
value. The upper 3-bits of the address are ignored. In
total, the 13-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 1FFFh
is reached, the counter will roll over to 0000h. 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. Following this instruction is a two-
byte address value. The upper 3-bits of the address
are ignored. In total, the 13-bits specify the address of
the first byte of the read operation. After the op-code
and address are complete, the SI line is ignored. The
bus master 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 1FFFh is reached, the counter will roll
over to 0000h. 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 pulls
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
Mar. 2005
7 of 13
RAMTRON [ RAMTRON INTERNATIONAL CORPORATION ]
