7
www.fairchildsemi.com
FM93CS56 Rev. C.1
FM93CS56 (MICROWIRE Bus Interface) 2048-Bit Serial EEPROM
with Data Protect and Sequential Read
5) Write Disable (WDS)
Write Disable (WDS) instruction disables all programming opera-
tions and is recommended to follow all programming operations.
Executing this instruction after a valid write instruction would
protect against accidental data disturb due to spurious noise,
glitches, inadvertent writes etc. Input information (Start bit, Opcode
and Address) for this WDS instruction should be issued as listed
under Table1. The device becomes write-disabled at the end of
this cycle when the CS signal is brought low.
Execution of a READ
instruction is independent of WDS instruction. Refer
Write Disable
cycle diagram.
Protect Register Instructions
Following five instructions, PRREAD, PREN, PRCLEAR,
PRWRITE and PRDS are specific to operations intended for
Protect Register. The PRE pin should be held high during these
instructions.
1) Protect Register Read (PRREAD)
This instruction reads the content of the internal Protect Register.
Content of this register is 8-bit wide and is the starting address of
the “write-protected” section of the memory array. All memory
locations greater than or equal to this address are write-protected.
Input information (Start bit, Opcode and Address) for this PRREAD
instruction should be issued as listed under Table 1. Upon
receiving a valid input information, decoding of the opcode and the
address is made, followed by data transfer (address information)
from the Protect Register. This 8-bit data is then shifted out on the
DO pin with the MSB first and the LSB last. Like the READ
instruction a dummy-bit (logical 0) precedes this 8-bit data output
string. Output data changes are initiated on the rising edge of the
SK clock. After reading the 8-bit data, the CS signal can be brought
low to end the PRREAD cycle. The PRE pin should be held high
during this cycle. Refer
Protect Register Read
cycle diagram.
Though the content of this register is 8-bit wide, only the last 7 bits
(LSB) are valid for FM93CS56 device.
2) Protect Register Enable (PREN)
This instruction is required to enable PRCLEAR, PRWRITE and
PRDS instructions and should be executed prior to executing
PRCLEAR, PRWRITE and PRDS instructions. However, this
PREN instruction is enabled (valid) only the following are true
■ Device is write-enabled (Refer WEN instruction)
■ PE pin is held high during this cycle
■ PRE pin is held high during this cycle
Input information (Start bit, Opcode and Address) for this PREN
instruction should be issued as listed under Table1. The Protect
Register becomes enabled for PRCLEAR, PRWRITE and PRDS
instructions at the end of this cycle when the CS signal is brought
low. Note that this PREN instruction must immediately precede
a PRCLEAR, PRWRITE or PRDS instruction. In other words, no
other instruction should be executed between a PREN instruction
and a PRCLEAR, PRWRITE or PRDS instruction. Refer
Protect
Register Enable
cycle diagram.
3) Protect Register Clear (PRCLEAR)
This instruction clears the content of the Protect register and
therefore enables write operations (WRITE or WRALL) to all
memory locations. Executing this instruction will program the
content of the Protect Register with a pattern of all 1s. However,
in this case, WRITE operation to the last memory address
(0x01111111) is still enabled. PRCLEAR instruction is enabled
(valid) only when the following are true:
■ PREN instruction was executed immediately prior to
PRCLEAR instruction
■ PE pin is held high during this cycle
■ PRE pin is held high during this cycle
Input information (Start bit, Opcode and Address) for this PRCLEAR
instruction should be issued as listed under Table1. After inputting
the last bit of address (A0 bit), CS signal must be brought low
before the next rising edge of the SK clock. This falling edge of the
CS initiates the self-timed clear cycle. It takes t
WP
time (Refer
appropriate DC and AC Electrical Characteristics table) for the
internal clear cycle to finish. During this time, the device remains
busy and is not ready for another instruction. Status of the internal
programming can be polled as described under WRITE instruction
description. While the device is busy, it is recommended that no
new instruction be issued. Refer
Protect Register Clear
cycle
diagram.
4) Protect Register Write (PRWRITE)
This instruction is used to write the starting address of the memory
section to be write-protected into the Protect register. After the
execution of PRWRITE instruction, all memory locations greater
than or equal to this address are write-protected. PRWRITE
instruction is enabled (valid) only the following are true:
■ PRCLEAR instruction was executed first (to clear the Protect
Register)
■ PREN instruction was executed immediately prior to
PRWRITE instruction
■ PE pin is held high during this cycle
■ PRE pin is held high during this cycle
Input information (Start bit, Opcode and Address) for this PRWRITE
instruction should be issued as listed under Table1. After inputting
the last bit of address (A0 bit), CS signal must be brought low
before the next rising edge of the SK clock. This falling edge of the
CS initiates the self-timed programming cycle. It takes t
WP
time
(Refer appropriate DC and AC Electrical Characteristics table) for
the internal programming cycle to finish. During this time, the
device remains busy and is not ready for another instruction.
Status of the internal programming can be polled as described
under WRITE instruction description. While the device is busy, it
is recommended that no new instruction be issued. Refer
Protect
Register Write
cycle diagram.
5) Protect Register Disable (PRDS)
Unlike all other instructions, this instruction is a one-time-only
instruction which when executed permanently write-protects
the Protect Register and renders it unalterable in the future. This
instruction is useful to safeguard vital data (typically read only
data) in the memory against any possible corruption. PRDS
instruction is enabled (valid) only the following are true:
■ PREN instruction was executed immediately prior to PRDS
instruction
■ PE pin is held high during this cycle
■ PRE pin is held high during this cycle
Input information (Start bit, Opcode and Address) for this PRDS
