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FM93CS66 Rev. C.1
FM93CS66 (MICROWIRE Bus Interface) 4096-Bit Serial EEPROM
with Data Protect and Sequential Read
Functional Description
A typical Microwire cycle starts by first selecting the device
(bringing the CS signal high). Once the device is selected, a valid
Start bit (“1”) should be issued to properly recognize the cycle.
Following this, the 2-bit opcode of appropriate instruction should
be issued. After the opcode bits, the 8-bit address information
should be issued. For certain instructions, some (or all) of these
8 bits are don’t care values (can be “0” or “1”), but they should still
be issued. Following the address information, depending on the
instruction (WRITE and WRALL), 16-Bit data is issued. Other-
wise, depending on the instruction (READ and PRREAD), the
device starts to drive the output data on the DO line. Other
instructions perform certain control functions and do not deal with
data bits. The Microwire cycle ends when the CS signal is brought
low. However during certain instructions, falling edge of the CS
signal initiates an internal cycle (Programming), and the device
remains busy till the completion of the internal cycle. Each of the
10 instructions is explained in detail in the following sections.
Memory Instructions
Following five instructions, READ, WEN, WRITE, WRALL and
WDS are specific to operations intended for memory array. The
PRE pin should be held low during these instructions.
1) Read and Sequential Read (READ)
READ instruction allows data to be read from a selected location
in the memory array. Input information (Start bit, Opcode and
Address) for this instruction should be issued as listed under
Table1. Upon receiving a valid input information, decoding of the
opcode and the address is made, followed by data transfer from
the selected memory location into a 16-bit serial-out shift register.
This 16-bit data is then shifted out on the DO pin. D15 bit (MSB)
is shifted out first and D0 bit (LSB) is shifted out last. A dummy-bit
(logical 0) precedes this 16-bit data output string. Output data
changes are initiated on the rising edge of the SK clock. After
reading the 16-bit data, the CS signal can be brought low to end
the Read cycle. The PRE pin should be held low during this cycle.
Refer
Read cycle diagram
.
This device also offers “sequential memory read” operation to
allow reading of data from the additional memory locations instead
of just one location. It is started in the same manner as normal read
but the cycle is continued to read further data (instead of terminat-
ing after reading the first 16-bit data). After providing 16-bit data,
the device automatically increments the address pointer to the
next location and continues to provide the data from that location.
Any number of locations can be read out in this manner, however,
after reading out from the last location, the address pointer points
back to the first location. If the cycle is continued further, data will
be read from this first location onward. In this mode of read, the
dummy-bit is present only when the very first data is read (like
normal read cycle) and is not present on subsequent data reads.
The PRE pin should be held low during this cycle. Refer
Sequen-
tial Read cycle diagram
.
2) Write Enable (WEN)
When V
CC
is applied to the part, it “powers up” in the Write Disable
(WDS) state. Therefore, all programming operations (for both
memory array and Protect Register) must be preceded by a Write
Enable (WEN) instruction. Once a Write Enable instruction is
executed, programming remains enabled until a Write Disable
(WDS) instruction is executed or V
CC
is completely removed from
the part. Input information (Start bit, Opcode and Address) for this
WEN instruction should be issued as listed under Table1. The
device becomes write-enabled at the end of this cycle when the
CS signal is brought low. The PRE pin should be held low during
this cycle. Execution of a READ instruction is independent of WEN
instruction. Refer
Write Enable cycle diagram.
3) Write (WRITE)
WRITE instruction allows write operation to a specified location in
the memory with a specified data. This instruction is valid only
when the following are true:
■ Device is write-enabled (Refer WEN instruction)
■ Address of the write location is not write-protected
■ PE pin is held high during this cycle
■ PRE pin should be held low during this cycle
Input information (Start bit, Opcode, Address and Data) for this
WRITE instruction should be issued as listed under Table1. After
inputting the last bit of data (D0 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.
The status of the internal programming cycle can be polled at any
time by bringing the CS signal high again, after t
CS
interval. When
CS signal is high, the DO pin indicates the READY/BUSY status
of the chip. DO = logical 0 indicates that the programming is still
in progress. DO = logical 1 indicates that the programming is
finished and the device is ready for another instruction. It is not
required to provide the SK clock during this status polling. While
the device is busy, it is recommended that no new instruction be
issued. Refer
Write cycle diagram.
It is also recommended to follow this instruction (after the device
becomes READY) with a Write Disable (WDS) instruction to
safeguard data against corruption due to spurious noise, inadvert-
ent writes etc.
4) Write All (WRALL)
Write all (WRALL) instruction is similar to the Write instruction
except that WRALL instruction will simultaneously program all
memory locations with the data pattern specified in the instruction.
This instruction is valid only when the following are true:
■ Protect Register has been cleared (Refer PRCLEAR
instruction)
■ Device is write-enabled (Refer WEN instruction)
■ PE pin is held high during this cycle
■ PRE pin should be held low during this cycle
Input information (Start bit, Opcode, Address and Data) for this
WRALL instruction should be issued as listed under Table1. After
inputting the last bit of data (D0 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
Write All
cycle diagram.
