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APPLICATION NOTE
A V A I LABLE
X25F128AN61 • AN75 • AN77 • AN79 • AN82
X25F128
16K x 8 Bits
SerialFlash™ Memory With Block Lock™ Protection
FEATURES
• 1MHz Clock Rate
• SPI Serial Interface
• 16K X 8 Bits
— 32 Byte Small Sector Program Mode
• Low Power CMOS
— <1µA Standby Current
— <5mA Active Current
• 1.8V – 3.6V or 5V “Univolt” Read and
Program Power Supply Versions
• Block Lock Protection
— Protect 1/4, 1/2 or all of E2PROM Array
• Built-in Inadvertent Program Protection
— Power-Up/Power-Down protection circuitry
— Program Enable Latch
— Program Protect Pin
• Self-Timed Program Cycle
— 5ms Program Cycle Time (Maximum)
• High Reliability
— Endurance: 100,000 cycles per byte
— Data Retention: 100 Years
— ESD protection: 2000V on all pins
• 8-Lead PDlP Package
• 16-Lead 150 mil SOIC Package
DESCRIPTION
The X25F128 is a 131,072-bit CMOS SerialFlash
memory, internally organized 16K X 8. It features a
“Univolt” Program and Read voltage, Serial Peripheral
Interface (SPI), and software protocol allowing operation
on a simple three-wire bus. The bus signals are a clock
input (SCK), plus separate data in (SI) and data out (SO)
lines. Access to the device is controlled through a chip
select (CS) input, allowing any number of devices to
share the same bus.
The X25F128 also features two additional inputs that
provide the end user with added flexibility. By asserting
the HOLD input, the X25F128 will ignore transitions on
its inputs, thus allowing the host to service higher priority
interrupts. The PP input can be used as a hardwire input
to the X25F128 disabling all program attempts to the
status register, thus providing a mechanism for limiting
end user capability of altering 0, 1/4, 1/2, or all of the
memory.
The X25F128 utilizes Xicor’s proprietary flash cell, pro-
viding a minimum endurance of 100,000 cycles and a
minimum data retention of 100 years.
FUNCTIONAL DIAGRAM
SI
SO
SCK
CS
HOLD
COMMAND
DECODE
AND CONTROL
LOGIC
X
DECODE
LOGIC
STATUS
REGISTER
DATA REGISTER
SECTOR DECODE LOGIC
32 8
MEMORY
ARRAY
PP
PROGRAMMING
CONTROL LOGIC
SerialFlash™ and Block Lock™ Protection are trademarks of Xicor, Inc.
© Xicor, Inc. 1995, 1996 Patents Pending
6829-1.9 4/7/97 T3/C0/D0 SH
1
HIGH VOLTAGE
CONTROL
6829 ILL F01.1
Characteristics subject to change without notice

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X25F128
PIN DESCRIPTIONS
Serial Output (SO)
SO is a push-pull serial data output pin. During a read
cycle, data is shifted out on this pin. Data is clocked out
by the falling edge of the serial clock.
Serial Input (SI)
SI is the serial data input pin. All opcodes, byte
addresses, and data to be written to the memory are
input on this pin. Data is latched by the rising edge of the
serial clock.
Serial Clock (SCK)
The Serial Clock controls the serial bus timing for data
input and output. Opcodes, addresses, or data present
on the SI pin are latched on the rising edge of the clock
input, while data on the SO pin change after the falling
edge of the clock input.
Chip Select (CS)
When CS is HIGH, the X25F128 is deselected and the
SO output pin is at high impedance and unless an
internal program operation is underway the X25F128
will be in the standby power mode. CS LOW enables
the X25F128, placing it in the active power mode. It
should be noted that after power-up, a HIGH to LOW
transition on CS is required prior to the start of any
operation.
Program Protect (PP)
When PP is LOW and the nonvolatile bit PPEN is “1”,
nonvolatile programming of the X25F128 status register
is disabled, but the part otherwise functions normally.
When PP is held HIGH, all functions, including nonvola-
tile programming operate normally. PP going LOW while
CS is still LOW will interrupt programming of the
X25F128 status register. If the internal program cycle
has already been initiated, PP going LOW will have no
effect on programming.
The PP pin function is blocked when the PPEN bit in
the status register is “0”. This allows the user to install the
X25F128 into a system with PP pin grounded and still
be able to program the status register. The PP pin
functions will be enabled when the PPEN bit is set “0”.
Hold (HOLD)
HOLD is used in conjunction with the CS pin to select the
device. Once the part is selected and a serial sequence
is underway, HOLD may be used to pause the serial
communication with the controller without resetting the
serial sequence. To pause, HOLD must be brought
LOW while SCK is LOW. To resume communication,
HOLD is brought HIGH, again while SCK is LOW. If the
pause feature is not used, HOLD should be held HIGH
at all times.
PIN CONFIGURATION
CS
SO
PP
VSS
8-LEAD DIP
18
27
X25F128
36
45
VCC
HOLD
SCK
SI
CS
SO
NC
NC
NC
NC
PP
VSS
16-LEAD SOIC
1 16
2 15
3 14
4 13
X25F128
5 12
6 11
7 10
89
VCC
HOLD
NC
NC
NC
NC
SCK
SI
6829 ILL F02.1
PIN NAMES
SYMBOL
CS
SO
SI
SCK
PP
VSS
VCC
HOLD
NC
DESCRIPTION
Chip Select Input
Serial Output
Serial Input
Serial Clock Input
Program Protect Input
Ground
Supply Voltage
Hold Input
No Connect
6829 PGM T01
2

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X25F128
PRINCIPLES OF OPERATION
The X25F128 is a SerialFlash Memory designed
to interface directly with the synchronous serial periph-
eral interface (SPI) of many popular microcontroller
families.
The X25F128 contains an 8-bit instruction register. It is
accessed via the SI input, with data being clocked in on
the rising SCK. CS must be LOW and the HOLD and PP
inputs must be HIGH during the entire operation. The PP
input is “Don’t Care” if PPEN is set “0”.
Table 1 contains a list of the instructions and their
operation codes. All instructions, addresses and data
are transferred MSB first.
Data input is sampled on the first rising edge of SCK after
CS goes LOW. SCK is static, allowing the user to stop
the clock and then resume operations. If the clock line is
shared with other peripheral devices on the SPI bus, the
user can assert the HOLD input to place the X25F128
into a “PAUSE” condition. After releasing HOLD, the
X25F128 will resume operation from the point when
HOLD was first asserted.
Program Enable Latch
The X25F128 contains a program enable latch. This
latch must be SET before a program operation will be
completed internally. The PREN instruction will set the
latch and the PRDI instruction will reset the latch. This
latch is automatically reset on power-up and after the
completion of a sector program or status register write
cycle.
Status Register
The RDSR instruction provides access to the status
register. The status register may be read at any time,
even during a program cycle. The status register is
formatted as follows:
7 654 3
2 10
PPEN X X X BL1 BL0 PEL PIP
6829 PGM T02
PPEN, BL0, and BL1 are set by the PRSR instruction.
PEL and PIP are “read-only” and automatically set by
other operations.
The Programming-In-Process (PIP) bit indicates
whether the X25F128 is busy with a program operation.
When set to a “1” programming is in progress, when
set to a “0” no programming is in progress. During
programming, all other bits are set to “1”.
The Program Enable Latch (PEL) bit indicates the
status of the program enable latch. When set to a “1” the
latch is set; when set to a “0” the latch is reset.
The Block Lock (BL0 and BL1) bits are nonvolatile and
allow the user to select one of four levels of protection.
The X25F128 array is divided into four equal segments.
One, two, or all four of the segments may be locked. That
is, the user may read the segments, but will be unable to
alter (program) data within the selected segments. The
partitioning is controlled as illustrated below.
Status Register Bits
BL1 BL0
Array Addresses
Locked
00
01
10
11
Program-Protect Enable
None
upper fourth
upper half
All
6829 PGM T03.1
The Program-Protect-Enable bit (PPEN) in the
X25F128 status register acts as an enable bit for the
PP pin.
Table 1. Instruction Set
Instruction Name Instruction Format*
Operation
PREN
0000 0110
Set the Program Enable Latch (Enable Program Operations)
PRDI
0000 0100
Reset the Program Enable Latch (Disable Program Operations)
RDSR
0000 0101
Read Status Register
PRSR
0000 0001
Program Status Register
READ
0000 0011
Read from Memory Array beginning at Selected Address
PROGRAM
0000 0010
Program Memory Array beginning at Selected Address
(32 Bytes)
*Instructions are shown MSB in leftmost position. Instructions are transferred MSB first.
6829 PGM T04.1
3

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X25F128
Locked Unlocked
PPEN PP PEL Blocks Blocks
Status
Register
0 X 0 Locked Locked
Locked
0 X 1 Locked Programmable Programmable
1 LOW 0 Locked Locked
Locked
1 LOW 1 Locked Programmable Locked
X HIGH 0 Locked Locked
Locked
X HIGH 1 Locked Programmable Programmable
6829 PGM T05
The Program Protect (PP) pin and the nonvolatile
Program Protect Enable (PPEN) bit in the Status Reg-
ister control the programmable hardware write protect
feature. Hardware program protection is enabled when
PP pin is LOW, and the PPEN bit is “1”. Hardware
program protection is disabled when either the PP pin is
HIGH or the PPEN bit is “0”. When the chip is hardware
program protected, nonvolatile programming of the Sta-
tus Register in disabled, including the Block Lock bits
and the PPEN bit itself, as well as the Block Lock
sections in the memory array. Only the sections of the
memory array that are not Block Locked can be pro-
grammed.
Note: Since the PPEN bit is program protected, it
cannot be changed back to a “0”, as long as
the PP pin is held LOW.
Clock and Data Timing
Data input on the SI line is latched on the rising edge of
SCK. Data is output on the SO line by the falling edge of
SCK.
Read Sequence
When reading from the SerialFlash memory array, CS is
first pulled LOW to select the device. The 8-bit READ
instruction is transmitted to the X25F128, followed by
the 16-bit address. After the read opcode and address
are sent, the data stored in the memory at the selected
address is shifted out on the SO line. The data stored in
memory at the next address can be read sequentially by
continuing to provide clock pulses. The address is
automatically incremented to the next higher address
after each byte of data is shifted out. When the highest
address is reached the address counter rolls over to
address $0000, allowing the read cycle to be continued
indefinitely. The read operation is terminated by taking
CS HIGH. Refer to the Read SerialFlash Memory Array
Operation Sequence illustrated in Figure 1.
To read the status register, the CS line is first pulled
LOW to select the device followed by the 8-bit instruc-
tion. After the read status register opcode is sent, the
contents of the status register are shifted out on the SO
line. The Read Status Register Sequence is illustrated
in Figure 2.
Programming Sequence
Prior to any attempt to program the X25F128, the
program enable latch must first be set by issuing the
PREN instruction (See Figure 3). CS is first taken LOW,
then the PREN instruction is clocked into the X25F128.
After all eight bits of the instruction are transmitted, CS
must then be taken HIGH. If the user continues the
programming operation without taking CS HIGH after
issuing the PREN instruction, the programming opera-
tion will be ignored.
To program the SerialFlash memory array, the user
issues the PROGRAM instruction, followed by the ad-
dress of the first location in the sector and then the data
to be programmed. The data is programmed in a 256-
clock operation. CS must go LOW and remain LOW for
the duration of the operation. The 32 bytes must reside
in the same sector and cannot cross sector boundaries.
If the address counter reaches the end of the sector
and the clock continues, or if fewer than 32 bytes are
clocked in, the contents of the sector cannot be guaranteed.
For the program operation to be completed, CS can only
be brought HIGH after bit 0 of data byte 32 is clocked in.
If it is brought HIGH at any other time, the program
operation will not be completed. Refer to Figure 4 below
for a detailed illustration of the programming sequence
and time frames in which CS going HIGH is valid.
To program the status register, the PRSR instruction is
followed by the data to be programmed. Data bits 0, 1,
4, 5 and 6 must be “0”. This sequence is shown in Figure 5.
While the program cycle is in progress, following a
status register or memory write sequence, the status
register may be read to check the PIP bit. During this
time the PIP bit will be HIGH.
Hold Operation
The HOLD input should be HIGH (at VIH) under normal
operation. If a data transfer is to be interrupted HOLD
can be pulled LOW to suspend the transfer until it can
be resumed. The only restriction is that the SCK input
must be LOW when HOLD is first pulled LOW and SCK
must also be LOW when HOLD is released.
The HOLD input may be tied HIGH either directly to VCC
or tied to VCC through a resistor.
4

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X25F128
Operational Notes
The device powers-up in the following state:
• The device is in the low power standby state.
• A HIGH to LOW transition on CS is required to
enter an active state and receive an instruction.
• SO pin is high impedance.
• The program enable latch is reset.
Data Protection
The following circuitry has been included to prevent
inadvertent programming:
• The program enable latch is reset upon power-up.
• A program enable instruction must be issued to set
the program enable latch.
CS must come HIGH at the proper clock count in
order to start a program cycle.
Figure 1. Read SerialFlash Memory Array Operation Sequence
CS
SCK
0 1 2 3 4 5 6 7 8 9 10
20 21 22 23 24 25 26 27 28 29 30
INSTRUCTION
16 BIT ADDRESS
SI
15 14 13
3210
HIGH IMPEDANCE
SO
76
MSB
DATA OUT
543210
6829 ILL F03
Figure 2. Read Status Register Operation Sequence
CS
SCK
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
INSTRUCTION
SI
HIGH IMPEDANCE
SO
76
MSB
DATA OUT
543210
6829 ILL F04
5