25AA640A(2008) データシートの表示（PDF） - Microchip Technology
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25AA640A Datasheet PDF : 30 Pages
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25AA640A/25LC640A
3.0 PIN DESCRIPTIONS
The descriptions of the pins are listed in Table 3-1.
TABLE 3-1: PIN FUNCTION TABLE
Name
Pin
X-Rotated
Number Pin Number
Function
CS
1
SO
2
WP
3
VSS
4
SI
5
SCK
6
HOLD
7
VCC
8
3
Chip Select Input
4
Serial Data Output
5
Write-Protect Pin
6
Ground
7
Serial Data Input
8
Serial Clock Input
1
Hold Input
2
Supply Voltage
3.1 Chip Select (CS)
A low level on this pin selects the device. A high level
deselects the device and forces it into Standby mode.
However, a programming cycle which is already
initiated or in progress will be completed, regardless of
the CS input signal. If CS is brought high during a
program cycle, the device will go into Standby mode as
soon as the programming cycle is complete. When the
device is deselected, SO goes to the high-impedance
state, allowing multiple parts to share the same SPI
bus. A low-to-high transition on CS after a valid write
sequence initiates an internal write cycle. After power-
up, a low level on CS is required prior to any sequence
being initiated.
3.2 Serial Output (SO)
The SO pin is used to transfer data out of the
25XX640A. During a read cycle, data is shifted out on
this pin after the falling edge of the serial clock.
3.3 Write-Protect (WP)
This pin is used in conjunction with the WPEN bit in the
STATUS register to prohibit writes to the nonvolatile
bits in the STATUS register. When WP is low and
WPEN is high, writing to the nonvolatile bits in the STA-
TUS register is disabled. All other operations function
normally. When WP is high, all functions, including
writes to the nonvolatile bits in the STATUS register
operate normally. If the WPEN bit is set, WP low during
a STATUS register write sequence will disable writing
to the STATUS register. If an internal write cycle has
already begun, WP going low will have no effect on the
write.
The WP pin function is blocked when the WPEN bit in
the STATUS register is low. This allows the user to
install the 25XX640A in a system with WP pin
grounded and still be able to write to the STATUS reg-
ister. The WP pin functions will be enabled when the
WPEN bit is set high.
3.4 Serial Input (SI)
The SI pin is used to transfer data into the device. It
receives instructions, addresses and data. Data is
latched on the rising edge of the serial clock.
3.5 Serial Clock (SCK)
The SCK is used to synchronize the communication
between a master and the 25XX640A. Instructions,
addresses or data present on the SI pin are latched on
the rising edge of the clock input, while data on the SO
pin is updated after the falling edge of the clock input.
3.6 Hold (HOLD)
The HOLD pin is used to suspend transmission to the
25XX640A while in the middle of a serial sequence
without having to retransmit the entire sequence again.
It must be held high any time this function is not being
used. Once the device is selected and a serial
sequence is underway, the HOLD pin may be pulled
low to pause further serial communication without
resetting the serial sequence. The HOLD pin must be
brought low while SCK is low, otherwise the HOLD
function will not be invoked until the next SCK high-to-
low transition. The 25XX640A must remain selected
during this sequence. The SI, SCK and SO pins are in
a high-impedance state during the time the device is
paused and transitions on these pins will be ignored. To
resume serial communication, HOLD must be brought
high while the SCK pin is low, otherwise serial
communication will not resume. Lowering the HOLD
line at any time will tri-state the SO line.
© 2008 Microchip Technology Inc.
DS21830D-page 13
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