25AA640/25LC640
DS21223H-page 6 © 2008 Microchip Technology Inc.
2.0 PIN DESCRIPTIONS
The descriptions of the pins are listed in Table 2-1.
TABLE 2-1: PIN FUNCTION TABLE
2.1 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, or remains
high during a program cycle, the device will go into
Standby mode when 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 high-to-low transition on CS
is
required prior to any sequence being initiated.
2.2 Serial Output (SO)
The SO pin is used to transfer data out of the 25XX640.
During a read cycle, data is shifted out on this pin after
the falling edge of the serial clock.
2.3 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 25XX640 in a system with WP pin grounded
and still be able to write to the STATUS register. The
WP
pin functions will be enabled when the WPEN bit is
set high.
2.4 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.
2.5 Serial Clock (SCK)
The SCK is used to synchronize the communication
between a master and the 25XX640. 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.
2.6 Hold (HOLD)
The HOLD pin is used to suspend transmission to the
25XX640 while in the middle of a serial sequence with-
out having to retransmit the entire sequence over
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 25XX640 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.
Name PDIP SOIC TSSOP Description
CS 1 1 3 Chip Select Input
SO 2 2 4 Serial Data Output
WP
3 3 5 Write-Protect Pin
V
SS 44 6Ground
SI 5 5 7 Serial Data Input
SCK 6 6 8 Serial Clock Input
HOLD
7 7 1 Hold Input
V
CC 8 8 2 Supply Voltage
