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 (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 X25640 is deselected and the SO
output pin is at high impedance and unless an internal
write operation is underway, the X25640 will be in the
standby power mode. CS LOW enables the X25640,
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.
Write Protect (WP)
When WP is LOW and the nonvolatile bit WPEN is high,
nonvolatile writes to the X25640 status register are
disabled, but the part otherwise functions normally.
When WP is held HIGH, all functions, including nonvola-
tile writes operate normally. WP going LOW while CS is
still LOW will interrupt a write to the X25640 status
register. If the internal write cycle has already been
initiated, WP going LOW will have no affect on a write.
The WP pin function is blocked when the WPEN bit in
the status register is LOW. This allows the user to install
the X25640 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 “1”.
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 com-
munication with the controller without resetting the serial
sequence. To pause, HOLD must be brought LOW while
SCK is LOW. To resume communication, HOLDis brought
HIGH, again while SCK is LOW. If the pause feature is not
used, HOLD should be held HIGH at all times.
4 X25640 11
3089 ILL F02.3
Chip Select Input
Serial Clock Input
Write Protect Input
3089 PGM T01