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24LC320-P Datasheet PDF : 12 Pages
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3.0 DEVICE OPERATION
3.1 Clock and Data Timing
Data input on the SI pin is latched on the rising edge of
SCK. Data is output on the SO pin after the falling edge
of SCK.
3.2 Read Sequence
The part is selected by pulling CS low. The 8-bit read
instruction is transmitted to the 25LC320 followed by
the 16-bit address, with the four MSBs of the address
being don’t care bits. After the correct read instruction
and address are sent, the data stored in the memory at
the selected address is shifted out on the SO pin. The
data stored in the memory at the next address can be
read sequentially by continuing to provide clock pulses.
The internal address pointer is automatically incre-
mented to the next higher address after each byte of
data is shifted out. When the highest address is
reached (0FFFh) the address counter rolls over to
address 0000h allowing the read cycle to be continued
indefinitely. The read operation is terminated by setting
CS high (Figure 3-1).
3.3 Write Sequence
Prior to any attempt to write data to the 25LC320, the
write enable latch must be set by issuing the WREN
instruction (Figure 3-2). This is done by setting CS low
and then clocking the proper instruction into the
25LC320. After all eight bits of the instruction are trans-
mitted, the CS must be brought high to set the write
enable latch. If the write operation is initiated
immediately after the WREN instruction without CS
being brought high, the data will not be written to the
array because the write enable latch will not have been
properly set.
Once the write enable latch is set, the user may pro-
ceed by setting the CS low, issuing a write instruction,
followed by the 16-bit address, with the four MSBs of
the address being don’t care bits, and then the data to
be written. Up to 32 bytes of data can be sent to the
25LC320 before a write cycle is necessary. The only
restriction is that all of the bytes must reside in the
same page. A page address begins with XXXX XXXX
XXX0 0000 and ends with XXXX XXXX XXX1 1111. If
the internal address counter reaches XXXX XXXX
XXX1 1111 and the clock continues, the counter will roll
back to the first address of the page and overwrite any
data in the page that may have been written.
25LC320
For the data to be actually written to the array, the CS
must be brought high after the least significant bit (D0)
of the nth data byte has been clocked in. If CS is brought
high at any other time, the write operation will not be
completed. Refer to Figure 3-3 and Figure 3-4 for more
detailed illustrations on the byte write sequence and the
page write sequence respectively.
While the write is in progress, the status register may
be read to check the status of the WPEN, WIP, WEL,
BP1, and BP0 bits. A read attempt of a memory array
location will not be possible during a write cycle. When
a write cycle is completed, the write enable latch is
reset.
3.4 Data Protection
The following protection has been implemented to
prevent inadvertent writes to the array:
• The write enable latch is reset on power-up.
• A write enable instruction must be issued to set
the write enable latch.
• After a successful byte write, page write, or status
register write, the write enable latch is reset.
• CS must be set high after the proper number of
clock cycles to start an internal write cycle.
• Access to the array during an internal write cycle
is ignored and programming is continued.
3.5 Power On State
The 25LC320 powers on in the following state:
• The device is in low power standby mode.
• The write enable latch is reset.
• SO is in high impedance state.
• A low level on CS is required to enter active state.
© 1996 Microchip Technology Inc.
Preliminary
DS21158B-page 7
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