© 2008 Microchip Technology Inc. DS21223H-page 7
25AA640/25LC640
3.0 FUNCTIONAL DESCRIPTION
3.1 Principles Of Operation
The 25XX640 is a 8192 byte Serial EEPROM designed
to interface directly with the Serial Peripheral Interface
(SPI) port of many of today’s popular microcontroller
families, including Microchip’s PIC16C6X/7X micro-
controllers. It may also interface with microcontrollers
that do not have a built-in SPI port by using discrete
I/O lines programmed properly with the software.
The 25XX640 contains an 8-bit instruction register. The
device is accessed via the SI pin, with data being
clocked in on the rising edge of SCK. The CS
pin must
be low and the HOLD
pin must be high for the entire
operation.
Table 3-1 contains a list of the possible instruction
bytes and format for device operation. All instructions,
addresses, and data are transferred MSB first, LSB
last.
Data is sampled on the first rising edge of SCK after CS
goes low. If the clock line is shared with other
peripheral devices on the SPI bus, the user can assert
the HOLD
input and place the 25XX640 in ‘HOLD’
mode. After releasing the HOLD
pin, operation will
resume from the point when the HOLD
was asserted.
3.2 Read Sequence
The device is selected by pulling CS low. The 8-bit
READ instruction is transmitted to the 25XX640 fol-
lowed by the 16-bit address with the three 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 pro-
vide clock pulses. The internal Address Pointer is auto-
matically incremented to the next higher address after
each byte of data is shifted out. When the highest
address is reached (1FFFh), the address counter rolls
over to address 0000h allowing the read cycle to be
continued indefinitely. The read operation is terminated
by raising the CS
pin (Figure 3-1).
3.3 Write Sequence
Prior to any attempt to write data to the 25XX640 array
or STATUS register, the write enable latch must be set
by issuing the WREN instruction (Figure 3-4). This is
done by setting CS
low and then clocking out the
proper instruction into the 25XX640. After all eight bits
of the instruction are transmitted, 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
proceed by setting the CS
low, issuing a WRITE
instruction, followed by the address, and then the data
to be written. Up to 32 bytes of data can be sent to the
25XX640 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 XXX0 0000
and ends with XXX1 1111. If the internal address
counter reaches 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.
For the data to be actually written to the array, the CS
must be brought high after the Least Significant bit (D0)
of the n
th
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 3-2 and Figure 3-3
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 (Figure 3-6). A read attempt of a
memory array location will not be possible during a
write cycle. When the write cycle is completed, the
write enable latch is reset.
TABLE 3-1: INSTRUCTION SET
Instruction Name Instruction Format Description
READ 0000 0011 Read data from memory array beginning at selected address
WRITE 0000 0010 Write data to memory array beginning at selected address
WREN 0000 0110 Set the write enable latch (enable write operations)
WRDI 0000 0100 Reset the write enable latch (disable write operations)
RDSR 0000 0101 Read STATUS register
WRSR 0000 0001 Write STATUS register
