© 2008 Microchip Technology Inc. DS21227F-page 7
25AA320/25LC320/25C320
3.0 FUNCTIONAL DESCRIPTION
3.1 Principles Of Operation
The 25XX320 are 4096 byte Serial EEPROMs
designed to interface directly with the Serial Peripheral
Interface (SPI) port of many of today’s popular
microcontroller families, including Microchip’s
PIC16C6X/7X microcontrollers. 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 25XX320 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 25XX320 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 25XX320 fol-
lowed 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 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 (0FFFh), 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 25XX320, 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
25XX320. 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 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 25XX320 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 over-
write 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
WRDI
0000 0100
Reset the write enable latch (disable write operations)
WREN
0000 0110
Set the write enable latch (enable write operations)
RDSR
0000 0101
Read STATUS register
WRSR
0000 0001
Write STATUS register
