© 1996 Microchip Technology Inc.

Preliminary

DS21147F-page 5

25C080/160

2.0

PRINCIPLES OF OPERATION

The 25C080/160 is an 1024/2048 byte EEPROM

designed to interface directly with the Serial Peripheral

Interface (SPI) port of many of today’s popular micro-

controller families, including Microchip’s midrange

PIC16CXX 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 soft-

ware.

The 25C080/160 contains an 8-bit instruction register.

The part 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. If the WPEN bit in the status register is set,

the WP pin must be held high to allow writing to the non-

volatile bits in the status register.

Table 2-1 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 25C080/160 in ‘HOLD’ mode. After

releasing the HOLD pin, operation will resume from the

point when the HOLD was asserted.

2.1

Write Enable (WREN) and Write

Disable (WRDI)

The 25C080/160 contains a write enable latch. This

latch must be set before any write operation will be

completed internally. The WREN instruction will set the

latch, and the WRDI will reset the latch. The following is

a list of conditions under which the write enable latch

will be reset:

• Power-up

• WRDI instruction successfully executed

• WRSR instruction successfully executed

• WRITE instruction successfully executed

2.2

Read Status Register (RDSR)

The RDSR instruction provides access to the status

register. The status register may be read at any time,

even during a write cycle. The status register is format-

ted as follows:

The Write-In-Process (WIP) bit indicates whether the

25C080/160 is busy with a write operation. When set to

a ‘1’ a write is in progress, when set to a ‘0’ no write is

in progress. This bit is read only.

The Write Enable Latch (WEL) bit indicates the status

of the write enable latch. When set to a ‘1’ the latch

allows writes to the array and status register, when set

to a ‘0’ the latch prohibits writes to the array and status

register. The state of this bit can always be updated via

7

654

3

2

1

0

WPEN

X

X

X

BP1

BP0

WEL

WIP

the WREN or WRDI commands regardless of the state

of write protection on the status register. This bit is read

only.

The Block Protection (BP0 and BP1) bits indicate

which blocks are currently write protected. These bits

are set by the user issuing the WRSR instruction.

These bits are non-volatile.

The Write Protect Enable (WPEN) bit is a non-volatile

bit that is available as an enable bit for the WP pin. The

Write Protect (WP) pin and the Write Protect Enable

(WPEN) bit in the status register control the program-

mable hardware write protect feature. Hardware write

protection is enabled when WP pin is low and the

WPEN bit is high. Hardware write protection is disabled

when either the WP pin is high or the WPEN bit is low.

When the chip is hardware write protected, only writes

to non-volatile bits in the status register are disabled.

See Table 2-2 for matrix of functionality on the WPEN

bit and Figure 2-1 for a flowchart of Table 2-2. See

Figure 3-5 for RDSR timing sequence.

TABLE 2-1:

INSTRUCTION SET

Instruction

Name

Instruction

Format

Description

WREN

0000 0110 Set the write enable latch

(enable write operations)

WRDI

0000 0100 Reset the write enable

latch (disable write opera-

tions)

RDSR

0000 0101 Read status register

WRSR

0000 0001 Write status register (write

protect enable and block

write protection bits)

READ

0000 0011 Read data from memory

array beginning at

selected address

WRITE

0000 0010 Write data to memory

array beginning at

selected address