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 non-volatile write. As soon as

the 8th bit of the status register is read out, the

ST95P04 enters a wait mode (data on D are not

decoded, Q is in Hi-Z) until it is deselected.

The status register format is as follows:

b7

b0

1

1

1

1

BP1

BP0

WEL

WIP

BP1, BP0: Read and Write bits

WEL, WIP: Read only bits.

During a non-volatile write to the memory array, all

bits BP1, BP0, WEL, WIP are valid and can be read.

During a non volatile write to the status register, the

only bits WEL and WIP are valid and can be read.

The values of BP1 and BP0 read at that time

correspond to the previous contents of the status

register.

The Write-In-Process (WIP) read only bit indicates

whether the ST95P04 is busy with a write opera-

tion. When set to a ’1’ a write is in progress, when

set to a ’0’ no write is in progress.

The Write Enable Latch (WEL) read only bit indi-

cates the status of the write enable latch. When set

to a ’1’ the latch is set, when set to a ’0’ the latch is

reset.

The Block Protect (BP0 and BP1) bits indicate the

extent of the protection employed. These bits are

set by the user issuing the WRSR instruction.

These bits are non-volatile.

Write Status Register (WRSR)

The WRSR instruction allows the user to select the

size of protected memory. The ST95P04 is divided

into four 1024 bit blocks. The user may read the

blocks but will be unable to write within the selected

blocks.

The blocks and respective WRSR control bits are

shown in Table 6.

When the WRSR instruction and the 8 bits of the

Status Register are latched-in, the internal write

cycle is then triggered by the rising edge of S. This

rising edge of S must appear after the 8th bit of the

Status Register content (it must not appear a 17th

clock pulse before the rising edge of S), otherwise

the internal write sequence is not performed.

Read Operation

The chip is first selected by putting S low. The serial

one byte read instruction is followed by a one byte

address (A7-A0), each bit being latched-in during

the rising edge of the clock (C). Bit 3 of the read

instruction contains address A8 (most significant

address bit). This bit is used to select the first or

second page of the device. Then, the data stored

in the memory at the selected address is shifted out

on the Q output pin; each bit being shifted out

during the falling edge of the clock (C). The data

stored in the memory at the next address can be

read in sequence by continuing to provide clock

Status Register Bits

Array Addresses

Protected

BP1

BP0

0

0

none

0

1

180h - 1FFh

1

0

100h - 1FFh

1

1

000h - 1FFh

Table 6. Array Addresses Protect

Instruction

Description

Instruction Format

WREN

Set Write Enable Latch

0000 X110

WRDI

Reset Write Enable Latch

0000 X100

RDSR

Read Status Register

0000 X101

WRSR

Write Status Register

0000 X001

READ

Read Data from Memory Array

0000 A011

WRITE

Write Data to Memory Array

0000 A010

Table 7. Instruction Set

Notes: A = 1, Upper page selected

A = 0, Lower page selected

X = Don’t care

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ST95P04