M28256 datasheet(4/21 Pages) STMICROELECTRONICS

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Mode

DQ0 - DQ7

Read

VIL

VIH

Data Out

Write

VIL

VIH

VIL

Data In

Standby / Write Inhibit

VIH

Hi-Z

Write Inhibit

VIH

Data Out or Hi-Z

Write Inhibit

VIL

Data Out or Hi-Z

Output Disable

VIH

Hi-Z

Notes: 1. X = VIH or VIL.

Table 3. Operating Modes (1)

The devices offer fast access time with low power

dissipation and requires a 5V or 3V power supply.

The circuit has been designed to offer a flexible

microcontroller interface featuring both hardware

and software handshaking with Data Polling and

Toggle Bit and access to a status register. The

devices support a 64 byte page write operation. A

Software Data Protection (SDP) is also possible

using the standard JEDEC algorithm.

PIN DESCRIPTION

Addresses (A0-A14). The address inputs select

an 8-bit memory location during a read or write

operation.

Chip Enable (E). The chip enable input must be

low to enable all read/write operations. When Chip

Enable is high, power consumption is reduced.

Output Enable (G). The Output Enable input con-

trols the data output buffers and is used to initiate

read operations.

Data In/ Out (DQ0- DQ7). Data is written to or read

from the memory through the I/O pins.

Write Enable (W). The Write Enable input controls

the writing of data to the memory.

OPERATIONS

Write Protection

In order to prevent data corruption and inadvertent

write operations; an internal VCC comparator inhib-

its Write operations if VCC is below VWI (see Table

7 andTable 9).Access to the memory in write mode

is allowed after a power-up as specified in Table 7

and Table 9.

Read

The device is accessed like a static RAM. When E

and G are low with W high, the data addressed is

presented on the I/O pins. The I/O pins are high

impedance when either G or E is high.

Write

Write operations are initiated when both W and E

are low and G is high.The device supports both E

and W controlled write cycles. The Address is

latched by the falling edge of E or W which ever

occurs last and the Data on the rising edge of E or

W which ever occurs first. Once initiated the write

operation is internally timed until completion and

the status of the Data Polling and the Toggle Bit

functions on DQ7 and DQ6 is controlled accord-

ingly.

Page Write

Page write allows up to 64 bytes within the same

page to be consecutively latched into the memory

prior to initiating a programming cycle. All bytes

must be located in a single page address, that is

A14-A6 must be the same for all bytes; if not, the

Page Write instruction is not executed. The page

write can be initiated by any byte write operation.

A page write is composed of successive Write

instructions which have to be sequenced with a

specific period of time between two consecutive

Write instructions, period of time which has to be

smaller than the tWHWH value (see Table 12 and

Table 13).

If this period of time exceeds the tWHWH value, the

internal programmingcycle will start. Once initiated

the write operation is internally timed until comple-

tion and the status of the Data Polling and the

Toggle Bit functions on DQ7 and DQ6 is controlled

accordingly.

DESCRIPTION (Cont’d)

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