28F256 datasheet(9/35 Pages) AMD

9 / 35 page

Am28F256

READ ONLY MODE

When VPP is less than VCC + 2 V, the command register

is inactive. The device can either read array or autose-

lect data, or be standby mode.

Read

The device functions as a read only memory when VPP

< VCC + 2 V. The device has two control functions. Both

must be satisfied in order to output data. CE# controls

power to the device. This pin should be used for spe-

cific device selection. OE# controls the device outputs

and should be used to gate data to the output pins if a

device is selected.

Address access time tACC is equal to the delay from

stable addresses to valid output data. The chip enable

access time tCE is the delay from stable addresses and

stable CE# to valid data at the output pins. The output

enable access time is the delay from the falling edge of

OE# to valid data at the output pins (assuming the ad-

dresses have been stable at least tACC–tOE).

Standby Mode

The device has two standby modes. The CMOS

standby mode (CE# input held at VCC ± 0.5 V), con-

sumes less than 100 µA of current. TTL standby mode

(CE# is held at VIH) reduces the current requirements

to less than 1mA. When in the standby mode the out-

puts are in a high impedance state, independent of the

OE# input.

If the device is deselected during erasure, program-

ming, or program/erase verification, the device will

draw active current until the operation is terminated.

Output Disable

Output from the device is disabled when OE# is at a

logic high level. When disabled, output pins are in a

high impedance state.

Auto Select

Flash memories can be programmed in-system or in a

standard PROM programmer. The device may be sol-

dered to the circuit board upon receipt of shipment and

programmed in-system. Alternatively, the device may

initially be programmed in a PROM programmer prior

to soldering the device to the board.

The Auto select mode allows the reading out of a binary

code from the device that will identify its manufacturer

and type. This mode is intended for the purpose

of automatically matching the device to be pro-

grammed with its corresponding programming algo-

r ith m. Th is mo de is f unc tio nal ove r t he en tir e

temperature range of the device.

Programming In A PROM Programmer

To activate this mode, the programming equipment

must force VID (11.5 V to 13.0 V) on address A9. Two

identifier bytes may then be sequenced from the device

outputs by toggling address A0 from VIL to VIH. All other

address lines must be held at VIL, and VPP must be

less than or equal to VCC + 2.0 V while using this Auto

select mode. Byte 0 (A0 = VIL) represents the manufac-

turer code and byte 1 (A0 = VIH) the device identifier

code. For the device these two bytes are given in Table

2 below. All identifiers for manufacturer and device

codes will exhibit odd parity with the MSB (DQ7) de-

fined as the parity bit.

Table 2.

Am28F256 Auto Select Code

Type

Code

(HEX)

Manufacturer Code

VIL

Device Code

VIH

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