HT27LC512 datasheet(7/10 Pages) HOLTEK

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Auto product identification

The Auto Product Identification mode allows

the reading out of a binary code from an

EPROM that will identify its manufacturer and

type. This mode is intended for use by the pro-

gramming equipment for the purpose of auto-

m atically

matching

the

device

programmed with its corresponding program-

ming algorithm. This mode is functional in the

°C±5°C ambient temperature range that is

required when programming the HT27LC512.

To activate this mode, the programming equip-

ment must force 12.0

±0.5V on the address line A9

of the HT27LC512. Two identifier bytes may then

be sequenced from the device outputs by toggling

address line A0 from VIL to VIH, when A1=VIH. All

other address lines must be held at VIH during

Auto Product Identification mode.

Byte 0 (A0=VIL) represents the manufacturer

code, and byte 1 (A0=VIH), the device code. For

HT27LC512, these two identifier bytes are shown

in the Mode Select Table. All identifiers for the

manufacturer and device codes will possess odd

parity, with the MSB (DQ7) defined as the parity

bit. When A1=VIL, the HT27LC512 will read out

the binary code of 7F, continuation code, to signify

the unavailability of manufacturer ID codes.

Read mode

The HT27LC512 has two control functions,

both of which must be logically satisfied in or-

der to obtain data at outputs. Chip Enable (CE)

is the power control and should be used for

device selection. Output Enable (OE) is the out-

put control and should be used to gate data to

the output pins, independent of device selec-

tion. Assuming that addresses are stable, ad-

dress access time (tACC) is equal to the delay

from CE to output (tCE). Data is available at the

outputs (tOE) after the falling edge of OE, as-

suming the CE has been LOW and addresses

have been stable for at least tACC-tOE.

Standby mode

The HT27LC512 has CMOS standby mode

which reduces the maximum VCC current to

µA. It is placed in CMOS standby when CE is

at VCC

±0.3V. The HT27LC512 also has a TTL-

standby mode which reduces the maximum

VCC current to 0.6mA. It is placed in TTL-

standby when CE is at VIH. When in standby

mode, the outputs are in a high-impedance

state, independent of the OE input.

Two-line output control function

To accommodate multiple memory connections,

a two-line control function is provided to allow

for:

• Low memory power consumption

• Assurance that output bus contention will not

occur.

It is recommended that CE be decoded and used

as the primary device-selection function, while

OE be made a common connection to all devices

in the array and connected to the READ line

from the system control bus. This assures that

all deselected memory devices are in their low-

power standby mode and that the output pins

are only active when data is desired from a

particular memory device.

System considerations

During the switch between active and standby

conditions, transient current peaks are pro-

duced on the rising and falling edges of Chip

Enable. The magnitude of these transient cur-

rent peaks is dependent on the output capaci-

tance loading of the device. At a minimum, a

0.1

µF ceramic capacitor (high frequency, low

inherent inductance) should be used on each

device between VCC and VPP to minimize tran-

sient effects. In addition, to overcome the volt-

age drop caused by the inductive effects of the

printed circuit board traces on EPROM arrays,

a 4.7

µF bulk electrolytic capacitor should be

used between VCC and VPP for each eight de-

vices. The location of the capacitor should be

close to where the power supply is connected to

the array.

HT27LC512

6th May ’99

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