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28C64A-15

64K (8K x 8) CMOS EEPROM

Microchip Technology 

28C64A

2.0 DEVICE OPERATION

The Microchip Technology Inc. 28C64A has four basic

modes of operation—read, standby, write inhibit, and

byte write—as outlined in the following table.

Operation

Mode

CE

OE

WE

I/O

Rdy/Busy

(1)

Read

L L H DOUT

H

Standby

H X X High Z

H

Write Inhibit H X X High Z

H

Write Inhibit X L X High Z

H

Write Inhibit X X H High Z

H

Byte Write L H L DIN

L

Byte Clear

Automatic Before Each “Write”

Note 1: Open drain output.

2: X = Any TTL level.

2.1 Read Mode

The 28C64A has two control functions, both of which

must be logically satisfied in order to obtain data at the

outputs. Chip enable (CE) is the power control and

should be used for device selection. Output Enable

(OE) is the output control and is used to gate data to

the output pins independent of device selection.

Assuming that addresses are stable, address access

time (tACC) is equal to the delay from CE to output

(tCE). Data is available at the output tOE after the fall-

ing edge of OE, assuming that CE has been low and

addresses have been stable for at least tACC-tOE.

2.2 Standby Mode

The 28C64A is placed in the standby mode by applying

a high signal to the CE input. When in the standby

mode, the outputs are in a high impedance state, inde-

pendent of the OE input.

2.3 Data Protection

In order to ensure data integrity, especially during criti-

cal power-up and power-down transitions, the following

enhanced data protection circuits are incorporated:

First, an internal VCC detect (3.3 volts typical) will inhibit

the initiation of non-volatile programming operation

when VCC is less than the VCC detect circuit trip.

Second, there is a WE filtering circuit that prevents WE

pulses of less than 10 ns duration from initiating a write

cycle.

Third, holding WE or CE high or OE low, inhibits a write

cycle during power-on and power-off (VCC).

2.4 Write Mode

The 28C64A has a write cycle similar to that of a Static

RAM. The write cycle is completely self-timed and ini-

tiated by a low going pulse on the WE pin. On the fall-

ing edge of WE, the address information is latched. On

rising edge, the data and the control pins (CE and OE)

are latched. The Ready/Busy pin goes to a logic low

level indicating that the 28C64A is in a write cycle which

signals the microprocessor host that the system bus is

free for other activity. When Ready/Busy goes back to

a high, the 28C64A has completed writing and is ready

to accept another cycle.

2.5 Data Polling

The 28C64A features Data polling to signal the comple-

tion of a byte write cycle. During a write cycle, an

attempted read of the last byte written results in the

data complement of I/O7 (I/O0 to I/O6 are indetermin-

able). After completion of the write cycle, true data is

available. Data polling allows a simple read/compare

operation to determine the status of the chip eliminat-

ing the need for external hardware.

2.6 Electronic Signature for Device

Identification

An extra row of 32 bytes of EEPROM memory is avail-

able to the user for device identification. By raising A9

to 12V ±0.5V and using address locations 1FEO to

1FFF, the additional bytes can be written to or read

from in the same manner as the regular memory array.

2.7 Chip Clear

All data may be cleared to 1's in a chip clear cycle by

raising OE to 12 volts and bringing the WE and CE low.

This procedure clears all data, except for the extra row.

DS11109H-page 6

© 1996 Microchip Technology Inc.

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