M87C257-10C3TR Просмотр технического описания (PDF) - STMicroelectronics

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M87C257-10C3TR

Address Latched 256 Kbit (32Kb x8) UV EPROM and OTP EPROM

STMicroelectronics 

M87C257

2

Device operation

Device operation

The modes of operation of the M87C257 are listed in the Operating Modes. A single power

supply is required in the read mode. All inputs are TTL levels except for VPP and 12V on A9

for Electronic Signature.

2.1

Read mode

The M87C257 has two control functions, both of which must be logically active in order to

obtain data at the outputs. Chip Enable (E) is the power control and should be used for

device selection. Output Enable (G) is the output control and should be used to gate data to

the output pins, independent of device selection. Assuming that the addresses are stable

(AS = VIH) or latched (AS = VIL), the address access time (tAVQV) is equal to the delay from

E to output (tELQV). Data is available at the output after delay of tGLQV from the falling edge

of G, assuming that E has been low and the addresses have been stable for at least tAVQV-

tGLQV.The M87C257 reduces the hardware interface in multiplexed address-data bus

systems. The processor multiplexed bus (AD0-AD7) may be tied to the M87C257's address

and data pins. No separate address latch is needed because the M87C257 latches all

address inputs when AS is low.

2.2

Standby mode

The M87C257 has a standby mode which reduces the active current from 30mA to 100µA

(Address Stable). The M87C257 is placed in the standby mode by applying a CMOS high

signal to the E input. When in the standby mode, the outputs are in a high impedance state,

independent of the G input.

2.3

Two Line Output Control

Because EPROMs are usually used in larger memory arrays, this product features a 2 line

control function which accommodates the use of multiple memory connection. The two line

control function allows:

● the lowest possible memory power dissipation,

● complete assurance that output bus contention will not occur.

For the most efficient use of these two control lines, E should be decoded and used as the

primary device selecting function, while G should be made a common connection to all

devices in the array and connected to the READ line from the system control bus. This

ensures 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.

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