W39L020

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The manufacturer and device codes may also be read via the command register, for instance, when

the W39L020 is erased or programmed in a system without access to high voltage on the A9 pin. The

command sequence is illustrated in "Auto-select Codes".

device identifier code (W39L020 = B5hex). All identifiers for manufacturer and device will exhibit odd

parity with DQ7 defined as the parity bit. In order to read the proper device codes when executing the

Auto-select, A1 must be low state.

Data Protection

The W39L020 is designed to offer protection against accidental erasure or programming caused by

spurious system level signals that may exist during power transitions. During power up the device

automatically resets the internal state machine in the Read mode. Also, with its control register

architecture, alteration of the memory contents only occurs after successful completion of specific

multi-bus cycle command sequences. The device also incorporates several features to prevent

Boot Block Operation

There are four alternatives to set the boot block. Either 16K-byte or 64K-byte in the top/bottom

location of this device can be locked as boot block, which can be used to store boot codes. It is

located in the last 16K/64K bytes or first 16K/64K bytes of the memory with the address range from

3C000/30000(hex) to 3FFFF(hex) for top location or 00000(hex) to 03FFF/0FFFF(hex) for bottom

location.

See Command Codes for Boot Block Lockout Enable for the specific code. Once this feature is set the

data for the designated block cannot be erased or programmed (programming lockout), other memory

locations can be changed by the regular programming method.

In order to detect whether the boot block feature is set on the first/last 16K/64K-bytes block or not,

users can perform software command sequence: enter the product identification mode (see

Command Codes for Identification/Boot Block Lockout Detection for specific code), and then read

from address 0002(hex) for first(bottom) location or 3FFF2(hex) for last(top) location.

If the

DQ0/DQ1 of output data is "1," the 64Kbytes/16Kbytes boot block programming lockout feature will be

activated; if the DQ0/DQ1 of output data is "0," the lockout feature will be inactivated and the block

can be erased/programmed.

To return to normal operation, perform a three-byte command sequence (or an alternate single-byte

command) to exit the identification mode. For the specific code, see Command Codes for

Identification/Boot Block Lockout Detection.

to prevent unintentional writes.

Write Pulse "Glitch" Protection

Noise pulses of less than 10 nS (typical) on #OE, #CE, or #WE will not initiate a write cycle.