Rainbow Electronics W29C020C - Manual (Page 3)

W29C020C
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FUNCTIONAL DESCRIPTION
Read Mode
The read operation of the W29C020C is controlled by CE and OE , both of which have to be low for the
host to obtain data from the outputs. CE is used for device selection. When CE is high, the chip is de-
selected and only standby power will be consumed. OE is the output control and is used to gate data
from the output pins. The data bus is in high impedance state when either CE or OE is high.
Refer to the read cycle timing waveforms for further details.
Page Write Mode
The W29C020C is written (erased/programmed) on a page basis. Every page contains 128 bytes of data.
If a byte of data within a page is to be changed, data for the entire page must be loaded into the device.
Any byte that is not loaded will be erased to "FF hex" during the write operation of the page.
The write operation is initiated by forcing CE and WE low and OE high. The write procedure consists
of two steps. Step 1 is the byte-load cycle, in which the host writes to the page buffer of the device.
Step 2 is an internal write (erase/program) cycle, during which the data in the page buffers are
simultaneously written into the memory array for non-volatile storage.
During the byte-load cycle, the addresses are latched by the falling edge of either CE or WE ,
whichever occurs last. The data are latched by the rising edge of either CE or WE , whichever occurs
first. If the host loads a second byte into the page buffer within a byte-load cycle time (T
BLC ) of 200
µ
S
after the initial byte-load cycle, the W29C020C will stay in the page load cycle. Additional bytes can
then be loaded consecutively. The page load cycle will be terminated and the internal write
(erase/program) cycle will start if no additional byte is loaded into the page buffer A7 to A17 specify the
page address. All bytes that are loaded into the page buffer must have the same page address. A0 to A6
specify the byte address within the page. The bytes may be loaded in any order; sequential loading is
not required.
In the internal write cycle, all data in the page buffers, i.e., 128 bytes of data, are written simultaneously
into the memory array. Before the completion of the internal write cycle, the host is free to perform other
tasks such as fetching data from other locations in the system to prepare to write the next page.
Software-protected Data Write
The device provides a JEDEC-approved optional software-protected data write. Once this scheme is
enabled, any write operation requires a three-byte command sequence (with specific data to a specific
address) to be performed before the data load operation. The three-byte load command sequence begins
the page load cycle, without which the write operation will not be activated. This write scheme provides
optimal protection against inadvertent write cycles, such as cycles triggered by noise during system
power-up and power-down.
The W29C020C is shipped with the software data protection enabled. To enable the software data
protection scheme, perform the three-byte command cycle at the beginning of a page load cycle. The
device will then enter the software data protection mode, and any subsequent write operation must be
preceded by the three-byte command sequence cycle. Once enabled, the software data protection will
remain enabled unless the disable commands are issued. A power transition will not reset the software
data protection feature. To reset the device to unprotected mode, a six-byte command sequence is
required. For information about specific codes, see the Command Codes for Software Data Protection in
the Table of Operating Modes. For information about timing waveforms, see the timing diagrams below.