AT89LP428 datasheet(12/149 Pages) ATMEL

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3654A–MICRO–8/09

AT89LP428/828

3.3.2

FDATA

The Flash data memory is a portion of the external memory space implemented as an internal

nonvolatile data memory. Flash data memory is enabled by setting the DMEN bit (MEMCON.3)

to one. When IAP = 0 and DMEN = 1, the Flash data memory is mapped into the FDATA space,

directly above the EDATA space near the bottom of the external memory address space.

(Addresses 0200H–03FFH on AT89LP428 and 0200H–05FFH on AT89LP828. See Figure 3-3

on page 11). MOVX instructions to this address range will access the internal nonvolatile mem-

ory. FDATA is not accessible while DMEN = 0. FDATA can be accessed only by 16-bit

(MOVX @DPTR) addresses. Addresses above the FDATA range are not implemented and

should not be accessed. MOVX instructions to FDATA require a minimum of 4 clock cycles.

3.3.2.1

Write Protocol

The FDATA address space accesses an internal nonvolatile data memory. This address space

can be read just like EDATA by issuing a MOVX A,@DPTR; however, writes to FDATA require a

more complex protocol and take several milliseconds to complete. The AT89LP428/828 uses an

idle-while-write architecture where the CPU is placed in an idle state while the write occurs.

When the write completes, the CPU will continue executing with the instruction after the

MOVX @DPTR,A instruction that started the write. All peripherals will continue to function during

the write cycle; however, interrupts will not be serviced until the write completes.

To enable write access to the nonvolatile data memory, the MWEN bit (MEMCON.4) must be set

to one. When MWEN = 1 and DMEN = 1, MOVX @DPTR,A may be used to write to FDATA.

FDATA uses Flash memory with a page-based programming model. Flash data memory differs

from traditional EEPROM data memory in the method of writing data. EEPROM generally can

update a single byte with any value. Flash memory splits programming into write and erase

operations. A Flash write can only program zeroes, i.e change ones into zeroes (). Any

ones in the write data are ignored. A Flash erase sets an entire page of data to ones so that all

bytes become FFH. Therefore after an erase, each byte in the page can be written once with

any possible value. Bytes can be overwritten without an erase as long as only ones are changed

into zeroes. However, if even a single bit needs updating from zero to one (

); then the con-

tents of the page must first be saved, the entire page must be erased and the zero bits in all

bytes (old and new data combined) must be written. Avoiding unnecessary page erases greatly

improves the endurance of the memory.

The AT89LP428/828 includes 8/16 data pages of 64 bytes each. One or more bytes in a page

may be written at one time. The AT89LP428/828 includes a temporary page buffer of 64 bytes,

so the maximum number of bytes written at one time is 64. The LDPG bit (MEMCON.5) allows

m u ltiple d a ta b yte s to b e lo a d ed to the tempor a ry p a ge bu ffer. While LDPG = 1 ,

MOVX @DPTR,A instructions will load data to the page buffer, but will not start a write

sequence. Note that a previously loaded byte must not be reloaded prior to the write sequence.

To write the page into the memory, LDPG must first be cleared and then a MOVX @DPTR,A

with the final data byte is issued. The address of the final MOVX determines which page will be

written. If a MOVX @DPTR,A instruction is issued while LDPG = 0 without loading any previous

bytes, only a single byte will be written. The page buffer is reset after each write operation. Fig-

ures 3-4 and 3-5 show the difference between byte writes and page writes.

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