AT89LP428 datasheet(11/149 Pages) ATMEL

11 / 149 page

3654A–MICRO–8/09

AT89LP428/828

3.3

External Data Memory

AT89LP microcontrollers support a 16-bit external data memory address space. The external

memory space is accessed with the MOVX instructions. The AT89LP428/828 does not support

an external memory interface. However, some internal data memory resources are mapped into

portions of the external address space as shown in Figure 3-3. These memory spaces may

require configuration before the CPU can access them. The AT89LP428/828 includes 512 bytes

of on-chip Extra RAM (EDATA) and 512/1024 bytes of nonvolatile Flash data memory (FDATA).

Figure 3-3.

External Data Memory Map

3.3.1

EDATA

The Extra RAM is a portion of the external memory space implemented as an internal 512-byte

auxiliary RAM. The Extra RAM is mapped into the EDATA space at the bottom of the external

memory address space, from 0000H to 01FFH. MOVX instructions to this address range will

access the internal Extra RAM. EDATA can be accessed with both 16-bit (MOVX @DPTR) and

8-bit (MOVX @Ri) addresses. When 8-bit addresses are used, the PAGE register (086H) sup-

plies the upper address bits. The PAGE register breaks EDATA into two 256-byte pages. A page

cannot be specified independently for MOVX @R0 and MOVX @R1. When 16-bit addresses are

used (DPTR), the IAP bit (MEMCON.7) must be zero to access EDATA. MOVX instructions to

EDATA require a minimum of 2 clock cycles.

Extra RAM

(EDATA)

0000

01FF

0200

03FF

Flash Data

(FDATA)

Extra RAM

(EDATA)

0000

01FF

0200

05FF

Flash Data

(FDATA)

AT89LP428

AT89LP828

Table 3-3.

PAGE

– EDATA Page Register

PAGE = 86H

Reset Value = XXXX XXX0B

Not Bit Addressable

––––

–––

PAGE.0

Bit

7654

321

Symbol

Function

PAGE0

Selects which 256-byte page of EDATA is currently accessible by MOVX @Ri instructions.

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