AT89LP51 datasheet(13/113 Pages) ATMEL

13 / 113 page

3709C–MICRO–5/11

AT89LP51/52 - Preliminary

Figure 3-3.

External Program Memory Fetches

In order for Fast mode to fetch externally, two wait states must be inserted for every clock cycle,

increasing the instruction execution time by a factor of 3. However, due to other optimizations,

external Fast mode instructions may still be 1/4 to 1/2 faster than their Compatibility mode equiv-

alents. Note that if ALE is allowed to toggle in Fast mode, there is a possibility that when the

CPU jumps from internal to external execution a short pulse may occur on ALE as shown in Fig-

ure 3-4. The setup time from the address to the falling edge of ALE remains the same. However,

this behavior can be avoided by setting the DISALE bit prior to any jump above the 8K border.

Figure 3-4.

Internal/External Program Memory Boundary (Fast Mode)

3.1.2

SIG

In addition to the 64K code space, the AT89LP51/52 also supports a 256-byte User Signature

Array and a 128-byte Atmel Signature Array that are accessible by the CPU. The Atmel Signa-

ture Array is initialized with the Device ID in the factory. The User Signature Array is available for

user identification codes or constant parameter data. Data stored in the signature array is not

secure. Security bits will disable writes to the array; however, reads by an external device pro-

grammer are always allowed.

In order to read from the signature arrays, the SIGEN bit (AUXR1.3) must be set (See Table 5-3

on page 28). While SIGEN is one, MOVC A,@A+DPTR will access the signature arrays. The

User Signature Array is mapped from addresses 0100h to 01FFh and the Atmel Signature Array

is mapped from addresses 0000h to 007Fh. SIGEN must be cleared before using MOVC to

CLK

ALE

PSEN

FLOAT

PCL

OUT

PCH OUT

DATA

SAMPLED

PCL

OUT

PCL

OUT

DATA

SAMPLED

DATA

SAMPLED

CLK

ALE

DISALE=0

PSEN

FLOAT

P0 SFR OUT

P2 SFR OUT

PCH OUT

PCL OUT

DATA

SAMPLED

SHORT

PULSE

ALE

DISALE=1

INTERNAL EXECUTION

EXTERNAL EXECUTION

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