AT89C51ED2-RDTUM datasheet(88/137 Pages) ATMEL

88 / 137 page

4235K–8051–05/08

AT89C51RD2/ED2

22. Reduced EMI Mode

The ALE signal is used to demultiplex address and data buses on port 0 when used with exter-

nal program or data memory. Nevertheless, during internal code execution, ALE signal is still

generated. In order to reduce EMI, ALE signal can be disabled by setting AO bit.

The AO bit is located in AUXR register at bit location 0. As soon as AO is set, ALE is no longer

output but remains active during MOVX and MOVC instructions and external fetches. During

ALE disabling, ALE pin is weakly pulled high.

Table 22-1.

AUXR Register

AUXR - Auxiliary Register (8Eh)

Reset Value = XX00 10’HSB. XRAM’0b

Not bit addressable

DPU

XRS2

XRS1

XRS0

EXTRAM

Bit

Number

Bit

Mnemonic

Description

DPU

Disable Weak Pull-up

Cleared by software to activate the permanent weak pull-up (default)

Set by software to disable the weak pull-up (reduce power consumption)

Reserved

The value read from this bit is indeterminate. Do not set this bit.

Pulse length

Cleared to stretch MOVX control: the RD and the WR pulse length is 6 clock periods

(default).

Set to stretch MOVX control: the RD and the WR pulse length is 30 clock periods.

XRS2

XRAM Size

XRS2 XRS1XRS0XRAM size

0256 bytes

1512 bytes

0768 bytes(default)

11024 bytes

01792 bytes

XRS1

XRS0

EXTRAM

EXTRAM bit

Cleared to access internal XRAM using MOVX @ Ri/ @ DPTR.

Set to access external memory.

Programmed by hardware after Power-up regarding Hardware Security Byte (HSB),

default setting, XRAM selected.

ALE Output bit

Cleared, ALE is emitted at a constant rate of 1/6 the oscillator frequency (or 1/3 if X2

mode is used) (default). Set, ALE is active only during a MOVX or MOVC instruction is

used.

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