AT89C51ID2_0309 Datasheet, PDF (116/159 Pages) ATMEL Corporation – 8-bit Flash Microcontroller

Reduced EMI Mode
The ALE signal is used to demultiplex address and data buses on port 0 when used with
external 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 86. AUXR Register
AUXR - Auxiliary Register (8Eh)
7
6
5
4
3
2
1
0
-
-
M0
XRS2
XRS1
XRS0
EXTRAM
AO
Bit
Bit
Number Mnemonic Description
Reserved
7
-
The value read from this bit is indeterminate. Do not set this bit.
Reserved
6
-
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
5
M0 periods (default).
Set to stretch MOVX control: the RD/ and the WR/ pulse length is 30 clock
periods.
4
XRS2 XRAM Size
XRS2 XRS1XRS0XRAM size
3
XRS1 _{0 0 0 256 bytes}
0 0 1 512 bytes
0 1 0 768 bytes(default)
2
XRS0 _{0 1 1 1024 bytes}
1 0 0 1792 bytes
EXTRAM bit
Cleared to access internal XRAM using movx @ Ri/ @ DPTR.
1
EXTRAM Set to access external memory.
Programmed by hardware after Power-up regarding Hardware Security Byte
(HSB), default setting, XRAM selected.
ALE Output bit
0
AO
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.
Reset Value = XX00 10’HSB. XRAM’0b
Not bit addressable
116 AT89C51ID2
4289A–8051–09/03