115
AT89C51ID2
4289C–8051–11/05
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 87. AUXR Register
AUXR - Auxiliary Register (8Eh)
Reset Value = XX00 10’HSB. XRAM’0b
Not bit addressable
76
543
210
-
M0
XRS2
XRS1
XRS0
EXTRAM
AO
Bit
Number
Bit
Mnemonic
Description
7-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
6-
Reserved
The value read from this bit is indeterminate. Do not set this bit.
5M0
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.
4XRS2
XRAM Size
XRS2 XRS1XRS0XRAM size
0 0 0 256 bytes
0 0 1 512 bytes
0 1 0 768 bytes(default)
0 1 1 1024 bytes
1 0 0 1792 bytes
3XRS1
2XRS0
1
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.
0AO
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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