Datasheet

106
AT89C51IC2
4301D–8051–02/08
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 83. AUXR Register
AUXR - Auxiliary Register (8Eh)
7 6 5 4 3 2 1 0
- - M0 - 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
5 M0
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.
4 -
Reserved
The value read from this bit is indeterminate. Do not set this bit
3 XRS1 XRAM Size
XRS1XRS0XRAM size
0 0256 bytes (default)
0 1512 bytes
1 0768 bytes
1 11024 bytes
2 XRS0
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.
0 AO
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
instructione is used.