AT89C51RB2/RC2
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 64. AUXR Register
AUXR - Auxiliary Register (8Eh)
7
6
-
5
4
-
3
2
1
0
DPU
M0
XRS1
XRS0
EXTRAM
AO
Bit
Bit
Number
Mnemonic Description
Disable Weak Pull-up
7
6
DPU
-
Cleared to activate the permanent weak pull up when latch data is logic 1
Set to disactive the weak pull-up.
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).
5
M0
Set to stretch MOVX control: the RD and the WR pulse length is 30 clock
periods.
Reserved
4
3
-
The value read from this bit is indeterminate. Do not set this bit.
XRS1
XRAM Size
XRS1 XRS0
XRAM size
0
0
1
1
0
1
0
1
256 Bytes (default)
512 Bytes
768 Bytes
1024 Bytes
2
XRS0
EXTRAM Bit
Cleared to access internal XRAM using movx @ Ri @ DPTR.
EXTRAM Set to access external memory.
Programmed by hardware after Power-up regarding Hardware Security Byte
1
0
(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.
AO
85
4180C–8051–12/03
ATMEL [ ATMEL ]
