这样除非执行了MOVX and MOVC指令，ALE有低电平脉冲外，ALE是内部上拉的。
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 (See Table 2-2). 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.
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