Careful with how you design in a backup supply. If the MCU is still driving
it's outputs you'll be pumping power to other IC's through the input
protection diodes. Shut down the MCU I/O's or have the backup supply power
all necessary IC's in your circuit.

At 04:28 PM 4/14/01 +0800, you wrote:
>    Use diode to supply Vcc to MCU, decoupled
>    with a reasonably large reservoir capacitor
>    that will keep the juice flowing to the MCU
>    for 2 second.
>
>    The diode is to prevent other part of the
>    system draining from the cap.
>
>    However, do take care of the reset requirement,
>    especially in brownout situation. Your MCU
>    might not be in reset, while other part of
>    your system already reset.
>
>Regards,
>Peter Tiang.
>
>----- Original Message -----
>From: "Wing Kent Kwan" <gzkwan@PUBLIC.GUANGZHOU.GD.CN>
>To: <PICLIST@MITVMA.MIT.EDU>
>Sent: Saturday, April 14, 2001 1:36 PM
>Subject: how use capacitor to keep Vcc to run 1-2 second
>
>
>> how can I do when power supply shut down 1-2second,
>> to keep the Vcc of MCU to Running 1-2second.
>> (use capacitor)

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