AVR-Chat

We have a pair of ATMEGA128s talking to each other using SPI.  The
slave is not always powered on.  The problem is when Vcc to the slave
is off it gets partially powered by the SPI lines.

It appears we could isolate them with optoisolators. But is there a
better/cheaper/smaller way?

Jim

On Sun, 02 Sep 2007 12:44:43 -0000
 "wd0e" <jim@coloradosatellite.com> wrote:
> We have a pair of ATMEGA128s talking to each other using
> SPI.  The
> slave is not always powered on.  The problem is when Vcc
> to the slave
> is off it gets partially powered by the SPI lines.
> 
> It appears we could isolate them with optoisolators. But
> is there a
> better/cheaper/smaller way?
> 
> Jim
> 
You could use a FET in each line with the pull-up powered
by  the local power for the second micro. That should be
smaller, cheaper, and lower power than opto isolators.

Jim Wagner
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Hi Jim,

On 9/2/07, wd0e <jim@coloradosatellite.com> wrote:
> We have a pair of ATMEGA128s talking to each other using SPI.  The
> slave is not always powered on.  The problem is when Vcc to the slave
> is off it gets partially powered by the SPI lines.

Configure the output SPi lines as GPIO and set them up as outputs
which get driven to ground. This should eliminate and leakage current
via that means. Input lines need to not have any pullups on them.

Have an input signal from the other processor which indicates when
it's on and when its not. You can then reconfigure the appropriate
lines when the other processor powered on versus not powered on.


-- 
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/

Hi 
  you can use a pull up ressistor (internal pul ups can be used) on the slave front for each input pin of spi (MOSI,SCK,SS) and a diode which anode is connected to slave pin and the cathode is connected to the corresponding pin of master.
   
  I dont have any idea about MISO pin.

wd0e <jim@coloradosatellite.com> wrote:
          We have a pair of ATMEGA128s talking to each other using SPI. The
slave is not always powered on. The problem is when Vcc to the slave
is off it gets partially powered by the SPI lines.

It appears we could isolate them with optoisolators. But is there a
better/cheaper/smaller way?

Jim



                         

       
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