Slave select for single slave SPI communication

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I want to make an ATmega (master) talk to another ATmega (slave) over SPI. Since there is only one slave I can get away without using  a slave select pin, and simply connecting the slave's SS pin to ground. This saves me a pin on the master device since I don't need a separate pin to drive the slave's SS pin. I tried both implementations and they both work but I was wondering whether they are the same performance wise. In Atmel's AVR151 application note (http://www.atmel.com/images/doc2...) it says "When SS is held low, the SPI is activated and MISO becomes output if configured so by the user." What does it mean for the SPI to become activated in terms of power consumption? Will the slave draw more current when SS is 0 than when it is 1? In this case it might make sense for me to use an extra pin on the master to make sure the SPI is only activated when I want to send or receive data to/from the slave.

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I tried both implementations

... and did you measure any difference in power consumption? Surely you are in the best position to be definitive about the implications...

Ross McKenzie ValuSoft Melbourne Australia

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valusoft wrote:

I tried both implementations

... and did you measure any difference in power consumption? Surely you are in the best position to be definitive about the implications...

I don't really have the proper tools for that I'm afraid. I was rather hoping that anyone had some experience with this already, or that there's some information on this supplied by Atmel that I had overlooked.