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Unpowered devices on SPI line (memories retaining contents)

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lapa678

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Hi folks,

I am working on a project where there are few SPI memories on the bus. To have a cold sparing, some memories are turned off with an aim to be turned on when there is a problem with the power ones.

I have tested few SPI memories, but whenever you have unpowered SPI device on the bus, the lines get loaded and the voltage drops to ~ 1.2V. SPI fails..

Any comment on that? For sure I could use buffers to 'cut off' the memory, but maybe there is some type of SPI memory that would be suitable?

Thanks!
Tom
 

Yes, indeed. But I managed to find memories which were partially working (only two out of three lines were loaded :) )... moreover, there is AUP family of TI buffers that different type of input which allows them to be unpowered and connected to the bus.

Maybe there are some automotive memories which support cold sparing you now of?

Thanks&regards,
Tom
 

Hi,

Don't disconnect power when SPI devices are connected to the bus, as long as it is not explicitely allowed in the datasheet.

I wonder what SPI memories you use, that draw so much power when disabled, that it makes sense to power them off.

Klaus
 

Well, the problem is not with power consumption. In my case the idea is to power them down not to make them prone to single event effects caused by space radiation, latch-ups. So the requirement of powering down it quite important.

Unless there are some memories that can handle the VCC=0V with bus connected, I might have to go with buffers.

Thanks,
Tom
 

Hi,

are you sure powering down a memory device like Flash or EEPROM ensures that it is immune against space radiation? Why?

My ideas:
* using rad hard devices
* using protection shield
* using some redundant memory techniques and software to recognize and correct the errors.

Klaus
 

Hi,

Powering down memory for sure will make it immune to latch-up :). Bit upsets will be handled by software means.

I am still looking for COTS components that have some high impedance state when powered down.

Thanks for the comment & interest though!
Tom
 

Hi,

you are more concerned about latch-up than data corruption inside the memory?
Latch-up only of the I/O buffers or latch-up of internal circuitry?

Klaus
 

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