Apply a logic high to a powered down processor. Will it damage it? 1

[jamesnguyen]

Hi everyone,

I have a processor board which has a power input and a SPI port at its external interface. The SPI port is connected directly to the processor.

What could happen to the processor if I just connect up the SPI port and apply a logic high to it without providing power to the processor board?

I did a test and found no damage so far. What do you think?

Thank you,
James

 

[MikeHalloran]

That should be covered in the data sheet, or in a referenced document.

Since you didn't identify the processor or the board, no one here can share their experience with the same hardware.


Mike Halloran
Pembroke Pines, FL, USA

 

[BrianG]

It depends on what device or system is providing the logic high condition. If its a normal TTL gate the curent is quite limited in the high state, but do not make this assumption for tri-state bus buffers as many can sink and source equally large currents.

Generally, driving a chip input with a voltage when the chip is powered off causes any input current to flow in the substrate diode of the bulk silicon. This current can then flow into the power rail of the remainder of the circuit via the driven chip's power supply pin.

If the source can provide sufficient current at the logic high state into the substrate diode, particularly with CMOS technology you may find that the processor and support chips can become powered via this unintended path, with sufficient voltage to become partially operational.

 

[benta]

An external interface that does not have a protection diode array is in my book a bad design.
There are many semicondutor companies that offer protection arrays in small packages.

Benta.

 

[MacGyverS2000]

Generally, driving a chip input with a voltage when the chip is powered off causes any input current to flow in the substrate diode of the bulk silicon. This current can then flow into the power rail of the remainder of the circuit via the driven chip's power supply pin.

If the source can provide sufficient current at the logic high state into the substrate diode, particularly with CMOS technology you may find that the processor and support chips can become powered via this unintended path, with sufficient voltage to become partially operational.

Brian beat me to it... I'll add that this back-powering can have really bad consequences. Erased/improperly programmed flash, RTCs go haywire, latchups on the next power cycle, etc.

Just because you see no immediate problem don't assume there isn't one. Back-powering is typically a no-no, intentional or not.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[jamesnguyen]

Thank you everyone for quick response!

How do module vendors (who make GPS, Bluetooth, processor, Ethernet, etc modules) protect their modules from this condition?

MikeHalloran: The processor is ADI's BF548 blackfin DSP.

benta: How does protection diode protect the processor? I thought they are for over/undervoltage or ESD protection...

Thanks!
James

 

[itsmoked]

It is over/undervoltage based on the rail voltages which would be zero when powered down.

You need to indeed protect those internal diodes of they exist. This is done all the time by including a series resistor of sufficient value to prevent damaging power dissipation to the diode.

For instance you can hook 120V to a PIC processor as long as the resistance is set correctly (and a bunch of other safety stuff is covered.)

Some processors skip providing the protection diodes. In this case you absolutely have to provide your own and, again, use the series resistance to protect the diodes.

That's how those makers protect their devices.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[benta]

"benta: How does protection diode protect the processor? I thought they are for over/undervoltage or ESD protection..."

Well, isn't input overvoltage exactly what you have here? Almost all MOS/CMOS circuits specify how high the input voltage may be compared to VDD, usually something like maximum allowed VIN is VDD + 0.5 V (and of course, also how low the input voltage may be compared to VSS).

The protection arrays conduct current from too high input voltages onto the positive supply rail to ensure that such an out-of-spec situation does not occur.

A collection can be found here, for instance:

http://www.vishay.com/diodes/protection-tvs-esd/arrays/

Benta.

 

[Comcokid]

I'm not familiar with ADs Blackfin family, but there are methods I've used for other processor families.

Careful reading of some data sheets may show that you only need a limiting resistor to accomplish what you need. This approach may result a little current being lost in the application.

Where power is critical (i.e. extreme battery operation) I've used Fairchild 'tiny logic' parts to buffer critical inputs. In addition to doing translation between 5V and 3V circuits (as a 3V proccessor connected to 5 volt A/D or D/A) the data sheet states that unpowered device inputs present a high-impedance to other logic.

 

[jimkirk]

There might not be immediate damage, but I'd be very concerned about the long term reliability. You should contact an applications engineer at ADI.

 

[jamesnguyen]

Thank you all for your reply. The processor board fortunately has RC filter (470R & 22p) on all of its external interface signals. So its I/O ports are somewhat "protected" by the 470R resistor. I will need to check with the vendor to see if there resistance is enough.

If not, I might need to add a pair of back-to-back "steering diodes" to each and every port (after the RC filter, of course).

What's the technical name for this "apply a logic high to a powered down processor" case. Is it called "cold-sparing"?

itsmoked: what is the typical resistor value you have seen?

 

[itsmoked]

1k is a pretty typical value I see for "I/O pin series protection resistors".

Most things being driven directly by processor pins have pretty high impedances so the 1k doesn't affect much. If things have low impedance then there's usually some interposing element like a FET or bipolar transistor to provide the needed current. In those cases you handle protecting those elements on a an individual basis based on their needs.

Keith Cress
kcress -

http://www.flaminsystems.com