Apply a logic high to a powered down processor. Will it damage it?
Apply a logic high to a powered down processor. Will it damage it?
(OP)
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
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





RE: Apply a logic high to a powered down processor. Will it damage it?
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
RE: Apply a logic high to a powered down processor. Will it damage it?
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.
RE: Apply a logic high to a powered down processor. Will it damage it?
There are many semicondutor companies that offer protection arrays in small packages.
Benta.
RE: Apply a logic high to a powered down processor. Will it damage it?
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
RE: Apply a logic high to a powered down processor. Will it damage it?
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
RE: Apply a logic high to a powered down processor. Will it damage it?
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
RE: Apply a logic high to a powered down processor. Will it damage it?
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:
Benta.
RE: Apply a logic high to a powered down processor. Will it damage it?
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.
RE: Apply a logic high to a powered down processor. Will it damage it?
RE: Apply a logic high to a powered down processor. Will it damage it?
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?
RE: Apply a logic high to a powered down processor. Will it damage it?
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