State of IC pins when not powered

[nickagian]

I have a quick question: When an IC (memory, opamp etc.) is not supplied what happens internally to its pins? Are they in High-Z? Are they pulled-up or -down?

 

[betwixt]

It depends on the chip manufacturer. If by "supplied" you mean with signal, often they build pull-up or pull-down resistors (or equivalent circuits) into the silicon to ensure the pins adopt one state or the other. An input pin without pull up/down will likely float and will be seen by the IC as a source of random signals.
If you mean supply voltage, in almost all cases the pins will sink current to ground. There are exceptions but usually, the pins have protection diodes, one to ground and one to supply to prevent voltages going out of range. Normally these do not conduct, they are there only for protection but if the supply is removed, the diode to the IC's internal supply will forward bias and current will sink into the IC.

Brian.

 

[nickagian]

Brian,

thanx for your reply. I mean supply voltage. What I have in mind is this: I have a processor connected to an external flash memory (M25P128) through SPI interface and to a FTDI IC (FT232R) through UART interface. When I put the processor in sleep mode, I turn-off the supply voltage to the memory and the FTDI and I want to find in what state should I put the GPIOs of the processor, in order to achieve low current consumption. I have tested with pins in input state and pull-up internal resistors, but I the processor needs about 3mA!! I then keep the power supply to both external ICs and the processor now needs around 300uA, but still this value is far away from the low current consumption stated in the datasheet. If I keep the pins to inputs with no pull-up / pull-down? Can this work?

 

[betwixt]

What type of processor are you using?
I'm basing this reply on PIC processors as these are the ones I'm most familiar with. There are often other features of the micro which have to be turned off, for example, ADCs or comparator modules, you should preserve their state in software if necessary then disable them before going into sleep mode. If you have an independent watchdog timer that might also be drawing current.

As for the IO ports, there is no point in using pull-up or pull-down resistors when the port is an output so they are normally only enabled when in input mode, you may have inadvertently increased the current by making the pins into inputs during sleep. What you need to do is decide the optimum logic levels on the peripherals to ensure they do not draw current from the processor then set the pins to that level before sleeping. For example (I have not checked this, I'm only suggesting) if the flash memory had pull-down on it's inputs, it would be better to drive it low than to sink current through your pull-up then into the flash's pull-down. Obviously you have to take care that you don't put the peripheral into an undesired state by leaving the pins at levels that have bad consequences.

Brian.

 

[nickagian]

What type of processor are you using?
I'm basing this reply on PIC processors as these are the ones I'm most familiar with. There are often other features of the micro which have to be turned off, for example, ADCs or comparator modules, you should preserve their state in software if necessary then disable them before going into sleep mode. If you have an independent watchdog timer that might also be drawing current.

As for the IO ports, there is no point in using pull-up or pull-down resistors when the port is an output so they are normally only enabled when in input mode, you may have inadvertently increased the current by making the pins into inputs during sleep. What you need to do is decide the optimum logic levels on the peripherals to ensure they do not draw current from the processor then set the pins to that level before sleeping. For example (I have not checked this, I'm only suggesting) if the flash memory had pull-down on it's inputs, it would be better to drive it low than to sink current through your pull-up then into the flash's pull-down. Obviously you have to take care that you don't put the peripheral into an undesired state by leaving the pins at levels that have bad consequences.

Brian.

Hello again Brian,

first of all the processor I am using is the EM250 from Ember. And I am using the API provided by the company, which is set to disable both ADC or any other internal modules during sleep. However, I need to check that the watchdog is also disabled and is not drawing any current. I think though that the WD is also disabled.

As for the IO ports, I have used pull-up / pull-down because I set the ports to input state. So you suggest to use output direction instead? So, ok... I will think of the correct logic states of the pins and then set them to the correct output state.

Nikos