0
\$\begingroup\$

Suppose I have an IC that has a standard level triggered input pin. I need this pin's state to be setup immediately at power on at a time when my MCU's GPIOs may not have established levels. What are my options for accomplishing this while still being able to control the pin level at a later time?

My way that of course comes to mind is using a pull up/down resistor however that can have high leakage current. During the times my MCU is online I will have no use for the resistors as the pin state can be set by the MCU so this leakage is highly undesiable. Are there any options i'm not thinking of?

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$

3 Answers 3

1
\$\begingroup\$

I once use megaohm pull downs and very low leakage mosfets as a cmos buffer. Also i think actel's igloo does the job with megaohm pull resistors.

\$\endgroup\$
0
\$\begingroup\$

It could probably be done, but the details would be very dependent on the actual circuit.

For example, suppose the microcontroller is reset by a supervisory circuit- you could have the pullup enabled by a hardware delay that stretches the reset signal. After the delay is over the pullup resistor is disconnected and the pin controls the input directly.

Another trick that is a bit more general and may not require any extra port pins is to use a keeper circuit- a positive feedback non-inverting gate that has small enough phase delay it won't oscillate. Also called a bus hold circuit... some parts ABTH type have it built into the input. As you can see the resistor draws negligible current if the input is open or tied to the high or low rails statically.

schematic

simulate this circuit – Schematic created using CircuitLab

\$\endgroup\$
0
\$\begingroup\$

If the peripheral has a reset line, you could connect that to I/O on the micro (you'd probably want to anyway), and then you could assert the state of these setup lines from the micro while performing a reset on the peripheral, at some point after power-up.

Are you absolutely sure the consumption via these pull-ups actually matters in your application?

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.