# Tag Info

33

Strong means low resistance. Weak means high resistance. Of course low and high are relative terms, and so are strong and weak. The reference for this relationship must be inferred from context. A strong or low resistance pull-up/down resistor is good because the time constant formed the load capacitance (often, the input gate capacitance, and the PCB trace ...

32

The answer depends on what you want the "default" configuration to be. For example, say you have a down-stream N-channel MOSFET, and you want it default off. Then you would use a pull-down resistor to ensure this behavior if the input becomes high impedance. simulate this circuit – Schematic created using CircuitLab On the other hand, suppose you ...

31

It's a pull down resistor, its purpose is to set the voltage of the Arduino input to 0 V when the button is open. It just sets a default voltage value to the connected node. But, why is such a resistor needed? Well, when the button is open, the voltage is not totally equal to zero due to antenna effects or surrounding components. It is then possible that the ...

30

When working with a digital circuitry that senses analog voltage, for example a microcontroller, or let's say Arduino, you are measuring voltages. However, without current, voltage cannot be present. For a voltage to be created on a component, there need to be a current flowing on it. According to Ohm's law; $V=I*R$, when $I=0$, the equation becomes; \\$...

30

There are a few possible reasons, such as Needing the resistor to be present during power-up, as the microcontroller will not yet have started executing. Needing a more precise resistance than the internal resistor. Internal pull-up/-down resistors have very wide tolerances. Needing a resistance larger or smaller than that provided internally. For example, ...

30

Electrically it makes sense because ground is the one common connection to all devices on a IIC bus. That's a lot less of a restriction than forcing power to be the common connection to all IIC devices, as would be required if the lines were driven high and floated low via pulldowns. Note that IIC devices don't all need to be powered from the same net or ...

29

tl;dr because there are real-world issues that prevent us from setting I/O standards and other specifications willy-nilly. If EE were so simple a lot of us wouldn't have jobs or thesis projects. To start, there are tons of practical reasons why we don't just increase I/O voltage. It's not as simple as some guy crossing out 3.3v on a piece of paper and ...

24

In the good old days, TTL drivers were much better at pulling a signal down than pulling it up. Therefore, protocols like I2C, but also interrupt lines, reset, and others, were all implemented using a pull-up with distributed pull-down.

19

If the signal doesn't already have a specification, use whichever one makes the most sense to you. It is your choice to make an input active-high or active-low. If it's buttons, make sure to use a debounce circuit (or do it in software). simulate this circuit – Schematic created using CircuitLab

17

Yes... You can... It's a proven feature. If you see pin structure in STM32 Reference manual, you can see that it has required components -- If this satisfies your requirements, then, you can do away with external components. Edit Remember that at Power up, Pins will be tristated. If it does not hamper your application, it is good! That's why it's good ...

17

In general, it is perfectly fine to use the internal pull-up or pull-down resistors. Disadvantage is that you can forget to enable it from software, that can cause a little headache. The external pull-ups will always be there. So do not forget to enable the internal ones and save a lot of PCB area by reducing component number. The STM32's internal pull-up ...

15

If your circuit design is such that you can choose - in other words you aren't required by the rest of the circuit to use a pull up or pull down - then you should consider safety and security in the case of failure. If your microcontroller fails, or just that output fails, the pull up or pull down will be in effect. How will this change the operation of ...

15

It's not a pull-down in the strictest sense that some other chip may be required to activate said line and fight against it. The chip in the OP's diagram is permanently chip-enabled as far as I can see and this is achieved by the 39 ohm resistor. If it were a zero ohm resistor would anybody quibble? So why use a 39 ohm resistor instead of a zero ohm ...

15

This looks like a pull-down resistor. a pull-[down] resistor is a resistor used to ensure a known state for a signal. https://en.wikipedia.org/wiki/Pull-up_resistor [They] are used in electronic logic circuits to ensure that inputs to the [microcontroller] settle at expected logic levels if external devices are disconnected or high-impedance. '...

15

Note that the current is wasted regardless of whether the circuit is "on" or "off" — when it is "on" the voltage drop across R11 is only slightly less than when it is "off". Using a PMOS transistor instead of the PNP would mean that the pulldown resistor could be on the order of megohms, reducing the "leakage" current to microamps. Or you could use a ...

14

Unless the pullup and pulldown resistors are active during reset (I suspect they aren't, but I'm not familiar with STM32 devices), there is an undefined pin state at reset. This is one major reason to use external pullup or pulldown resistors.

14

It's easier to use ground as a common reference among subsystems that might have varying supply voltages. If you use PNP transistors to pull up to a supply voltage, all subsystems would have to be connected to the same supply.

13

Honest truth: What you're trying to do is easier implemented just by using a microcontroller with enough pins. It's probably even cheaper than an Attiny85. Who knows. But: If you really must, you can do various things to get more output out of a single line: Buy an IO expander that uses the 1-Wire (pseudo)standard, and implement a 1-Wire transmitter on the ...

13

Yes, it smells. Use a transistor. This is the normal way to invert a signal. The OE input is a sourcing input. These are designed to be used with a switch which is either open or switched to common. It's not meant to have Vcc connected directly to the input! You have a switch (your header), which would be perfect, but you want the behaviour inverted - ...

12

You do need a resistor in series with the base but a better design would also put a resistor in series with the SSR's LED to limit the current unambiguously and not be reliant on hFE of the transistor. So, I would make the base series resistor 1 kohm and this would force about 2.5 mA into the base. This will likely saturate the collector down to about 0.1 ...

11

From a TI datasheet, this is the equivalent circuit for a 7407 buffer: Note that the input is relatively low impedance (having a 6k pullup to Vcc through the emitter-base junction (which effectively looks like a diode) of the transistor shown). The datasheet says it requires 1.6mA to pull the input low (parameter IIL). This is a lot of current compared ...

11

Ideally, there should no apparent difference, but I always put this "guard" resistor close to the pin which it will guard. There are two reasons: If you put the pull-down resistor close to your MCU, and have a long long trace from your MCU to the converter. If your converter's enable pin will source some current, the current will flow through the long trace ...

11

The resistor to ground is exactly as the description says so - to prevent the pin floating high and turning on the FET, thus making the motor move erratically/uncontrolled. Floating MOSFETs are bad, and because the "on" control of the FET is essentially just a capacitor with very low capacitance, it is quite easy for it to float up and turn itself on. ...

11

TTL logic and its descendants all source current into the inputs when low. In the case of LSTTL it will source about 0.4mA - this will cause a voltage to be developed across the 10k resistor such that the input is seen as a logic '1'. If you change the resistor to 1.5k or less it should work. (As corrected by Trevor) The recommend way to interface ...

11

It has been solved already. Some microcontrollers have built in pull-up and/or pull-down resistors that can be enabled via software. But these are not active when microcontroller has no firmware so for safety reasons you might still want to have external resistors to keep stable state during powerup, reset or firmware download.

10

As others have said, internal pull-ups and pull-downs are fine as long as: you remember to configure them, they are strong enough, you do not end up fighting them with an external pull, and the voltage at start-up and reset is unimportant. On the last point, remember that CMOS inputs at mid-voltage level can cause significant shoot-through current on their ...

10

Because it's good practice to never leave logic pins purely floating. Brief TI Overview Detailed TI overview Amusing dramatized war story of a real-world example

10

Use a second DIO to control the pull-up voltage. The output is normally low. Set it high momentarily, read the DIO input, set the output low. simulate this circuit – Schematic created using CircuitLab

9

From the STM32F405/7 datasheet Since you didn't say what processor it uses, I guessed.

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