A Pullup Resistor brings a line up to a voltage (Typically VCC), while allowing the line to be pulled low.

learn more… | top users | synonyms

0
votes
0answers
16 views

mixing parasitic powered 1-wire devices with others which are not. All in a star topology

I'm designing a 1-wire network with star topology. Number of branches will be 4 - 6. Each branch will connect two devices. One temperature sensor and one PIO-device 5 V will be supplied to the ...
2
votes
3answers
362 views

Why are pull-up resistors required to be connected to the 2 lines of I2C? [duplicate]

Without pull-up resistors connected to the 2 lines of I2C, I2C communication fails. Why is there a need to have pull-up resistors for I2C? In contrast, UART and SPI do not need pull-up resistors. ...
0
votes
2answers
55 views

Does an active low pin need a pull up resistor?

I am interfacing a PIC24 microcontroller with this nRF24L01 2.4GHz transceiver. The transceiver has an active low interrupt pin that is toggled on transmit and a few other instances. Does this pin ...
0
votes
2answers
64 views

Controlling pull-up pins from external circuit

I have a circuit with an Arduino controlling a 2-channel, normal-off, opto-isolated relay board, 5V input signals, turning relay ON when the Arduino digital pin is set LOW. The relays are controlled ...
3
votes
2answers
53 views

Use GPIO for switchable Pullups of I2C

I'm designing a general purpose interface board and want to make available as many of the peripherals available and usable on an expansion header without the need for permanent (soldering) changes. ...
0
votes
1answer
22 views

does CompactRIO modules have builtin pull-up/down resistors

I have Compact RIO 9076 with 4 different modules: ni-9205 (Analog Input), ni-9264(Analog Output), ni-9402 (Bidirectional Digital I/O), ni-9403 (Bidirectional Digital I/O). I have several 3rd party ...
1
vote
1answer
77 views

FPGA floating pins, when place pull-up/down resistor on Input or Output

I've looked my FPGA datasheet and found that there is no pull-up/down resistors on it's pins(just a pull-up but that need to be enabled). So, when I power up my circuit I've for a "big" amount of time ...
0
votes
0answers
57 views

Problem with pull-up resister

I am interfacing DS18S20 with SIM800. I am able to interface one DS18S20 to SIM800 and measure the temperature. But when I interface two DS18S20 on the same line, the data line is not pulling high. ...
0
votes
1answer
80 views

Is it possible to detect high impedance at FPGA input?

I am using a FPGA development board which have neither pull-up nor pull-down resister at the input side. VHDL code that I am developing has to detect either logic zero or logic one and to do ...
1
vote
1answer
45 views

Finding the Pull-up Resistor Value

I am designing a wireless sensor node that will be operating off of 1.8V. I am using an Atmel ATtiny861a MCU with a CC2520EMK radio and a 23A1024 External memory chip. I have a chip select pin for ...
0
votes
2answers
23 views

Low leakage current methods of initializing pin inputs at power up

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 ...
1
vote
3answers
107 views

pull down resistor in PIR circuit

I have an motion sensor circuit. The circuit calls for a voltage of 2-15 dc volts. On the PIR data sheet the source pins calls for a pull down resistor of 47K at 5 volts. My question is if I use 8 ...
4
votes
2answers
125 views

How to default logic high after a voltage divider?

I have a 5V logic pin coming in that I want to cut down to 3.3V. When the 5V pin is detached, however, I want the pin to default logic high (3.3V). How can I choose R_pu appropriately? V_in = 5V ...
1
vote
1answer
28 views

Atmega 328 with DS1620 thermometer chip. Why is a pull-up resistor needed on the DQ line

I think understand the purpose of pull up resistors in general but in this particular case the DS1620 datasheet says that in the output mode it sources a current of 1ma (see footnote 5 on the last ...
0
votes
1answer
35 views

multiple mosfets controlling individual circuits sharing one pull up res?

i'd like to control several individual circuits using an array of MOSFETs and wondered if they have to have individual pull-up resistors or if they can share one connecting them to ground. Or do the ...
0
votes
1answer
101 views

PICkit3 MCLR to Vdd Pull-Up Resistor

I'll preempt this by saying that I'm a software person whose education (unfortunately) didn't even touch on the most basics of hardware or circuits. I'm attempting to program a PIC16f1709 microchip ...
0
votes
2answers
66 views

Why does my mainboard power the cathode to turn the Power-LED off?

Close to all desktop motherboards have a pin header that is supposed to power an LED on the front of an enclosure with to indicate that the computer is on. In the documentation for my board, these ...
2
votes
2answers
119 views

What use is a pull-up/pull-down with a push-pull output?

For at least some STM32F4 MCUs, push-pull + pull-up/pull-down is a valid GPIO configuration, but what would you ever use it for, and why? I assume there's some saner reason than "I just really felt I ...
15
votes
9answers
2k views

Why does I²C only have pull-up resistors (interview question)?

The interviewer wanted to know why pull-up resistors are used on SDA and SCL when the opposite logic can also be implemented. Is there an explanation for why pull-up resistor usage is the chosen ...
0
votes
0answers
83 views

Why are pullups needed on eMMC data lines?

The JEDEC spec for eMMC shows pullups on both CMD and DAT lines. The pullup for the CMD line is obviously necessary as it sometimes operates in open drain mode, this necessity is represented by Rod ...
0
votes
3answers
46 views

Why does switch change parallel current after junction?

This is an image demonstrating the use of a pull up resistor. I've just started studying electronics and have been learning about ohm's law. Ohm's law says that current within a circuit is equal to ...
0
votes
2answers
140 views

Calculate pull-up resistor for phototransistors

I have made a sensor which uses six IR phototransistors, three per sensor block, opposite it is a matched IR diode(940nm). This is in turn connected to a MCU. The function of the circuit is an ...
1
vote
2answers
164 views

Why are pull-up resistors not drawn this way?

So, I've always been using a pull-up resistor in the configuration that usually appears in textbooks but I can't tell why it isn't drawn in another way. So, basically a pull-up resistor is usually ...
0
votes
2answers
120 views

How to detect input voltage change on specific pin of atmega32? DC jack sensing

I am trying to make an LCD circuit that, eventually, will control an amplifier and display data to the user through the LCD. One of the features I want to include is the ability to show if the device ...
0
votes
3answers
104 views

How does digital pullups work?

I have lame question. How does digital pullups work? I'm asking because I don't understand DHT11 driver code. Sample code: ...
0
votes
3answers
150 views

Why does a closed circuit with a pull-up resistor output “0”?

I have a switch setup with a pull-up resistor. With the switch open I get a "1" or HIGH output and with the switch closed I get a "0" or LOW output. I understand why I am getting the "1" with it open ...
1
vote
1answer
167 views

i2c communication between cc3200 and at42qt2120

I am trying to do i2c communication between TI's cc3200 and atmel's touch IC AT42QT2120. I am able to read and write to different memory locations of the touch IC, but i am unable to get it's change ...
1
vote
1answer
120 views

Connecting Open-Drain inputs or outputs together with one resistor

I had a couple of quick questions. I am designing a board using 10 IO expanders which I'll use as inputs. Each has an !INT pin which is open drain and pulled high to 5v through a resistor. Can I ...
0
votes
0answers
66 views

STM23 and TLK105 Ethernet acts strange

I am conntecting STM32F107 and TLK105 from Texas Instruments and it works fine most of the time. The problem is a very strange behavior such as: - TLK software reset is not done correctly - Reading ...
0
votes
3answers
218 views

Why use a resistor for Pull-Up? [duplicate]

I just got my first Arduino and had a question about pull-up resistors. I was trying to connect a push-button to the Arduino at pin 2, and if it's HIGH then I write HIGH to pin 13. Now, in the ...
1
vote
2answers
102 views

Is it wrong to write HIGH to a pin used as an open collector?

I'm a software engineer playing with PICs and have an old PS/2 mouse that I am using (successfully), which has CLK and DATA connections as open collectors. So I have configured the internal pullups ...
0
votes
2answers
223 views

Pull up resistor on Schmitt trigger IC input?

I have two different circuits on two different boards. The output of board A, let's say, is optocoupler based and hence, it has a pull up on it. This signal is connected to board B through an ...
0
votes
3answers
94 views

Turn pull-up into pull-down

I have come across the following problem a few times in the past: You have a break-out board where some pins have already been pulled up or down for convenience, e.g. RESET pulled high to keep the ...
4
votes
1answer
442 views

Placement/Location of pull up / pull down resistor?

I have connected a microcontroller GPIO pin, which is intended to be an output, to a DC-DC converter's active high Enable input pin. Since this pin is active high, and because I do not want this ...
4
votes
3answers
708 views

Experiment with pull up/down resistors gives unexpected results

The wikipedia article on pull-up/down resistors provides this image: along with some explanatory text: ... the circuit shown [in the picture] uses 5 V logic level inputs to actuate a relay. ...
1
vote
1answer
116 views

Pull up does not work

I got a breadboard with some logic on it. Including it's own power suppply. It is connected to my Arduino Mega 2560 to fill a shiftregister with some bits. Whenevery my arduino is in a undefind state ...
0
votes
1answer
263 views

Output of a NAND gate

Above is a TTL totem pole output NAND gate. There is a 120 ohm pull up resistor there. Since it is called pull up, can we say that the HI output will be connected to a very high input impedance? If ...
0
votes
1answer
168 views

A question on pull up resistors

I'm wondering why pull up resistors in text books are told such that they pull up the voltage to \$V_{CC}\$. Here is a diagram: When the button is open what will be the input voltage to the micro ...
0
votes
2answers
143 views

Voltage Dividers in a pull-up resistor circuit (why does the pin go to low)

I understand that when the button is in its open state the voltage on the input pin should be fairly close to \$ V_{cc} \$ provided \$ R_1 << R_2 \$ as \$ V_{in} = V_{cc} \frac{R_2}{R_1 + ...
0
votes
1answer
499 views

Arduino Mega & LIN Transceiver (MPC2004)

I am trying to connect an Arduino Mega with another device using a LIN Transceiver. I have decided to use a LIN MPC2004 Transceiver for this. I am now trying to understand the reference design of ...
0
votes
0answers
64 views

Resistor and LED Bias

My question pertains to this sketch of a project called the Quiz-O-Tron 3000. I wired up the proto-sheild and the 4 button switches. When I went back through my inspection and testing of the units, ...
3
votes
2answers
765 views

Using internal pull-ups for I2C on PIC18F

I'm working with a PIC18F46J53 and it was working quite well without any troubles. Suddenly, I found a hindrance on my path. I used RB4 and RB5 pins as I2C bus pins for communicating with an EEPROM ...
0
votes
2answers
297 views

LM339 : why output +5V

This circuit diagram below is given in my book ( written by Coughlin and Driscoll ) . simulate this circuit – Schematic created using CircuitLab Here it is said that at this condition ...
1
vote
2answers
167 views

Can a digital input having internal pull up resistor drive a load?

simulate this circuit – Schematic created using CircuitLab I am working on MC56F8006 DSC . On power up or reset the default state of all GPIOs is inputs with internal pullups enabled. I ...
0
votes
1answer
212 views

GPIO Pin configurations [duplicate]

I went through the data sheet of STM32F4 and found the pin configuration of GPIOs. I dont understand the difference between these configurations. • Input floating • Input pull-up • Input-pull-down • ...
0
votes
3answers
545 views

Can I do without I²C pull-up resistors?

I am trying to produce a small circuit, and I am looking to reduce the part count. I recently added an I²C EEPROM which is used once when the system boots up, along with this EEPROM I added two ...
1
vote
3answers
186 views

Do I need pull-down resistor on Enable pin cd74hc4051

Can I directly connect enable pin to ground? I am not going to use it. datasheet Also can I connect all unused pins of LM324 op-amp to ground through single resistor or what configuration does it ...
19
votes
5answers
12k views

When to use pull-down vs. pull-up resistors

After learning and experimenting with microcontrollers, the concept of pull-up- and pull-down resistors has been understood. I now understand when and how to use them, and how they work. I've mainly ...
1
vote
0answers
45 views

Internal pull up resistors in a micro-controller vs external pull ups [duplicate]

Micrcontrollers such as PIC describe their pull up resistors as weak pull up resistors, what does weak signify? When/why would I want to use external pull-up resistors on I/O pins that support ...
2
votes
2answers
146 views

How can I design this pull up without wasting current/performance?

I want to move the low side enable switch of my H-bridge onto the high side, because for safety reasons it's better to turn off the tap than plug the drain. I'm powering a 24V, 35W BLDC, so it's ...