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

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56 views

How to get the full Vcc output from comparator?

I am working on a circuit using a comparator for a school project, but in testing it is not acting as expected. Specifically, the output is not going all the way to the higher rail voltage attached ...
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6answers
1k views

What happens if I omit the pullup resistors on I2C lines?

Just now I realized that the I2C data and clock lines (SDA and SCL) must have pullup resistors. Well, I've built a couple of clocks using the DS1307 RTC (see datasheet) according to the schematic ...
1
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2answers
80 views

SPI and pullups. Is it ever used in open collector configuration

Is SPI ever wired as open collector in devices calling for the use of pullup resistors? Everything seems to point to it usually being totem pole but I wonder if there are exceptions. The only pullup ...
1
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1answer
96 views

How do you size pull-up/down resistors? [duplicate]

The attached photo is pulled from a Piccolo DSP reference design from TI. They use 57.6k pull-up resistors and 2.2k pull-down resistors to allow configuration of boot pins at startup. I've seen all ...
2
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2answers
115 views

Convert microcontroller 3.3V output to 6V

I have built an h-bridge from 4 MOSFETS (PHP21N06LT) shown below. This bridge is used for motor speed control in either direction and fast stop. I have a dev board with C8051F020 microcontroller on ...
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1answer
41 views

Detect current on intercom

I'm trying to get an input from an intercom and apparently there is GND and another wire for the call. I'd like to know when there's voltage between the call wire and the ground. I'm thinking of ...
3
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5answers
136 views

How much current is used with a μC GPIO input and pullup resistor?

I'm trying to understand power budgets a bit better and I have a question regarding pullups. I have a input to a microcontroller that needs to be logic high at all times. The micro goes to sleep now ...
-1
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1answer
70 views

how to, best way to use a linear actuator vs gear system? [closed]

I am wondering what is the best and cheap option to use in a crane like system. a linear actuator or a gear rig system with an engine. the weight at the end could be up to 100 pounds. I was ...
0
votes
1answer
33 views

GPIO pull up to different level

I am using MSP430F235 which is supplied with 2.5V on DVCC. So, all its I/Os will be in 2.5V level. Now, I want to enable a LDO in which EN pin is tied to a different level (>2.5V is the battery supply ...
2
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4answers
94 views

Pull up circuit current flows?

Im trying to understand how pull up/down circuits works . Let's consider this schematic : I understand that there are two possible current flows : When the button is not pressed, the current ...
3
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1answer
115 views

When a pin needs to stay LOW or HIGH, is it okay to connect directly to VCC or GND?

For example, I'm using an Exar Exar SP3220EBCY-L RS-232 transceiver and the typical application loop back test shows that the EN pin is connected to GND and the SHDN pin is connected to VCC. Should I ...
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1answer
53 views

CMOS 4066 pull up resistor

I'm controlling three 4066 (Quad Bilateral Switch) with two chained 74HC595 (8-bit serial-in, serial or parallel-out shift register with output latches; 3-state) Do I need to use pull up resistor ...
2
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1answer
74 views

I2C Pull-ups with Multiple Power Supplies

If I have an I2C Master with multiple I2C Slaves all on separate 3V3 lines (i.e. 3v3_Master, 3V3_Slave1, 3V3_Slave2... etc.) does it matter which 3V3 line the pull-up resistors on the line connect to? ...
1
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0answers
379 views

How to assign a pull-up/down resistor in Verilog for inputs?

As a newbie in FPGA world, I realized that it is possible to set pull-up/down resistors in Verilog but I don't know how. I have written my code that works just fine but when I connect my XC3S400 to ...
1
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3answers
151 views

How does this MOSFET circuit work?

The following circuit from a textbook models an inverter implemented by a MOSFET switch. I understand how in the first case Vout is zero, but in the second case(the switch is open), the textbook ...
1
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4answers
313 views

i2C : pull-up resistors “design pattern”, Shielded cable and connector?

I'm designing an arduino based board which will uses I2C port a lot. I want to be able to connect several slave devices, and/or connect several mother boards together (at least 4 would be great). The ...
1
vote
1answer
174 views

Arduino serial communication RX high

I'm trying to interface with a device via async serial. Here is the power on requirement from the device datasheet: Power-On Reset Requirements When applying power to the DNT900, the /RESET Pin ...
0
votes
1answer
105 views

LM311 Pull Up Resistor

If I use an LM311 Comparator, then we are required to connect a pull up resistor at the output. Does the square wave output depends on the value of the voltage the resistor pulls up to? Would it be ...
1
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2answers
122 views

Why is pull-up resistor disabled in OUTPUT mode

I noticed in a lot of micro datasheets that the internal pull-up resistor is disabled automatically when the GPIO is in OUTPUT mode, but I'm not sure I understand why. Is a pull up resistor never ...
1
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1answer
89 views

pin is pulled up to something

I was reading 17xx series` data sheet, and I had trouble understanding a specific explanation given for a major number of pins: 5 V tolerant pad providing digital I/O with TTL levels and ...
4
votes
0answers
243 views

How should I alter my PISO shift register circuit so that it works with 3.3v instead of 5v?

I've followed this Arduino guide to build a circuit using a CD4021BE PISO shift register, and 10k pulldown resistors. It works perfectly fine with the Arduino's 5v supply. I then attempted to use ...
1
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2answers
141 views

Basic misunderstanding about dropping down voltage going in a TTL IC using resistors

I'm having serious problems figuring this out. Below is a part of an 8-bit computer schematic. The 62256-70 is a static ram chip. I understand these 8K2 resistors are there to provide a default value ...
3
votes
3answers
141 views

Is it common for internal pull-up resistors to fail? or what would cause them to become intermittent?

I have a board based on an ASIC ARM Cortex-M3 that after months of work suddenly started to report spurious button presses. The ASIC is not our design, but a reputable company's. The buttons ...
8
votes
1answer
211 views

Pull Up and Pull Down Resistor Values [duplicate]

I usually use 10K Ohm resistors for pulling up or pulling down micro-controller lines. This is only because I have tons of them. Recently while looking up pick and place surface mount information I ...
1
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1answer
354 views

ATTiny45 Quadrature Decoder Pull-Up resistors

I am trying to use the ATTiny Quadrature Decoder from here. The schematic that I have attached here has two external 10K Pull-up resistors. I have looked into the ATTiny45 datasheet and it seems that ...
4
votes
2answers
279 views

How to deal with multiple pull-up resistors on modules

I was thinking about doing a modules for a development board I did a few days ago that will include spi/i2c interfaces. The problem is that I didn't included pull-up resistors in the MCU pins in the ...
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2answers
133 views

How and why is “floating input” a valid state for RF/IR encoder IC's?

RF/IF encoder/decoder IC's such as this one, will accept the address-pin in one of 3 states: Floating High Low As per this excellent answer, I believe "Floating" input can assume either High or ...
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votes
2answers
1k views

I2C pull-up resistors calculation & where to place them

I'm connecting a PIC18F46K22 (master) to a PIC18F4620 (slave) using I2C. The wire between the devices is ~30cm. I'm using 400kHz without slew rate control. Later on, I'll add another 18F4620 on ~5cm ...
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votes
5answers
477 views

Connecting push button to a controller pin

This problem is decades old but I do not see a direct answer to it on this site. I want the following: This circuit just allows to check the state of the button from inside the micro-controller. ...
5
votes
2answers
158 views

i2c external pullup kills communication

I am using an MCP23017 (which shouldn't matter tho) to extend the number of GPIO pins of an arm based linux computer using i2c-dev bitbanging. It all works fine with wires of a few centimetres. The ...
5
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1answer
244 views

i2c on different logic level devices

I'm having a problem making LPC2148 work with SRF10 sensor. LPC is 3.3v device with 5v compatible i2c (at least user docu claims that). On the other side there is SRF10 device which is 5v. I have ...
8
votes
1answer
256 views

Weak internal pullups on microcontrollers and EMI susceptibility

What disadvantages can I run into when using weak internal pullups (100k) on microcontrollers? I'm wondering how susceptible lines (with only parasitic trace/component capacitance) become to EMI ...
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votes
5answers
2k views

Pull-up and Pull-down Resistor Usage on Input or Output MCU Pins

Are pull-up/down resistors (whether internal or external) only needed for MCU INPUT pins? In contrast, an MCU pin configured as an OUTPUT "knows what level it's at" because it does the driving - a ...
4
votes
3answers
312 views

DS1307-breakout-board

I want to build an RTC clock for the Raspberry Pi. Adafruit has a guide to accomplish this with Dallas DS1307. On the schematic they put a 2.2K resistor between VCC and SCL and do the same for VCC and ...
6
votes
2answers
1k views

I2C pullup resistor calculations

I am trying to drive this I2C LCD display with an ATxmega16A4U microcontroller. Both devices are listed to work with I2C clock frequencies up to 400 kHz. The two devices are the only devices on the ...
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2answers
696 views

Heuristic explanation of the usage of a pull-up (or pull-down) resistor needed

First, I acknowledge that there are several questions regarding this topic in the forum, however, the answers assume too much background knowledge of electronics to be of use to a true beginner (like ...
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vote
1answer
248 views

i2c problem on LPC2148

I have problem with getting my LPC2148 to work with i2c devices. LPC is on 3.3v bus and i2c devices are on 5v bus with pull-up of 3k6 ohm on either 3.3v or 5v (jumper selection). What im experiencing ...
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2answers
464 views

Must IO pins each have their own pull-up/down resistor?

A very basic question: Must each IO pin of a microcontroller that is serving as an input from a switch or jumper have its own pull-up/down resistor? In this example, each pin is pulled down by a ...
1
vote
3answers
257 views

I2C - Where's the ACK

I'm working on I2C master driver for interfacing a particular peripheral. The problem I have, is that when the 9th clock arrives, the slave device doesn't pull down the SDA line. I checked the ...
3
votes
3answers
589 views

Choosing a transistor and resistor for PIC input pin protection from Car Power

Further to my previous question, Using a PIC to perform switch on and timed shutdowns in automotive environment I've been pointed at zener diodes and voltage dividers, but I'm now confused by the ...
2
votes
1answer
319 views

Need of External Pull Up/Pull Down for Processor I/O pins

For a particular I/O pin, its been mentioned in the processor data manual that there is Internal pull down, after power on reset, in the device with a default drive state of the pin as tristate(Z). I ...
3
votes
1answer
182 views

pull up resistor issue?

I have three i2c devices connected to AVR microcontroller. On the i2c bus on master side there are two pull ups with 2k7 ohm. What im experiencing is that when i power up the microcontroller devices ...
0
votes
1answer
1k views

TLC 5940 PWM driver blank input and pull-up resistor

I've read some of the data sheet for the TLC 5940 and it states the following about the BLANK input: Blank all outputs. When BLANK = H, all OUTn outputs are forced OFF. GS counter is also reset. ...
3
votes
4answers
2k views

Pulldown resistor on output pin, how does output still work?

On a part I'm working with, it says that you can configure the part to operate in a particular mode by "strapping" a particular pin to ground. That is, it wants you to use a pulldown resistor to pull ...
2
votes
3answers
3k views

Why is this MOSFET's “pullup” resistor necessary?

I'm reading an basic electronics textbook, the chapter on MOSFETs, and it has started with a simple model of the MOSFET as a switch (the "S model"). It shows a circuit like this: And says: Here we ...
1
vote
1answer
326 views

Pulling a pin to 12 V using 3.3V

I'm building a boost converter and want to implement a shutdown feature. The particular chip I'm using, LM3478, has a shutdown feature which is activated by a high level on the frequency adjust pin. ...
9
votes
1answer
2k views

Plugging a LDR into GPIO pins of a Raspberry Pi

I want to connect a LDR into the GPIO pins of my Raspberry Pi, I know that Raspberry Pi doesn't have an analog-to-digital converter so what I want to do is to signal a HIGH signal (3.3v) on the GPIO ...
2
votes
4answers
467 views

Sharing a pull-up resistor

I have two IC chips. On one chip I have one 10K ohm pull-up resistor and on another chip I have three 10K ohm pull-up resistors. At first I was using four through hole pull-up resistors. Then I ...
5
votes
3answers
808 views

Why would an input pin have both a pull-up and pull-down resistor?

In his answer detailing the various types of input pins, Russell McMahon leaves the following note [referring to input pins]: there are special cases where a resistor to high and low at once is ...
4
votes
3answers
5k views

Logic Conversion 3.3V to 5V (Current Related) 74HC 74HCT

A beginner style question. I'm attempting to perform some logic level conversions on 3.3V signals to 5V using the 74HC4050D powered at 3.3V and 74HC541 powered at 5V. Is it OK to directly connect the ...