On the necessity to add pull-down resistors to ICs - Page 1
Forum section: electronics / beginners

author: Watth
author_rank: Frequent Contributor
timestamp: October 12, 2016, 03:18:39 pm
content: Considering logic inputs (and outputs) of ICs, I was wondering if I really need to add pull-down resistors.Is there something to look for in the datasheets?It might depend on the source of the signal, (a µCU, or a mechanical switch).Or, to be sure, and especially if the source of the signal is not definitive, is it advisable to add one anyway ?

author: Audioguru
author_rank: Super Contributor
timestamp: October 12, 2016, 04:05:25 pm
content: The datasheet of a logic IC will show the calculation of pullup or pull down resistors and if they are needed.I have never seen an output pull-down resistor but a pullup resistor is needed if the output is open-collector.Old TTL logic had a high input current that needed a low value pull-down resistor to make it a logic low if it is not driven low.Cmos picks up all kinds of interference if it has a floating input.

author: MosherIV
author_rank: Super Contributor
timestamp: October 12, 2016, 04:58:19 pm
content: QuoteWith microcontrollers, consider what would happen if the micro fails to start ?What would the outputs of the micro do (usually they will be in high impedance mode) to control circuits down stream ?I have seen a real world project where a system re-boot caused the microcontroller to reboot but not start.One of the micro outputs was driving a motor, and the motor was pushing toner powder into the image drum toner transfer hopper.After running for the whole weekend like this, it completely emptied the toner all over the office !Glad I did not have to clean up the mess(We never determined why the micro did not re-start, we could not re-create the failed re-start )

author: w2aew
author_rank: Super Contributor
timestamp: October 12, 2016, 08:06:45 pm
content: Quote from: Audioguru on October 12, 2016, 04:05:25 pmPull-down resistors are needed on the outputs of ECL / PECL logic, since these are emitter follower outputs. Although, often a split-termination is used to provide a match to 50 ohms, and a thevenin equivalent termination voltage of VCC-2V.

author: sahko123
author_rank: Frequent Contributor
timestamp: October 12, 2016, 09:20:36 pm
content: In my experience I have found that pull down or ups are used to have a set parameter or to eliminate high impedance which can cause unwanted and unpredicted out comesHope this hekped .

author: Brumby
author_rank: Unknown
timestamp: October 13, 2016, 01:00:06 am
content: "On the necessity to add pull-down resistors to ICs" - it is simple. The circuitry after the pull-down (or pull-up) resistor needs a clearly defined stateat all timesin order to operate consistently and reliably.But why?To understand where this came from, let's go back to a time when this was not a problem in a classic garden-variety type of IC - the early 7400 series of TTL.The output of these chips was either a logic high or a logic low - there was no 'in-between' or 'undefined'. This made cascaded stages easy to design and construct since outputs were connected directly to inputs, with no ambiguity for the following circuitry. But it had limitations. For example, you could not directly parallel outputs and the fan-out was limited to 10.The solution which first showed itself in CMOS chips (like the 4000 series) is where some high impedance interface circuitry was introduced, including a third state for outputs. It wasn't a high or low logic level - but more like an open circuit. This feature is known as Tri-State. Such chips offered this high impedance state as selectable - it could output a defined logic stateorit could be made to be effectively disconnected. This was great. You could now have a dozen chips wanting to drive the same circuit and there would be no problems - you just select the chip that was to take control and put the rest into the high impedance state.With a high impedance on the inputs to the gates, the fan-out went from 10 to, quite literally, thousands! This meant you could pretty much forget about this limitation when working within this family of components.BUT ... now we can have a problem...........With a high impedance input, if it is not connected to anything or is connected to an output that is in a high impedance state, there is NO real influence on the voltage of this input that is directly controlled by the circuit. It will, in fact, pick up any stray EMI, leakage current or possibly static in a completely unpredictable manner. This is where you need something like a pull-up or pull-down resistor - so the (logic) state is defined.

author: theenggprojects
author_rank: Newbie
timestamp: October 13, 2016, 01:05:40 am
content: It totally depends on your system. For example, in your project you want HIGH output then you should add the Pull Down resistance so now when you get the HIGH output from microcontroller you will do your work and when the output is not coming from microcontroller then this PULL down resistance will simply make it LOW. Similarly, if you are working on inverse logic then you should PULL UP the output.

author: Jeroen3
author_rank: Super Contributor
timestamp: October 21, 2016, 09:52:56 pm
content: Quote from: sahko123 on October 12, 2016, 09:20:36 pmSome guy at work once made relay controller. With 32 relays and some transistors. If the card boots, the relays do "rrrt". If the chip doesn't boot, as happens when supply voltage is within 90-100 Vac, the relays are semi-switched on. You hear rrrrrrrt, and they aren't on, but they aren't off either. Relays with load burn out due to contact resistance.Turns out, there was no pull down on the transistor gate or mcu pin.Always have the PCB define a safe pin state. And not the software. Because there are a dozen reasons the sofware might not work.- The software isn't loaded yet.- The jtag is attached.- The supply voltage isn't high enough. (high enough to start, source too much, and drop)- The crystal is damaged.- The flash get corrupted.- The chip is too hot.- The bootloader failed. (customer calls: "my unit started smoking during firmware upgrade, is that normal?")...