For those working on ECU design for the first time. Leaving IC input TERMINALS unused can cause unexpected malfunctions and noise. Many people use resistors on input TERMINALS based on other circuits, but may be doing so without knowing why they are necessary. Understanding the meaning and importance of TERMINALS treatment will enable more stable designs. This article explains why TERMINALS treatment is necessary, explains the basics of pull-up and pull-down, and includes actual experimental results in an easy-to-understand explanation even for beginners.
Why is it necessary to leave IC input TERMINALS unused?
In ECU design, leaving IC input TERMINALS unused can cause unexpected malfunctions and noise. CMOS input TERMINALS, in particular, have unstable potentials when left open, making them susceptible to external noise and static electricity. Properly treating unused TERMINALS can ensure stable operation and noise immunity. In fact, failure to treat TERMINALS can result in LEDs lighting up unstably or ICs failing to correctly recognize Hi/Low signals. While these types of malfunctions are often overlooked during the design stage, they can also lead to problems after mass production, so it's important for even beginners to understand the importance of TERMINALS treatment.
Pull-up and pull-down basics
Not all IC input TERMINALS necessarily require pull-up or pull-down. Depending on the IC's specifications, pull-up and pull-down resistors may be implemented internally, and in some applications, there may be no problem if they are left unconnected. When designing, carefully check the datasheet and circuit diagram of the IC you are using, and add external resistors as needed to perform pull-up or pull-down operations. Resistance values generally range from 1kΩ to 10kΩ, but you should also consider noise resistance and power consumption when selecting a resistor. Having the right knowledge and treating TERMINALS according to the situation will lead to a stable ECU design.
Pull-up connects the input TERMINALS to the Vcc (POWER SUPPLIES) side with a resistor and normally maintains a high level, while pull-down connects the input TERMINALS to the GND (ground) side with a resistor and normally maintains a low level.
The following diagram shows an example circuit and the IC input TERMINALS voltage when pull-up and pull-down resistors are added to the input TERMINALS of an IC. With pull-up and pull-down resistors, the voltage level of the IC input TERMINALS remains stable regardless of the ON/OFF state of SWITCHES.
Circuit and IC input TERMINALS voltage when pull-up and pull-down are present
On the other hand, if there are no pull-up or pull-down resistors, the IC input TERMINALS will be "open" when SWITCHES is off, and will be in a floating state with no electrical connection. In this case, the potential (voltage) of the input TERMINALS will be unstable (undefined), making it susceptible to the influence of weak currents and voltages such as external noise and static electricity from the human body.
Circuit and IC input TERMINALS voltage when there is no pull-up/pull-down
Why does the voltage at the input TERMINALS become unstable?
"Unstable" refers to an indeterminate state where the potential of the input TERMINALS is neither 0V (Low) nor POWER SUPPLIES voltage (Hi), but cannot be determined as either. In this state, the IC's internal circuits do not function properly, making it susceptible to malfunctions and noise. When an input TERMINALS is in this open state, TERMINALS potential fluctuates to unpredictable values due to surrounding electromagnetic waves, static electricity, leakage current from other circuits, etc. The IC will mistakenly interpret this as "Hi" or "Low," causing unintended behavior such as flickering LEDs. To ensure stable operation, it is important to clarify TERMINALS potential by providing pull-up and pull-down as necessary.
Experiment: Comparison of behavior with and without pull-up/pull-down
Creating an IC input TERMINALS mock circuit
We conducted an experiment to compare behavior with and without pull-up/pull-down. To make it easier to understand visually, we created a simulation circuit that uses an LED to indicate the potential state of the IC input TERMINALS. In this IC input TERMINALS simulation circuit, a green LED lights up when a high signal is input to the input TERMINALS, and a red LED lights up when a low signal is input.
| MOS FETS | 2N7000 |
|---|---|
| Green LED | OSXX5154A-VV |
| Red LED | OSR6LU5B64A-5V |
Parts List
An IC input TERMINALS simulation circuit that displays the input TERMINALS potential state with an LED
Definition of Hi/Low for input TERMINALS and key points for stable operation
Some books describe the state when POWER SUPPLIES voltage is applied to the input TERMINALS as Hi, and the state when 0V is applied as Low. However, this description alone does not fully explain why the potential becomes unstable when the input TERMINALS is "open," that is, when it is not connected to anything.
The Hi/Low state of an IC's input TERMINALS is determined by whether TERMINALS 's potential exceeds the IC's internal threshold, but the "open" state can be better understood by imagining the current flowing through the input TERMINALS. In the Hi state, current flows from POWER SUPPLIES (Vcc) to the input TERMINALS. In the Low state, current flows from the input TERMINALS to GND.
*In IC datasheets, the Hi threshold is described as a voltage range used to determine the Hi/Low state of the input TERMINALS signal, with specific values based on POWER SUPPLIES voltage, such as "Vih = 0.7*Vcc or higher" and "Vil = 0.3*Vcc or lower."
Without pull-up and pull-down, the input TERMINALS becomes open, the current flow is interrupted, and TERMINALS cannot maintain a stable potential, making the input TERMINALS voltage unstable. If a small current such as external noise or static electricity is applied to TERMINALS in this state, the potential of the input TERMINALS will easily fluctuate, and the IC will not be able to determine whether this unstable potential is high or low, which can cause it to malfunction.
On the other hand, if a pull-up or pull-down is installed, a constant, albeit weak, current always flows "from POWER SUPPLIES to the input TERMINALS" or "from the input TERMINALS to GND" via a resistor, allowing TERMINALS to maintain a stable potential.
In this way, if the pull-up and pull-down resistors are installed correctly, the current required to stabilize TERMINALS potential can be ensured even when the input signal is OFF, preventing malfunctions due to external noise or static electricity and achieving stable operation.
Image of the current flowing through the input TERMINALS
Experiment by adding pull-up and pull-down to the input TERMINALS simulation circuit
I created a circuit that added SWITCHES and pull-up/pull-down to the IC input TERMINALS simulation and checked the behavior of the LED when SWITCHES is on/off.
With pull-up, the green LED lit up (Hi input) when SWITCHES was off, and the red LED lit up (Low input) when SWITCHES was on, showing the expected behavior. With pull-down, the red LED lit up (Low input) when SWITCHES was off, and the green LED lit up (Hi input) when SWITCHES was on, showing the expected results as well. This is an example of how pull-up and pull-down allow the input TERMINALS to maintain a stable potential even when SWITCHES is off.
Experiment with the input TERMINALS simulation circuit, excluding the pull-up
How would the LED behave if there were no pull-ups or pull-downs?
We added only SWITCHES on the GND side to the IC input simulation circuit, created a circuit without a pull-up, and confirmed its operation.
Without pull-ups, the LEDs lit unstably regardless of button operation, and simply touching the input TERMINALS with a finger caused the brightness of the green and red LEDs to change unpredictably. In the example photo above, when SWITCHES is OFF and the input TERMINALS is touched with a finger, only one of the green LED (high input) or red LED (low input) should light up, but both LEDs light up at an incomplete brightness. This is because the input TERMINALS is open, and external noise and static electricity from the human body cause the potential to fluctuate. This same unstable behavior occurs when pull-downs are not implemented. These experimental results demonstrate that the presence or absence of pull-ups and pull-downs is directly related to the stable operation of IC input TERMINALS. Even beginners should try this experiment to experience its importance.
Acts as a current limiting resistor
Pull-up and pull-down resistors are not only used to prevent input signals from becoming "unstable," but are also sometimes used to limit and protect the current flowing through the printed circuit board. For example, if the resistance value is too low when SWITCHES is turned on, excessive current will flow through the printed circuit board, posing the risk of fire in the worst case scenario. Conversely, if the resistance value is too high, it will not be possible to ensure sufficient current to stabilize the potential of the input TERMINALS, reducing noise immunity and making it more susceptible to malfunctions caused by minute external noise.
Current limiting resistor example
Current limiting resistors generally have resistance values of around 1 kΩ to 10 kΩ, but the optimum value will vary depending on the circuit specifications and the IC being used. When designing, it is important to always check the IC datasheet and reference circuit to select an appropriate resistance value. When selecting a resistance value, consideration should be given to a comprehensive range of factors, including power consumption, noise resistance, and the IC's input current tolerance.
Pull-up and pull-down resistors are not just for TERMINALS processing; they also contribute significantly to the longevity and stable operation of ICs. Selecting the correct resistance value will improve the reliability of the entire circuit.
Summary
Leaving IC input TERMINALS unused is essential for stable ECU design. By properly selecting and placing pull-up and pull-down resistors, you can prevent malfunctions and the effects of noise.
Even beginners can experience its importance through experiments and use it in their future designs.
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