When designing an ECU (electronic control unit), have you ever struggled with choosing the pull-up and pull-down resistance values for IC input TERMINALS?
This article explains how to determine the pull-up and pull-down resistance values for IC input TERMINALS in an easy-to-understand manner, even for beginners, along with knowledge that will be useful in actual design situations. Following on from the previous article, which introduced "Why TERMINALS processing is necessary," this time we will delve into the practical topic of "selecting resistance values."
Previous article: Don't fail in ECU design! Learn stable design through IC input TERMINALS pull-up/pull-down procedures and experiments
Effect of pull-up and pull-down resistor values on the circuit
The pull-up and pull-down resistors attached to IC input TERMINALS have a role beyond simply processing TERMINALS.
For example, if the resistance value is too high, power consumption will be reduced, but the device will be vulnerable to external noise, and the potential of TERMINALS will easily change due to small amounts of static electricity or leakage current.
Conversely, if the resistance value is too small, the circuit will be more resistant to noise, but the current that flows when SWITCHES is turned on will increase too much, and there is a risk that it will exceed the allowable current of the IC or board.
As such, it is important to strike a balance between stable operation and safety when selecting pull-up and pull-down resistor values.
How to determine and calculate resistance values
When selecting pull-up and pull-down resistor values for an IC input TERMINALS, the first basic principle is Ohm's law.
Ohm's Law
Here, V is voltage (V), I is current (A), and R is resistance (Ω). From this formula, when the voltage V is constant, increasing the resistance R will decrease the current I that flows.
When the resistance value is extremely large
For example, if you set the pull-up resistor value to a very large value of 100MΩ, when POWER SUPPLIES Vcc is 5V and SWITCHES is open,
Only current can flow.
If the pull-up resistor value is too large, almost no current will flow.
This current is equivalent to "almost no current flowing" to the IC, and the voltage at the input TERMINALS becomes susceptible to external noise and static electricity, resulting in an unstable state.
In other words, if the resistance value is too large, even if you think you have pulled up TERMINALS voltage, it will not actually be stable.
When the resistance value is extremely small
Conversely, if the resistance is set to an extremely small value of 0.1 Ω, when Vcc is 5 V and SWITCHES is closed,
A large current flows.
This is far beyond the wiring rating of a normal printed circuit board (approximately 1A per 1mm width), and in the worst case scenario, the board may short-circuit, causing damage or fire.
If the pull-up resistor value is too small, it will almost become a short circuit.
The same applies to pull-down resistors.
The same idea applies to pull-down resistors.
As such, if the pull-up and pull-down resistor values are too large or too small, the IC will not operate correctly and stably. By selecting the appropriate resistance values, you can prevent malfunctions caused by noise and static electricity, and prevent damage to components and boards caused by excessive current.
Selection points at actual design sites
Resistance values commonly used in design
Resistance values of around 10 kΩ to 100 kΩ are often used in design, but the optimum value varies depending on the application and IC specifications. Generally, a slightly larger value is chosen if you want to reduce power consumption, and a slightly smaller value is chosen if you prioritize noise resistance.
We have compiled a list of pull-up and pull-down resistor values commonly used for LOGIC ICs and general digital inputs, categorized by application and usage environment.
Commonly used pull-up and pull-down resistor values for different applications and environments (for LOGIC ICs and general digital inputs)
| IC applications and usage environments | Commonly used pull-up and pull-down resistor values | comment |
|---|---|---|
| General-purpose (balanced type) | 10kΩ to 100kΩ | Best balance between noise immunity and power consumption. Most common in practice. |
| Slightly noisy environment | 4.7kΩ to 10kΩ | When you want a strong effect. A standard EMI countermeasure. |
| Very noisy environment | 1kΩ to 4.7kΩ | Use in environments where noise is severe. However, be careful of the increase in current. |
| Power saving (battery operated) | 47kΩ to 100kΩ | Reduces current consumption through resistors. Suitable for ICs with low leakage such as CMOS inputs. |
| No particular restrictions (general CMOS LOGIC) | 10kΩ to 100kΩ | CMOS inputs have very low leakage and are therefore used in a wide range of ICs. |
Consider the input capacitance and rise time of the IC
When selecting pull-up and pull-down resistor values, you must also consider the input capacitance and rise time of the IC. If high-speed signal transmission is required, using resistance that is too large will cause signal delays and lead to timing problems, so you must also take the input capacitance and rise time into consideration.
Furthermore, as explained in the previous chapter, it is important to avoid extreme values when selecting resistor values.
So what is the appropriate resistance value?
The important thing here is the IC datasheet, which contains the voltage required to determine whether the input TERMINALS is high or low, the allowable leakage current, the input capacitance, etc. Using this information, you can calculate the resistance value.
For reference, the input high level threshold and input leakage current specifications are defined as follows in the datasheet for CMOS LOGIC ICs:
| Symbol | Parameter | Condition | Min | Max | Unit |
|---|---|---|---|---|---|
| Vih | High level input voltage | Vcc = 4.5 to 5.5 V | 0.7 * Vcc | - | V |
| Iin | Input leakage current | Vi = 5.5V or GND; Vcc = 0 ~ 5.5V | - | ±1 | uA |
IC datasheet (excerpt)
How to determine the upper limit of the pull-up resistor from static conditions
First, determine the upper limit of the pull-up resistor from the static electrical characteristics information in the datasheet. The upper limit of the pull-up resistor is determined mainly based on whether the IC input TERMINALS is reliably recognized as a high level (logical high) and whether it is resistant to noise and disturbances.
Basic Concept
-
1. Consider the input leakage current of the IC
Check the maximum input leakage current I IN(max) listed on the IC datasheet. -
2. High level threshold V IH(min) must be met
To ensure that an input TERMINALS is recognized as high, a sufficient voltage must be applied to the input TERMINALS through the pull-up resistor. -
3. Calculation formula
Using Ohm's law, we can find the upper limit as follows:
Vcc: POWER SUPPLIES voltage
V IH(min): The minimum voltage that the IC recognizes as high
I IN(max): Maximum input leakage current
Calculation example
As an example, let's consider the pull-up resistor required when the electrical characteristics of an input TERMINALS are "POWER SUPPLIES Vcc = 5V, V IH(min) = 0.7 x Vcc, I IN(max) = 1uA." V IH(min) is the high voltage threshold, the input voltage at which the input TERMINALS can reliably determine a high level. Since characteristics change depending on the Vcc voltage range, a value appropriate for the Vcc voltage is used. I IN(max) is the maximum input leakage current, a small amount of current that flows even when it should not due to the internal structure of an IC input TERMINALS. If an input leakage current of I IN(max) flows through an input TERMINALS with a pull-up resistor, let's calculate the resistance value R max so that the input voltage becomes V IH(min).
From the calculation results, we can see that if the pull-up resistance value is set to a maximum of 1.5 MΩ, the IC will recognize the input TERMINALS as High.
Calculate the pull-up resistor value from the leakage current and high-level threshold
How to determine the upper limit of pull-up resistors based on dynamic conditions
The upper limit of the pull-up resistance, R max 1.5 MΩ, calculated from I IN(max) and V IH(min), is merely the upper limit for determining a High signal under static conditions. However, in an actual circuit, the input TERMINALS has input capacitance C in, which causes the signal to rise slowly. When selecting a pull-up resistor, the RC time constant (τ = R∙C in), which determines the signal rise time and is combined with the input capacitance C in, is an important factor.
The voltage at the input TERMINALS with a pull-up resistor does not go high immediately when SWITCHES is opened, but rather rises slowly according to the RC charging curve.
If this ramp-up is too slow, the following problems will occur:
- The signal rises slowly, and it takes time for the IC to determine the logic high value.
- Noise is easily introduced, which can cause false judgments
- High speed signals can cause timing issues
Generally, it takes about 3 to 5τ for a digital input to be recognized as stable and high. Most microcontrollers and LOGIC ICs have an input capacitance of about 5 to 20 pF.
Here, we use a typical value of C in = 10 pF and calculate the signal rise time when the pull-up resistor value is changed from 1 kΩ to 1 MΩ.
| Resistance value | time constant | |
|---|---|---|
| R | τ | 5τ (almost stable) |
| 1 MΩ | 10 µs | 50 µs |
| 100 kΩ | 1 µs | 5 µs |
| 10 kΩ | 0.1 µs | 0.5 µs |
| 1 kΩ | 0.01 µs | 0.05 µs |
Relationship between resistance and time constant
A 1 MΩ pull-up resistor takes 50 µs to rise, making it unstable in noisy environments or with high-speed signals. On the other hand, a 10 kΩ resistor stabilizes in around 0.5 µs, providing a sufficient response time for many ECUs and digital circuits.
The upper limit of the pull-up resistance, Rmax, calculated from I IN(max) and V IH(min) was 1.5 MΩ, but when the rise time due to the RC time constant is taken into account, the practical upper limit is around 10 kΩ to 100 kΩ. In particularly noisy environments, making the input TERMINALS impedance too high makes it more susceptible to the effects of external noise, so a value of around 1 kΩ to 10 kΩ is often chosen to provide some leeway.
How to determine the lower limit of the pull-up resistor
The lower limit of the pull-up resistance is determined mainly based on the criterion that the current that flows when SWITCHES or IC is turned ON (low) does not exceed the allowable value of the IC or SWITCHES.
If the resistance value is too small, the current that flows when SWITCHES is turned on will be too large, and there is a risk that it will exceed the ratings of the IC, SWITCHES element, and even the wiring on the printed circuit board.
Basic Concept
-
1. Check the allowable current of ICs and SWITCHES
Check the datasheet for items such as "maximum input current" and "maximum sink current." -
2. Calculate the lower limit of the resistance value from the maximum current
Ohm's Lawis used to calculate the following formula:
Calculation example
As an example, let's consider the pull-up resistance R min required when the electrical characteristics of the input TERMINALS are "POWER SUPPLIES Vcc = 5V, I max = 5 mA." I max is the allowable TERMINALS current, the maximum current that can flow into the IC input TERMINALS, and if more current flows than this the IC will be damaged.
Let's calculate the resistance value R min.
In this case, if R min is set to a resistance value less than 1 kΩ, a current exceeding 5 mA will flow when SWITCHES is ON, increasing the risk of damage to the IC and SWITCHES. The current rating of the wiring on the printed circuit board must also be taken into consideration. OTHERS, power consumption will also increase, so if the resistance value is too small, it will cause heat generation and power loss.
Set the pull-up resistor within the upper and lower limits.
It is important to set the pull-up resistor value within the range of two values: the lower limit determined from the allowable current of the IC and SWITCHES, and the upper limit determined from the input leakage current and high-level threshold.
Within this range, both stable circuit operation and safety can be achieved. When translating into actual values, a trade-off between the required system specifications and safety design is determined.
As such, if the pull-up/pull-down resistors are too large or too small, they will not operate stably.
In actual design work, you should keep these things in mind when reading data sheets.
- IC specifications (determination voltage, leakage current, input capacitance, etc.)
- Circuit use and operating environment (presence or absence of noise sources, wiring length, etc.)
- Verification through experiments and simulations
It is important to select the resistance value by taking all of the above into consideration.
Select the optimum resistance value by considering not only the recommended value on the data sheet but also the actual circuit operation and surrounding environment.
Summary
Selection of pull-up and pull-down resistor values for IC input TERMINALS is the basis for stable ECU design.
By understanding "why a particular value is necessary" and "in what situations it is useful," and then selecting the optimal value from both a theoretical and practical perspective, you can prevent malfunctions and the effects of noise, and achieve a highly reliable circuit design.
Please refer to the contents of this article when carrying out future design work or selecting products.
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