100. Summary of Pull-up, Pull-down, Active Low, and Active High (Basic Concepts in Embedded Software Development)

It feels like it’s been a while since I wrote an article related to the basics of embedded systems.

Today, let’s take some time to clarify the terms ‘Pull-up,’ ‘Pull-down,’ ‘Active Low,’ and ‘Active High.’

The term ‘Pull-up’ implies that something always has a high voltage connected to it, right? (laughs)

‘Pull-down’ sounds like it means that ground is connected, doesn’t it?

MCUs are structured with pins as shown below. Depending on the purpose of the pin, it can be used as an input, where it reads voltage, or as an output, where it outputs voltage (either 0V or 3.3V).

Pull-up and pull-down are used in the input section where voltage is read.

When configuring a digital input, you can set pull-up or pull-down.

Currently, it is set to pull-up.

If the pin is open, with pull-up enabled, it will read a high state.

To set pull-down, it would be configured as follows.

In that case, it will initially read the default value of 0.

If this value is read incorrectly when the pin is in a floating (open) state, there is a risk of significant issues during the process of creating control logic. Therefore, the internal pull-up and pull-down circuits are used to eliminate this risk.

Let’s look at it from the perspective of an integrated circuit (IC). Imagine configuring a digital output in an open-drain format, where only a connection to ground is possible. From the MCU’s perspective, it can be either connected to ground or in a floating state.

When it is in a floating state, attaching a pull-up circuit to the IC ensures that the IC initially receives the voltage from Vcc (e.g., 3.3V). When the MCU subsequently grounds it, the IC receives 0V. Thus, to ensure a clear voltage in a floating situation, pull-up and pull-down circuits are necessary.

Here, I can provide additional explanation about the concepts of Active Low and Active High.

Now, let’s examine the IC.

Consider a pin on the IC that serves as the Enable pin for the IC. In this case, the Enable pin of the IC is designed to be activated by a Low input for the IC to operate properly.

It operates when it’s Low — this is known as Active Low.

‘This IC is Active Low, so remember to ground control it from the MCU for it to function properly!’

This is a common topic discussed during development.

Therefore, an IC operating in the Active Low mode typically has a pull-up circuit attached. (It’s not always a necessity, though!)

So, what about the case of Active High?

As shown below, the IC has a pull-down circuit attached by default.

In this state, the IC is disabled.

However, when the MCU sets the Digital Output to High,

3.3V is supplied to the IC,

and the IC starts to operate.

It needs to be High for the IC to function.

This IC is considered Active High.

Here’s an example:

The current TC275 MCU includes a built-in CAN transceiver IC (TLE9251V).

To achieve Normal Operation and ensure proper functioning of the CAN transceiver,

the STB (Standby) pin needs to be pulled Low.

It seems that internally, this chip has VIO pulled up. (laughs)

And when the MCU pulls this pin low,

the CAN transceiver IC operates normally.

In this case, we can conclude that this transceiver operates as an Active Low IC.

In summary,

Pull-up, pull-down, active low, and active high have been outlined in a general sense.