This is sometimes referred to as noise margin. You will also notice that there is cushion of 0.7 V between the output of one device and the input of another. The minimum input HIGH voltage (V IH) is 2 V, or basically any voltage that is at least 2 V will be read in as a logic 1 (HIGH) to a TTL device. Basically, this means that output voltage of the device driving HIGH will always be at least 2.7 V. You will notice that the minimum output HIGH voltage (V OH) is 2.7 V. So if an active-high input is NOTTED, then it is now active-low. When something is NOTTED, it changes to the opposite state. The line is used to represent NOT (also known as bar). Just be sure to double check for pin names that have a line over them. Many ICs will have both active-low and active-high pins intermingled. If, however, the CE pin doesn't have a line over it, then it is active high, and it needs to be pulled HIGH in order to enable the pin. The CE pin would need to be pulled to GND in order for the chip to become enabled. If you see the CE pin anywhere in the datasheet with a line over it like this, CE, then that pin is active-low. For an active high pin, you connect it to your HIGH voltage (usually 3.3V/5V).įor example, let's say you have a shift register that has a chip enable pin, CE. If it's an active-low pin, you must 'pull' that pin LOW by connecting it to ground. Simply put, this just describes how the pin is activated.

When working with ICs and microcontrollers, you'll likely encounter pins that are active-low and pins that are active-high. The most common standard is TTL or Transistor-Transistor Logic. How is a logic 0 (LOW) or a logic 1 (HIGH) defined? Manufacturers of chips generally define these in their spec sheets. The strength of a signal is typically described by its voltage level. A binary 1 is also referred to as a HIGH signal and a binary 0 is referred to as a LOW signal. This is commonly translated as a binary 1 or binary 0. Binary Logic refers to one of two states - ON or OFF. Logic 0 or Logic 1ĭigital electronics rely on binary logic to store, process, and transmit data or information. There are several different technologies that have evolved over the past 30 years in electronics to define the various voltage levels. In Arduino, we call these signals HIGH or LOW, respectively. Represented in binary, an ON translates to a binary 1, and an OFF translates to a binary 0.

We often refer to the two states in a digital circuit to be ON or OFF. Put simply, a logic level is a specific voltage or a state in which a signal can exist.

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