Lesson : Digital Electronics

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In chapter 1 and 2, we learned about Ohm’s law, Power law, Kirchhoff’s law and resistors. Chapter 1 and 2 were about analog electronics. There is also the world of digital electronics which is a lot different to analog electronics. In analog electronics, voltages can take multiples values like shown on Figure 1. In digital electronics, the voltage will have defined values. On Figure 2, we can see that the voltage has two states (0V or 3.3V). Theses states are called logic level (“Low” and “High”). Often, “Low” logic level is called “0 (zero)” and “High” logic level is called “1 (one)”. In Figure 2, the “Low” logic level is 0V and the “High” logic level is 3.3V. The voltages level for a “Low” logic level and a “High” logic level will change between different device. In Figure 2, we decided that the “Low” voltage is 0V and the “High” voltage is 3.3V just for the example.

Digital electronics is used to transfer information in a reliable way since analog voltage tends to be noisy and it makes transferring datas challenging. Digital electronics fix this issue by having two or more defined states being far apart from each others on the voltage scale which creates a better noise immunity. A lot of popular and common communications protocols uses digital electronics to transfer datas reliably. Do note that almost all systems will have analog and digital electronics nowadays with some exceptions.

Binary number :

You may have noted that we have two defined states in the Figure 2 example. “0” (0V in our example) or “1” (3.3V in our example) which is a binary number. A binary number is a number expressed in base 2 number system which means they are only two digits (“0” and “1”). We commonly uses the base 10 number system often called the decimal system which has ten digits (0, 1, 2, 3, 4, 5, 6, 7, 8, 9). Counting and calculating with the binary system is just a different way of working with numbers than with the decimal system. In later lessons, we will sees how the binary system works in details. The important bits for now is to understand that in digital electronics, the binary system is used.

Logic level :

The voltage level used to defined a “0” or “1” are called logic level. You will have a zone called the logic level “High” and another called the logic level “Low” as shown on Figure 3. There is also a gray zone where the logic level is undefined and this could create an unwanted behavior of the circuit if the voltage is in this area. The behavior of this gray logic level will vary with each device/circuit and it is difficult to predict its behavior so it is strongly not recommended to operate in this area. There are some important information for the logic level in this figure. The $V_{IH}$ and $V_{OH}$ are used to defined the input voltage and output voltage for a logic high of a circuit or chip. The $V_{IL}$ and $V_{OL}$ are used to defined the input voltage and output voltage for a logic low of a circuit or chip. These informations can often be found in the datasheet of digital components. The ground (GND) and VCC (Common collector supply voltage or simply the voltage source of a device) are in general the minimum and maximum voltage the logic low and logic high will support. In some specific applications, VCC and GND could be replace by a specific voltage level.

$V_{IH}$ stand for “High” Level Input Voltage. This is the minimum voltage level used to detect a logic “High” or “1” for a specific device like a chip for example.
$V_{IL}$ stand for “Low” Level Input Voltage. This is the maximum voltage level used to detect a logic “Low” or “0” for a specific device like a chip for example.
$V_{OH}$ stand for “High” Level Output Voltage. This is the minimum voltage level used to send a logic “High” or “1” for a specific device like a chip for example.
$V_{OL}$ stand for “Low” Level Output Voltage. This is the maximum voltage level used to send a logic “Low” or “0” for a specific device like a chip for example.

The $V_{IH}$ , $V_{IL}$ , $V_{OH}$ and $V_{OL}$ are often found in datasheet. There are some standards that exists to define these levels to have compatibility between various chips maker. You will often hear about these standards. Some of the commons standards are CMOS and TTL which you will see quite often. We won’t dive into the details of these standard in this chapter but keep in mind that standards exists for logic level. These standards are quite useful to avoid having chip or device that aren’t compatible with each others. For chapter 3, we won’t go into the detail of theses parameters since we will mostly work with “1s” and “0s” when analyzing circuit instead of voltages. Since we are beginning to explore digital electronics, working with “0s” and “1s” will make it much simpler for analysis. You just need to understand that in real-life applications, they are voltage level that defines a “1” or “0”. In the next lesson, we are going to see our first logic gate and how it works.