In this blog post, we’ll explore why transit card readers are equipped with clocks by examining the principles behind electronic clocks and quartz oscillators.
Electronic Clocks and Quartz Oscillators
Just a few decades ago, clocks were considered luxury items, but those days are long gone. Every room has at least one wall clock, and clocks are built into computers, smartphones, and tablets as standard features. Even beyond expensive electronic devices like computers and smartphones, a surprisingly large number of electronic devices come equipped with a clock function—and even the transit card readers on buses have clocks.
Of course, having a clock isn’t a bad thing. The clock on a card reader is quite useful for gauging whether you’re running late while crammed onto a packed bus on your way to school. However, the card reader would still function just fine without the clock feature. So why bother including a clock on the card reader?
To put it simply, it’s because both electronic devices and clocks use a key component called a quartz oscillator. To understand this in more detail, let’s first examine the principles behind electronic clocks and quartz oscillators.
Almost all clocks in use today are electronic. They are commonly called quartz watches, a name derived from their use of a quartz oscillator. Quartz is a material that exhibits the piezoelectric effect: when a voltage is applied, it changes shape; conversely, when its shape changes, it generates an electric field that produces a voltage. By utilizing this property and shaping the quartz into a specific form, it can be made to vibrate at a constant frequency, much like a spring—and this is precisely what a quartz oscillator is.
The oscillation frequency of a quartz crystal is extremely consistent, resulting in minimal error. Therefore, time can be measured simply by accurately counting how many times the quartz crystal oscillates in one second. For example, if a quartz crystal oscillates once per second, a watch could be made simply by counting the number of oscillations. However, such a slow oscillation results in poor accuracy, so it is not used in practice. A typical quartz watch is designed so that the crystal oscillator vibrates 32,768 times (2¹⁵) per second, and by counting these vibrations, it ensures that one second elapses after a certain number of counts.
So, what is the relationship between this crystal oscillator and electronic devices?
The Relationship Between Electronic Devices and Quartz Oscillators
Electronic devices refer to devices—such as computers, smartphones, and transit card readers—that process, store, or output information input from an external source via electronic circuits. Taking a transit card reader as an example, when a card is recognized, the information stored on the card is transmitted to the reader. Based on this information, the reader calculates the fare, deducts the amount from the balance, saves the updated balance, displays it on the screen, and communicates the necessary information to a central server. Computers and smartphones also operate on the same principle, as they are devices that process and store information entered via keyboards, mice, or touchscreens, and then output the results through screens or speakers.
A particularly important aspect of electronic devices is their computational capability. The circuits responsible for computation can be broadly divided into components that perform calculations and those that temporarily store information.
Computational components actually perform the calculations. They receive inputs where a high voltage represents 1 and a low voltage represents 0, and they perform operations such as addition, subtraction, logical operations, or selecting specific signals. However, since these computational components lack the ability to store information, the output value changes immediately when the input value changes.
Take the AND gate, a representative computational element, as an example: it outputs a 1 only when both inputs are 1, and outputs a 0 if either input is 0. Therefore, the moment the input values change, the output value changes within a very short time as well, so the calculation result cannot be stored.
However, during the operation of electronic devices, it is often necessary to temporarily store calculation results for the next operation. The component used for this purpose is the flip-flop. A typical example is the D flip-flop, which receives a data input (D) and stores it at the output (Q); once stored, the value remains unchanged even if the input changes. However, to store data, the timing of the storage must be determined by a clock signal. In other words, a signal that changes at a constant cycle is absolutely necessary.
This is where the crystal oscillator comes into play. Since a crystal oscillator oscillates by fluctuating in voltage at a very consistent frequency, it can provide the reference signal for the flip-flop to store data. This periodic signal is called a clock, and most clock signals used in electronic devices are generated using crystal oscillators.
Computer advertisements often use phrases like “2.5 GHz clock”; the clock frequency referred to here denotes the speed of the reference signal that governs the CPU’s operation. Since a large number of flip-flops are used inside a CPU, a higher clock frequency allows it to process more tasks. Of course, actual performance is influenced by various factors—such as CPU architecture, design, and instruction processing efficiency—so performance cannot be judged by clock frequency alone, and simply increasing the clock frequency does not necessarily improve performance.
Why Transportation Card Readers Have Clocks
To summarize what we’ve covered so far, we can see that electronic clocks contain quartz crystals, and electronic devices also incorporate quartz crystals. Transportation card readers also require quartz crystals because they use various digital circuits and flip-flops to read card information, calculate fares, store balances, and communicate with central servers.
Meanwhile, the clock function has a structure simple enough to be implemented simply by counting the number of oscillations of the quartz crystal. All that is required is to store the current time and increment it by one second every set number of oscillations; in today’s environment, with advanced integrated circuit technology, the space and cost required to implement this are minimal.
Ultimately, adding a clock to a transit card reader does not incur significant costs. Since the crystal oscillator required for the device’s operation is already built in, implementing the clock function alongside it is not difficult and provides convenience to users. Therefore, the clock on a transit card reader can be viewed as a small service—much like the mints offered at restaurants—that costs almost nothing but is helpful to have.
