What is the timing of the microcontroller? The timing of the timing

Crystal Oscillator

Quartz Oscillator 3225 20M OSC

Timing is a fundamental concept in microcontroller operation. We know that the microcontroller executes instructions by fetching them sequentially from memory and performing a series of micro-operations. These operations must occur in a strict time order, which defines the timing of the microcontroller. Think of it like the school bell that regulates classroom activities — everything happens in coordination with the sound. So how does the microcontroller manage its timing? Let's dive deeper.

The basic unit of timing in a computer is the machine cycle. This is the time it takes for the computer to access memory. Just like seconds in our daily lives, one machine cycle typically consists of 12 oscillator cycles. The oscillator period is the time between two consecutive oscillations of the crystal. For a 12MHz crystal, the period is T = 1/f = 1/12μs. Therefore, a machine cycle would be 12 × 1/12μs = 1μs.

Inside the MCS-51 microcontroller, there’s an internal high-gain inverting amplifier connected to pins XTAL1 and XTAL2. To create a stable oscillator, we connect a crystal and two capacitors. The choice of crystal frequency determines the speed of the microcontroller, while the capacitor size affects the stability and startup time of the oscillator. It’s common to use ceramic or trimmer capacitors in the range of 10–30pF. Also, placing the crystal and capacitors close to the chip helps reduce PCB parasitic capacitance and improves signal integrity.

While the internal clock is commonly used, external clocking is also possible, especially when multiple MCUs are involved. In such cases, a single external pulse can synchronize all devices. If no external signal is used, the XTAL2 pin remains unused, as shown in the diagram. This setup ensures consistent timing across the system.

Not all instructions take the same amount of time to execute. Some require just one machine cycle, while others take two, three, or even four. For example, the DJNZ instruction in the 89C51 requires two machine cycles. At 12MHz, each cycle is 1μs, so two cycles equal 2μs. If you're using this for a delay, say 62,500 iterations, the total delay would be 62,500 × 2μs = 125ms. That’s why LEDs flash quickly if the delay is too short.

In summary, the clock circuit is essential for the proper functioning of the microcontroller. It provides the timing reference that keeps all operations synchronized. Whether using an internal or external clock, ensuring stability and accuracy is key to reliable performance. Understanding these basics will help you design more efficient and robust embedded systems.