Basic principles of microcontroller timer/counter

This article mainly studies the basic concepts of timers/counters. First, we start with the concepts of counting and timing in daily life to understand the implementation logic of timer/counter functions. An important knowledge point is overflow. When the timer/counter is full, an "overflow" action will occur, which will generate an interrupt. At the same time, we can further understand the three main methods of implementing timing or counting.

Timing and counting are the most common and widespread problems in daily life and production.

The timer and counter functions are implemented using essentially the same logic, and both functions involve counting input signals, essentially counting pulses. Counters count and indicate the number of input signals (events) in any interval, while timers count the number of input signals in a specified interval to indicate the elapsed time.

In a single-chip microcomputer, when the timer/counter is used for timing, it counts the machine cycle (obtained by dividing the crystal oscillator of the single-chip microcomputer by 12). Since the cycle of each count is fixed, the counting time can be easily calculated based on the number of counts. As shown in Figure 1.

 

Let's start with a routine in life: a basin is under a faucet, the faucet is not closed tightly, and water drips into the basin. The capacity of the basin is limited, the water drops continue to fall, and the water in the basin continues to fill up. Eventually, a drop of water makes the basin full of water, which is "overflow".

If an empty basin needs 10,000 drops of water to be full, you can put some water in before starting to drip, which is called the initial count value. If you want to count 9,000 now, you can put 1,000 drops of water in first, that is, the initial count value is 1,000, and then count 9,000 to overflow and generate an interrupt.

The method of counting initial values ​​is usually used in single-chip microcomputers. If each pulse is 1 microsecond, it takes 256 microseconds to count 256 pulses. If you want to time 100 microseconds, just put 156 in the counter first, and then count 100 to generate an interrupt when the overflow occurs. As shown in Figure 2.

There are three main ways to achieve timing or counting.

(1) Software delay

Software delay is achieved by using a microprocessor to execute a delay program segment. Because it takes a certain amount of time for the microprocessor to execute each instruction, software delay is achieved through the loop of instructions. Software timing has the characteristic of not using hardware, but it takes up a lot of CPU time. In addition, the accuracy of software timing is not high. Under different system clock frequencies, the time to execute an instruction is different, and the timing time of the same software delay program will also be different.

(2) Hardware timing

Hardware timing uses the frequency divider in the digital circuit to appropriately divide the system clock to generate the required timing signal. It can also use a monostable circuit or a simple timing circuit (such as the commonly used 555 timer) to control the timing time by an external RC (resistance, capacitance) circuit. Such a timing circuit is relatively simple, and the timing time can be changed within a certain range by using different frequency division multiples or changing the resistance value and capacitance value.

(3) Programmable hardware timing

The biggest feature of programmable timer/counter is that it can realize the change of timing time through software programming, and complete the timing function or counting function through interruption or query method. This circuit not only can change the timing value and timing range through program, but also has multiple working modes, can output multiple control signals, and has strong functions.