The instruction cycle refers to the time it takes for a microcontroller to execute a single instruction. It is typically an integer multiple of the machine cycle, which itself depends on the clock frequency. In traditional 80C51 microcontrollers, most instructions are executed in a single machine cycle, meaning the instruction cycle equals the machine cycle. However, some complex instructions, like multiplication or division, may take two or more machine cycles.
Modern microcontrollers, as of 2012, have evolved to eliminate frequency division, allowing for a one-to-one relationship between the instruction cycle, machine cycle, and clock cycle. This simplifies timing calculations and improves efficiency.
When examining the external signals of an 8051 microcontroller, such as ALE and PSEN, their frequency changes occur less frequently than a full machine cycle. Simulating these signals using C language can be challenging, so developers often rely on matching the timing specifications provided in the datasheet.
An instruction cycle is not fixed; it varies depending on the complexity of the instruction. For example, simple single-byte instructions may complete in just one machine cycle, while more complex operations might require up to four. The number of machine cycles needed per instruction determines whether it’s classified as a single-cycle, double-cycle, or multi-cycle instruction.
In most 51-series microcontrollers, one machine cycle consists of 12 clock cycles. However, some microcontrollers may define a clock cycle differently, such as 1 clock cycle equaling 2 oscillation cycles.
For PIC microcontrollers, each instruction cycle corresponds to four clock cycles. For instance, with an 8MHz crystal oscillator, the instruction cycle would be 0.5 microseconds (1/(8/4)).
Let’s look at some examples:
- Example 1: 35μs with an 8MHz crystal, 8-bit timer, prescaler 1:2, initial value E4.
- Example 2: 156.25μs with a 32768Hz crystal, 8-bit timer, prescaler 1:32, initial value FC.
Calculation method 1:
35 = (256 - initial value) * frequency division * 4 / crystal oscillator + 14 / frequency division
= (256 - E4) * 2 * 4 / 8,000,000 + 14 / 2
Calculation method 2:
0.015625 = (256 - initial value) * frequency division * 4 / crystal oscillator
= (256 - FC) * 32 * 4 / 32768
Note: When using a prescaler ratio of 1-8, you must add 14 / divide to the calculation.
Prescalers and postscalers are used in conjunction with counters. A prescaler divides the input signal before it reaches the counter, while a postscaler does the same after the counter has incremented. For example, if TMR2 has a 1:2 postscaler, it will trigger the overflow flag only after two overflows.
Modern development tools like Keil allow programmers to debug and measure execution times directly. By observing the timing of each instruction, developers can fine-tune their code for optimal performance and accuracy.
Understanding how to calculate instruction cycles is essential for real-time applications, especially in embedded systems where precise timing is critical. Whether working with 8051, PIC, or other microcontroller architectures, knowing the relationship between clock, machine, and instruction cycles ensures accurate control and efficient operation.
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