The instruction cycle refers to the time required for a microcontroller to execute a single instruction. This duration is typically an integer multiple of the machine cycle. In traditional 80C51 microcontrollers, most instructions are executed in a single machine cycle, meaning the instruction cycle equals the machine cycle. However, some instructions, like multiplication or division, may take two or more machine cycles.
Modern microcontrollers, as of 2012, have advanced to eliminate frequency division, allowing for a one-cycle instruction execution where the instruction cycle, machine cycle, and clock cycle are all aligned. This improves efficiency and performance significantly.
Looking at external signals such as ALE and PSEN on an 8051 microcontroller, their frequency changes occur less frequently than one machine cycle. Simulating these signals using C language can be challenging, so timing must be matched carefully, often extending the cycle for accurate results.
An instruction cycle is not fixed because it depends on the number of machine cycles required by the specific instruction. For example, a simple instruction might require just one machine cycle, while complex ones could need up to four. In general, for most 51-series microcontrollers, one machine cycle equals 12 clock cycles.
Some microcontrollers have different relationships between clock cycles and oscillation periods. For instance, one clock cycle might consist of two oscillation cycles, depending on the architecture.
In summary, the instruction cycle is defined by the number of machine cycles needed to complete the instruction. Simple instructions usually take one machine cycle, while more complex operations require multiple cycles.
For PIC microcontrollers, every four clock cycles make up one internal instruction cycle. For example, with an 8 MHz crystal, the instruction cycle would be 1/(8/4) = 0.5 μs.
Examples include:
- 35 μs with an 8 MHz crystal, 8-bit timer, 1/2 division, and initial value E4.
- 156.25 μs with a 32768 Hz crystal, 8-bit timer, 1/32 division, and initial value FC.
Calculation methods vary, but they generally involve the formula: (256 - initial value) × division ratio × 4 / crystal frequency.
Special notes mention that for division ratios from 1 to 8, an additional 14/division factor should be added.
Prescalers and postscalers are used in conjunction with counters. The prescaler affects the counting process before the counter updates, while the postscaler acts after the counter has changed. For example, TMR2’s postscaler delays the overflow flag until a set number of overflows occur.
Modern development tools like Keil allow developers to simulate and debug programs, helping them determine the exact time each instruction takes. This makes it easier to understand and optimize timing in real-time applications.
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