Instruction Set and Timing

In order to understand--and better make use of--the 8051, it is necessary to understand some underlying information concerning timing.

The 8051 operates based on an external crystal. This is an electrical device which, when energy is applied, emits pulses at a fixed frequency. One can find crystals of virtually any frequency depending on the application requirements. When using an 8051, the most common crystal frequencies are 12 megahertz and 11.059 megahertz--with 11.059 being much more common. Why would anyone pick such an odd-ball frequency? Theres a real reason for it--it has to do with generating baud rates and well talk more about it in the Serial Communication chapter. For the remainder of this discussion well assume that were using an 11.059Mhz crystal.

Microcontrollers (and many other electrical systems) use crystals to syncrhronize operations. The 8051 uses the crystal for precisely that: to synchronize its operation. Effectively, the 8051 operates using what are called "machine cycles." A single machine cycle is the minimum amount of time in which a single 8051 instruction can be executed. although many instructions take multiple cycles.

A cycle is, in reality, 12 pulses of the crystal. That is to say, if an instruction takes one machine cycle to execute, it will take 12 pulses of the crystal to execute. Since we know the crystal is pulsing 11,059,000 times per second and that one machine cycle is 12 pulses, we can calculate how many instruction cycles the 8051 can execute per second:

11,059,000 / 12 = 921,583

This means that the 8051 can execute 921,583 single-cycle instructions per second. Since a large number of 8051 instructions are single-cycle instructions it is often considered that the 8051 can execute roughly 1 million instructions per second, although in reality it is less--and, depending on the instructions being used, an estimate of about 600,000 instructions per second is more realistic.

For example, if you are using exclusively 2-cycle instructions you would find that the 8051 would execute 460,791 instructions per second. The 8051 also has two really slow instructions that require a full 4 cycles to execute--if you were to execute nothing but those instructions youd find performance to be about 230,395 instructions per second.

It is again important to emphasize that not all instructions execute in the same amount of time. The fastest instructions require one machine cycle (12 crystal pulses), many others require two machine cycles (24 crystal pulses), and the two very slow math operations require four machine cycles (48 crystal pulses).


Many 8051 derivative chips change instruction timing. For example, many optimized versions of the 8051 execute instructions in 4 oscillator cycles instead of 12; such a chip would be effectively 3 times faster than the 8051 when used with the same 11.059 Mhz crystal.

Since all the instructions require different amounts of time to execute a very obvious question comes to mind: How can one keep track of time in a time-critical application if we have no reference to time in the outside world?

Luckily, the 8051 includes timers which allow us to time events with high precision--which is the topic of the next chapter.

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