Книга: Introduction to Microprocessors and Microcontrollers

How to make a microprocessor go faster?

Increase the clock speed

This seems the obvious answer and generating the necessary square wave shown in Figure 6.4 is no problem. In a modern microprocessor-based system there are two clocks that we need to consider. There is a square-wave clock that controls the internal operation of the microprocessor. This is the headline speed seen in the adverts, ‘the 2 GHz Pentium’. There is also the operating clock, about 133 MHz, for the system to control the external devices and memory. This saves us from upgrading all the external devices to match each new processor.

Internal clock speeds will probably continue to increase at least into the low gigahertz range but there are limiting factors that will make continuous increases in speed difficult to achieve. It would be a trivial electronic problem to generate a square wave of 1000 GHz or more, so there is obviously more to it. And there is.

Power dissipation

Heat is a form of power and is an unwanted by-product of any activity inside a microprocessor.

Power = voltage ? current

So, to reduce heat production, we have to reduce either the voltage or the current, or both.

We will start with voltage since it is a little more straightforward. The early microprocessors used a 15 V supply that has been steadily reduced and the latest designs are pushing at 1.5 V. How much further can we go along this line? It appears as if we have nearly reached the limit. The integrated circuits are made from a semiconductor called silicon. This passes electricity under the control of electric charges. The applied voltage creates this charge. In silicon, the simplest device needs at least 0.6 V to operate although, by adding minute traces of other materials, this figure can be reduced a little. A single transistor can do little with voltages less than 1 V so in a complex circuit it is already amazing that the total voltages can be as low as they are. The chances of a microprocessor running on less than 1 V is slight indeed. If I were braver, I would say impossible. Another point with regard to the voltage is that we must not forget the effects of random electrical noise as we saw earlier in Figure 2.2. Sudden changes in current flow in nearby circuits can cause random changes. If the voltages are reduced too far, the microprocessor will become more prone to random errors.

Bursts of current are promoted by the vertical leading and trailing edges of the clock pulses so the higher the clock frequency, the more edges per second and the more current will flow and hence more heat will be generated. To reduce the heat generated simply reduce the clock speed, which is exactly what we don’t want to do.

Size of architecture and its effects

As the electric charges move through the transistors inside the microprocessor it takes a finite time. It follows then, that if we reduce the size of the transistors we can move data around faster and this is true.

The smallest feature that could be fabricated in a microprocessor had an initial size of about 10 ?m when microprocessors were first produced; it has now been reduced to 0.13 ?m, a significant reduction. This reduction has two drawbacks. Firstly, it is much more difficult and expensive to manufacture without accepting enormous failure rates. Secondly, the heat generated has not changed, since it is a feature of voltage and current but not size. This means that its temperature will increase unless we can dissipate the power. An unfortunate problem with semiconductors is that they are heat sensitive and will auto-destruct if the temperature rises too far. We do our best with heat sinks, which are basically slabs of aluminum with fins to increase the surface area, and fans to keep the heat moving. A typical operating range is 0–85°C when measured in the centre of the outer case of the microprocessor (not the heat sink).

To increase the system speed

When we look at the overall system, it is apparent that not all things have progressed at the same rate. The largest bottleneck is the memory. We already use a slower clock speed but the microprocessor still spends a lot of its time humming a tune and bending paper clips waiting for information to arrive from the memory. During the life of the microprocessor, the clock speed has increased from 0.1 MHz to about 3 GHz, an increase of about 30 000 times. During this time, these DRAM memories have got bigger but only about 2000 times faster.

Modern microprocessors have about 128 kbyte of on-board RAM, called a cache. When the microprocessor has to go to the external memory for information, it saves a copy of the address and the information in case it is needed again. It also saves the address and information from the next memory location. The reasoning behind this is that since nearly all languages are procedural then the next location is likely to be accessed next. If not, the program may jump back to a previous address to repeat part of a program as in a counting loop to produce a delay. When the microprocessor next requires access to the memory, it first checks the high speed cache to see if the information is stored, if it is, we have scored a ‘hit’ and the system has increased its speed. If it is not there, it is a ‘miss’ and the main memory is used. This new information is then stored in the cache for later.

This cache is sometimes called a level 1 cache, or L1 cache. This implies that there may be a level 2 cache – and there is. The L2 cache is usually 256 kbyte.

When data is needed, the microprocessor checks cache level 1, then 2 and lastly, the main memory.

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