Книга: Introduction to Microprocessors and Microcontrollers

Two types of RAM

Ram chips can be designed in two different forms which we call static RAM (SRAM) and dynamic RAM (DRAM), as seen in Figure 6.16.

Figure 6.16 The two types of RAM

Static RAM

These are constructed of flip-flops. The problem with the flip-flop is that it draws current all the time. Therefore, it tends to get rather warm and, on a single chip, the components cannot be packed together very tightly. The benefit is that they are very fast and are used where speed of access is important. Static RAM is often called SRAM.

Dynamic RAM

These store the information in capacitors, which are small components that store an electrical charge in the form of static electricity. They are called ‘dynamic’ owing to one of its drawbacks. In use, the electricity stored in each capacitor leaks away because of the imperfect insulation. So, after a little while the charge has to be replaced otherwise the DRAM will be empty and all the stored information will be lost. This replacing is called ‘refreshing’ and has to be performed at intervals of about 2 ms by a DRAM control circuit. To prevent any interference with the operation of the microprocessor system, the refreshing is done in the background whenever the DRAM is not being used.

Once the static charge is stored, no further current is required (except for refreshing), therefore less heat is being generated internally and we can pack more memory into a given space. We say it has a high packing density.

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