Книга: Introduction to Microprocessors and Microcontrollers

A practical microprocessor system

Apart from in the most minimal of circuits, some RAM is needed. Even if the microprocessor-based system is controlling an oven, we still need the facility to vary the instructions to change the temperature, the time cycle, the fan speed etc., so some RAM must be added. Some microprocessors have a small amount of RAM included internally, enough for this sort of system to work but still quite limited. If we add some external RAM, the microprocessor is controlling the operation of three chips: ROM, RAM and I/O. To control the flow of information it needs to send chip select and read/write information along the control bus.

Read/write signals tell the RAM and the I/O chip whether they have to read, i.e. accept information from the data bus or to write information onto the data bus.

Chip select is the on/off switch for each of the chips and we have to be very careful to ensure that only one set of information is being connected to the data bus at one time.

If a ROM chip were to be putting a binary 0 onto one of the data bus connections and, at the same time, another ROM or a RAM chip was applying a binary 1, there would be a disagreement between the two chips. What would happen? It is sad but the likely outcome is a fight to the finish with one or other of the chips being condemned to the waste bin.

To prevent this from happening, an address decoder circuit samples the address bus and selects the appropriate chip. If the microprocessor wishes to send some data to the RAM chip, it applies a suitable address to the address bus that is applied to the ROM, RAM and the I/O controller but there are no problems at the moment since all these chips are switched off. The address decoder applies the inputs from the address bus to an array of logic gates that have been organized to comply with the memory map of the system. The output from the address decoder then switches the RAM chip on and the ROM and I/O chips off. The design of the address decoder can be modified to control any number of external chips in the system. An upgraded system is shown in Figure 7.6.

Figure 7.6 A complete microprocessor system

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