Книга: Introduction to Microprocessors and Microcontrollers

How it all works

To demonstrate its operation we can ask it to perform a simple task. Instruction: Send the number 25H which is in the ROM and store it in the RAM at address 2500H. This is what happens – follow the action on Figure 7.6.

1 The microprocessor has to collect the instruction from an address in ROM. It does this by putting the address onto the address bus.

2 The address is applied to the ROM and the RAM as well as the address decoder. This will not cause any problems because all the chip selects will be switched off at the moment. When the logic gates within the address decoder responds to the input from the address bus the result will be that the ROM is switched on and the other two are kept off.

3 Switching on the ROM will mean that it takes in the address from the address bus. Inside the ROM chip, the row and column decoders activate one of the memory locations and the binary number stored at that location is placed on the data bus by switching on the tri-state buffers. As soon as the information is read, the chip select will switch the ROM chip off.

4 The information which is now on the data bus is read by the microprocessor. It is an instruction which can be interpreted as ‘go to address F600H and read the number that is stored in that address’.

5 In response to this instruction, the microprocessor puts the address F600H onto the address bus.

6 The address decoder applies this number to its logic gates and this results in the chip select of the ROM chip being switched on again. The ROM chip accepts the address F600H into its row and column decoders and then puts the number 25H onto the data bus.

7 This number is stored temporarily in the microprocessor.

8 The microprocessor then puts the number 2500H onto the address bus and the address decoder puts a signal on the chip select of the RAM chip to switch it on. It then sends a logic 1 on the read/write line. The RAM is switched on and it is told to read the data on the data bus. The read/write line goes to the I/O chip as well but again, this causes no problem because its chip select line is keeping it switched off.

The number 25 is now safely stored in the RAM chip and will remain there until it is over-written with new information or the power is switched off.

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