John Crisp - Introduction to Microprocessors and Microcontrollers

Designing a decoding circuit

Let’s imagine that we have a microprocessor-based system using eight memory chips, ROM or RAM it doesn’t matter. Each of the chips holds 8 kbytes of memory. From Table 7.1 we can see that an 8 kbytes chip will require 13 address lines in order to access each of their internal locations. Assume too, that the microprocessor that we are using has a 16-bit address bus so we have the situation shown in Figure 7.7. The address lines are numbered from A0 (address line number 0) to A15. The 13 bits A0–A12 are heading off towards the ROM and RAM chips. The remaining three address lines, A13–A15, are used by the address decoder.

Figure 7.7 There are three ‘spare’ address lines

The decoding chip

The decoder circuit can be made from separate logic gates or can be bought ready-built in a single integrated circuit. For ease of construction, most designers opt for this choice for the result is smaller, dissipates less heat and is less expensive (and it works first time). There is very little to be said for the build-it-yourself approach.

The basic requirements are three input address lines and eight output lines each connected to one of the chip select pins on a memory chip.

To switch the chips on, the chip select must be taken to a logic 0 voltage. A logic 1 voltage level will switch the chip off. It is vital, of course, that only one chip can be switched on at the same time otherwise they will load competing data onto the data bus and are likely to be destroyed. The three addresses can result in 2³=8 different inputs to the logic gates built into the decoder chip. The internal design ensures that when the address pins are all at zero, the first output goes to a logic 0 and all the others remain high. The memory chip to which this first output is connected is switched on and all the others are on. When the next combination of inputs 0, 0, 1 is applied, the second memory chip is switched on and the others are off. The next combination switches on the next memory chip and so on until the three input wires have switched on each of the memory chips with a single combination of addresses (see Figure 7.8). With three inputs and eight outputs, it is referred to, reasonably enough, as a 3 to 8 decoder.

Figure 7.8 The operation of the address decoder

Table 7.2 looks a lot worse than it really is. It is really just a summary of the decoder chip outputs. If the microprocessor put the address C2F1H on the address bus, then in binary it would be: 1100 0010 1111 0001. It has been broken up into groups of four Table 7.2 The 3–8 decoder can control eight memory chips just to make it a little easier to read. The most significant bit, A15, is on the left-hand end.

Table 7.2 The 3–8 decoder can control eight memory chips

In Table 7.3, we can see that 13 out of the 16 address lines go to the memory chip and the other three are fed to the decoder chip. The three lines going to the decoder chip carry the data 1 1 0. We can see that the values C=1, B=1 and A=0 occur near the bottom of the table. These values result in Chip 6 receiving a logic 0 value and thus being selected for use. All other chips are deselected by the logic 1.

Table 7.3 In full decoding, every address line is used A

Full decoding

In the above example, the 8 kbyte memory chips used 13 address lines and the decoder used three. This makes a total of 16 lines used out of a 16-bit address bus. There are no unused lines and this is referred to as ‘full decoding’.

Partial decoding

Now let’s make a small change. The memory chips used are 4 kbyte each rather than 8 kbyte. What effect would this have?

The first result would be that the number of address lines going to the chips would be reduced to 12. There are still only eight chips to be selected so a 3–8 decoder is enough. So what have we got now?

Twelve address lines to the chips and three to the decoder and one left over and unused. If nothing is connected to this line, then it cannot matter what voltage it carries (see Table 7.4).

Table 7.4 One address line is unused

We will look at our previous address C2F1H. If it happened to go to a value of 1, the address would change from:

1 1 0 0 0 0 1 0 1 1 1 1 0 0 0 1 (C2F1H)

to

1 1 0 10 0 1 0 1 1 1 1 0 0 0 1 (D2F1H).

We now have two numbers that can be placed on the address bus which will result in access to the same memory location since all the bits that are actually used are identical. If we instructed the microprocessor to store some information in the address C2F1H and then to recover the information from address D2F1H, we would get the same information again. The address D2F1H is referred to as a ghost address or an image address. It is important to appreciate that ghost or image addresses have no effect at all on the operation of the microprocessor system. They are merely alternative names for a single address. Incomplete or partial decoding always gives rise to image addresses, their number and their addresses are easily worked out. In technospeak, we say that partial decoding results in more than one software addresses pointing to the same hardware address.