Microcomputer Circuits and Processes

If you get the chance, look up 74154 IC in a databook and check the
logic inside it. The decoder's 4 inputs are connected to 4 bits of the extra
address bus, and. the outputs to the memory board enables, only the
first 4 being used here to get our lK of memory.
Figure 2.15 shows these connections. Remember we also had to
decide how to connect the read (RD) and write (WR) control signals.
That is simple, since each memory board only responds to RD and WR
when it is selected by its enable via the decoder. The decoder only
selects one board at a time, so the RD and WR signals only reach one
board at any time. Stated simply, the enable signal overrides both RD
and WR signals.
16-bit
address
bus
4 bits
8 bits
o
E
C
o
oE
R
t--------+--t-l~
E
Figure 2.15
Use of a decoder to address memory boards. Here, 4 extra memory lines are decoded,
giving a total of 16 addressable memory boards.
The use of a decoder to select the correct memory board is just one
example of decoding addresses. The same technique can be used in
selecting input and output chips, as you will see. Some commercial
equipment uses this technique for selecting whole subsystems, such as
data acquisition systems, measurement modules, robot control boards,
advanced graphics boards, and the like.
TIMING
DIAGRAMS
We have seen how a microprocessor interacts with some memory via
the address and the .data buses and via the RD and WR signals of the
control bus. For the microprocessor to work properly things must
happen at the right time: for example, it is no good sending data along
the data bus to memory, then some time later sending the address where
the data was to be stored. Obviously the correct sequence is first to send
out the address on the address bus, and then send out the data. Finally,
a pulse has to be sent out on WR to actually write the data into the
memory. This is an example of timing.
It is the job of the CPU to do the timing. That is why there is a clock
on the CPU chip producing a square wave of frequency around 2 MHz.
These clock pulses drive complex logic circuits inside the CPU which
decide when to output address, data, RD, WR, and other signals onto
the correct bus. Connecting an oscilloscope with several traces to the
CPU signals is the next best thing to stripping down a CPU and
dissecting its logic. Figure 2.16 shows the format of the display.
16