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However, if you want, you could use the SPI Bus outside
the PCB at low speeds, only that this is not practical: the SPI Bus requires 3
or 4 lines, which are a bit too many, when compared to 1 or 2 lines needed to
communicate, efficiently, with field devices outside the PCB.
Anyway, on the
PCB the SPI Bus is very good, because we can practically attach to the Bus as
many IC chips (or devices) as we want. Please excuse me for not providing a
picture of the SPI Bus, but rest assured that you do not need one: the SPI Bus
is so simple that you will understand everything in words.
The next question is: “What is this SPI Bus good for?” Besides from
exchanging data between various IC chips, the SPI Bus is a method of multiplying
microcontroller’s pins. In other words, if you have a tiny 8 pins
microcontroller, you could control with that little monster few hundreds of
digital Inputs and Outputs. This is impressive, and I am certain that many doubt
my words; let’s explain this.
The SPI Bus contains three lines, and they can be on any I/O controller pins;
these lines are: Clock, Data-In, and Data-Out. In addition, each IC connected to
the SPI Bus needs an individual Enable line. Things work like this: suppose that
we have four devices, A, B, C, and D, all wired to the SPI Bus, and the Bus
itself is wired to seven controller pins--this is 3 Bus lines plus the 4 Enable
ones. When we want to send a message to device C, we enable its Enable line
first, than we send the message serially, one bit at a time. In the same time
devices A, B, and D do exactly nothing, because they are not enabled.
The beauty with the SPI Bus is that it is Synchronous, meaning that when the
controller sends the message to one IC it is also able to receive data from that
IC, in the same time. This particular aspect of the SPI protocol is very well
suited for microcontroller-to-microcontroller communications.
Now, we have seen that a small 8 pins microcontroller can control 4 devices
(ICs) using 7 pins. Taking into account that one device of type A, B, C, and D
could have eight or sixteen I/O ports this is still far from the hundreds Inputs
and Outputs I promised to you. The second beautiful thing about SPI is, one
device IC can be serialized with many more. For example, we could have B1, B2,
B3, B4, B5, and so on. All of them are serialized together, and they require
only 4 microcontroller pins to make them work: the Enable line is common to all
of them. Now it is clear, I hope, that, indeed, we can have hundreds of I/O
lines on a small 8 pins controller.
Further from this general presentation of the SPI Bus, you should be aware
that almost all ICs implement the SIP protocol in a particular way. For detailed
and practical applications I suggest that you visit my home site at Corollary
Theorems. There you are going to discover a good tutorial book about working
with hardware, firmware, and software design, in general, and about few
practical implementations of the SPI Bus, in particular.
Many microcontrollers have built-in SPI Bus hardware modules, but I was never
interested too much about using them. What I do, I always design--on the PCB and
for one microcontroller--one, two or more custom SPI Busses, because my custom
implementations are far more flexible. Besides, practical implementation of a
custom SPI Bus it is really simple--trust me with this one!
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