Tim, "What does control the voltage?"
Tim, I will try to address some of your questions.
Of course, even an introductory explanation would
require few full-hour lectures. Anyway, I will try
to link the long chain from MOSFET transistors
to a computer/microprocessor.
1. The basic element of every microcircuit is
an invertor. It takes two transistors connected
in series between ground and Vcc, with their gates
connected together. These two transistors have
differnt switching features and are called "complimentary". Nice animated illustration of this can be found here:
tech-www.informatik.uni-hamburg.de
Another nice site, more advanced, shows how
individual transistors are connected to form
boolean logical functions, or "logical gates"
(do not mix them up with transistor gates!!!):
eng.uci.edu
2. In final design, every input of every gate is connected
to output of another internal gate, or to some
input buffers, to perform communication (I/O)
with external world, usually via a collection of
signals known as busses. A bus usually have several
address lines ( to select external memory location),
plus data and control lines.
The internal gates form bunch of complex
structures - flip-flops, bigger storage and
control registers, even bigger logical blocks, local memories, etc.
Every such structure is usually controlled by a
a special self-contained circuitry called
"state machine". Depending on variety of input signals, it uses input clock as "initial voltage
switch", and hops between variety of states that
control (send low or high voltages to) other
logical blocks. See e.g.
ee.ntu.edu.au
The chip usually has several
other external control signals that must be
provided by a mainboard. The most important signals
are RESET and abovementioned CLOCK.
The RESET signal usually is generated
once upon the power-on event, or can be issued
manually. The main purpose of this signal is to
ensure that the chip starts from a known pre-defined
state, and is kept in this state until RESET goes off
and all state machines start to use the CLOCK and
logically analyze input signals.
3. For a microprocessor chip, the first thing it
does it initiates a external read operation from
a predefined memory location. In PC, the first thing a CPU
does is a fetch of instruction from address FFFFFFF0.
Always, that's all it can do alone.
To force a CPU to do more meaningful job, a board
designer has to place a memory chip on the bus, which
would respond to this initial address, and make
sure it contains a proper chain of instruction for the CPU. This memory chip usually contains a program that performs
inital configurations of whatever needs to be
configured, provides basic input/output
subroutines to communicate with peripheral devices
such as floppy and videocard, and therefors is
called BIOS.
4. I'll stop here for now. For another drink.
- Ali |
